University of Melbourne Chemistry Worksheet

REPORT COVER SHEET AND DECLARATIONSchool of Chemistry
The University of Melbourne
Laboratory Report Cover Sheet
Student Name:
Student Number:
Subject Name & Code:
Demonstrator:
Experiment Title:
F5: Chemical Equilibrium
Due Date: _____________________________
By submitting work for assessment, I hereby declare that I understand the University’s policy
on academic integrity and I declare that:
• This laboratory report is my own original work and does not involve plagiarism or
unauthorised collusion, except where due credit is given to the work of others. The report is
based on results and spectra obtained by me during my laboratory session.
• This laboratory report has not previously been submitted for assessment in this or any other
subject.
For the purposes of assessment, I give the assessor of this assignment the permission to:
• Reproduce this laboratory report and provide a copy to another member of staff; and
• Take steps to authenticate the assignment/laboratory report, including communicating a copy
of this assignment to a checking service (which may retain a copy of the assignment on its
database for future plagiarism checking).
Feedback on Report: Feedback on your report and the mark you received will be available on the
Online Practical Assignments page on Canvas.
Plagiarism:
Plagiarism is the act of representing as one’s own original work the creative works of another,
without appropriate acknowledgment of the author or source.
Collusion:
Collusion is the presentation by a student of an assignment as his or her own work, but which is in fact
the result in whole or in part of unauthorised collaboration with another person or persons. Collusion
involves the cooperation of two or more students in plagiarism or other forms of academic
misconduct.
Both collusion and plagiarism can even occur in group work. For examples of plagiarism, collusion and
academic misconduct in group work please see the University’s policy on Academic Honesty and
Plagiarism: https://academichonesty.unimelb.edu.au
Plagiarism and collusion constitute cheating. Disciplinary action will be taken against students who
engage in plagiarism and collusion as outlined in University policy. Proven involvement in plagiarism
or collusion may be recorded on your academic file in accordance with Statute 13.1.18.
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
1
Experiment F5. Chemical Equilibrium
Author(s):
Day/Time/Group number:
Abstract:
(Summary of what you did and what you found out)
Introduction and Aim:
(What is a dynamic equilibrium? Why is it of interest? How are you measuring it? and
what are you trying to find out?)
Experimental:
(How did you perform your experiment? A full method is not required. Make sure to
reference the source, year and publisher.)
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
2
Results and Calculations:
(Tables of data and calculations)
PART A: CHEMICAL EQUILIBRIUM SHIFTS FOR [Fe(SCN)]2+ COMPLEX IONS SYSTEM
1. The Equilibrium System of [Fe(SCN)]2+ Complex Ions
Fe3+(aq)
+
SCN– (aq)
⥫⥬
[Fe(SCN)] 2+(aq)
iron(III)
thiocyanate
iron(III) thiocyanate
ions
ions
complex ions
SOLUTION
COLOUR
Fe(NO3)3 (aq)
KSCN (aq)
[Fe(SCN)]2+ (aq)
2. Chemical Equilibrium Shifts.
The test tube reactions are listed in Table 1.
Record your observations in Table 1 – colour changes and any precipitation observed. (Column 3)
Question 1 (Column 4)
For each test tube reaction 2 – 9:
For each test, the equilibrium is disturbed and the system then re-equilibrates.
Using the colour of solution (Part A1 above), identify which reaction is favoured, forward or reverse
for the reaction:
Fe3+(aq)
+
SCN– (aq)
⥫⥬
[Fe(SCN)] 2+(aq)
H -ve
– does the reaction shift to the right to make more product or to the left to make more reactant?
Provide a concise explanation (see example in table).
Tips:
Fe(OH)3 is insoluble in water;
AgSCN is insoluble in water;
NH3 is good base that means it undergoes the following reaction in water:
NH3 (aq) + H2O (l) → NH4+ (aq) + OH- (aq)
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
3
Table 1. Chemical Equilibrium Shifts: Fe3+(aq) + SCN–(aq) ⥫⥬ [Fe(SCN)]2+(aq) H -ve
TEST TUBE
NUMBER
TEST
SOLUTION
ADDED
(4 to 5 drops)
OBSERVATIONS
(e.g. colour changes/
precipitation/
colour of precipitate)
example
0.5M K3PO4
Solution becomes
light yellow
QUESTION 1
Reaction shifts towards products (right) or reactants(left)?
Circle/highlight your answer.
Explain.
Reaction shifts to left(reactants) / right(products).
Insoluble FePO4 forms, less Fe3+ ions in the solution. The
reaction shifts to the left to form more Fe3+ to get back to
equilibrium. {write reaction equation into Table 2:
Fe3+(aq) + PO4 3–(aq) → FePO4 (s) }
Reaction shifts to left(reactants) / right(products).
2
0.2 M Fe(NO3)3
Reaction shifts to left(reactants) / right(products).
3
0.2 M KSCN
Reaction shifts to left(reactants) / right(products).
4
0.1 M AgNO3
Reaction shifts to left(reactants) / right(products).
5
0.2 M KCl
Reaction shifts to left(reactants) / right(products).
6
1.0 M NH3
Reaction shifts to left(reactants) / right(products).
7
2.0 M NaOH
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
4
TEST TUBE
NUMBER
TEST
SOLUTION
ADDED
(4 to 5 drops)
OBSERVATIONS
(e.g. colour changes/
precipitation/
colour of precipitate)
QUESTION 1
Reaction shifts towards products (right) or reactants(left)?
Circle/highlight your answer.
Explain.
Reaction shifts to left(reactants) / right(products).
H -ve means reaction is ……thermic
8
ice bath
Reaction shifts to left(reactants) / right(products).
H -ve means reaction is ……thermic
9
hot water bath
Question 2
Write an ionic equation only for any precipitation reaction you observe.
Table 2. Precipitation reactions observed
Reaction
number
Reaction equation for precipitation reaction observed
Example
Fe3+(aq) + PO4 3–(aq) → FePO4 (s)
PART B: DETERMINATION OF EQUILIBRIUM CONSTANT (Kc)
The equilibrium constant, K, for the following reaction:
Fe3+(aq) +
SCN –(aq)
⥫⥫
[Fe(SCN)] 2+(aq)
Is written as
𝐾=
[[𝐹𝑒(𝑆𝐶𝑁)]2+ ]
[𝐹𝑒3+ ][𝑆𝐶𝑁 – ]
Four different sets of concentrations of the reactants are prepared and the concentration of
[Fe(SCN)] 2+(aq) at equilibrium is measured in each solution using absorbance spectrophotometry.
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
5
1. Standard Solution for [Fe(SCN)]2+ Complex Ions
Fe3+(aq)
+
large excess
SCN –(aq)
⥫⥫
limiting reactant
[Fe(SCN)] 2+(aq)
A standard solution of [Fe(SCN)] 2+(aq) is prepared.
As the [Fe3+]initial concentration is in large excess (100 fold), according to Le Chatelier’s Principle, the
equilibrium will shift to the product side until all the SCN– is converted to [Fe(SCN)] 2+.
Thus the equilibrium concentration [Fe(SCN)] 2+ in the standard solution (now called [Fe(SCN)] 2+standard) is
approximately equal to the initial concentration of SCN– (i.e. [SCN–]initial) in the 20.00 mL solution.
[Fe(SCN)] 2+standard = [SCN–]initial
Question 3
a. Calculate the concentration of [SCN-] initial in standard solution (20.00 mL). (Show
working, use c1V1 = c2V2)
b. What is the concentration of [Fe(SCN)] 2+ standard?
2. Finding the concentration of [Fe(SCN)]2+ n in solutions A to D using Absorbance measurements.
Four different mixtures of Fe(NO3)3 and KSCN are prepared according to the table below:
Conical Flask
Fe(NO3)3
(mL)
KSCN
(mL)
Distilled Water
(mL)
Total Volume
(mL)
A
5.0
2.0
3.0
10.0
B
5.0
3.0
2.0
10.0
C
5.0
4.0
1.0
10.0
D
5.0
5.0
0.0
10.0
Student Notes Table F5.1 Reaction mixture in each flask.
The iron(III) thiocyanate complex absorbs light at 460nm. The Beer-Lambert Law given in equation
F5.1 for the absorbance of light by [Fe(SCN)]2+ at 460 nm may be written as:
Absorbance (A) 𝜶 𝒄𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏
=εxlxc
= ε x l x [[Fe(SCN)] 2+]
(F 5.3)
showing a linear relationship between A and [Fe(SCN)]2+ where ε and l are constants;
l is the pathlength of the cuvette which is 1.00 cm
ε is the molar absorption coefficient for [[Fe(SCN)] 2+] at 460 nm
{a measure of how well [[Fe(SCN)] 2+absorbs light at this wavelength}
Since Abs is proportional to concentration, we can say that
Abs (solution1)/conc (solution 1) = Abs(solution2)/conc (solution 2)
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
6
Since we treat ε x l as constants, we can solve for ε x l using our standard solution of known
concentration:
Rearrange equation F5.3:
𝑨𝒔𝒕𝒂𝒏𝒅𝒂𝒓𝒅
= 𝜺 .𝒍
(F 5.4)
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+
]
𝒔𝒕𝒂𝒏𝒅𝒂𝒓𝒅
The equilibrium concentrations of iron(III) thiocyanate [Fe(SCN)]2+n for each solution A to D, can be
calculated using the absorbance of each solution, An and the constants, 𝜺 𝒂𝒏𝒅 𝒍:
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒏 =
𝑨𝒏
𝜺 .𝒍
(F 5.5)
Question 4 Complete Table 3
Table 3 Absorbance measurement for standard solution, [Fe(SCN)] 2+standard
Concentration of [Fe(SCN)] 2+standard
(from Q3b)
Absorbance of [Fe(SCN)] 2+standard
at 460 nm
Value of constant,
ε x l (M-1)
Question 5 Enter the Absorbance measurements for each solution A to D into Table 4 and
complete the calculations of [[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒏, showing all working.
Table 4. Determination of Equilibrium Concentrations of [Fe(SCN)]2+
[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒏 =
REACTION
FLASK
ABSORBANCE
(An)
εxl
(from Table 3)
𝑨𝒏
𝜺 .𝒍
( F 5.5)
𝑨
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ] 𝒏 = 𝜺 𝒏.𝒍
A
B
C
D
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
7
3. Calculating the value of K for each of the four solutions, A to D
For each solution, the equilibrium can be expressed in the following ICE table:
Fe3+(aq)
+
SCN –(aq)
⥫⥫
[Fe(SCN)] 2+ (aq)
Initial
[Fe3+]initial
[SCN–]initial
0
Change
-x
-x
+x
Equilibrium
[Fe3+]initial – x
=[Fe3+]eq
[SCN–]initial – x
=[SCN–]eq
𝑲
+x
= [Fe(SCN)]2+eq.
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒆𝒒.
= [𝑭𝒆𝟑+]𝒆𝒒
(F 5.6)
. [𝑺𝑪𝑵− ]𝒆𝒒
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒏.
=
([𝐅𝐞𝟑+ ]𝒊𝒏𝒊𝒕𝒊𝒂𝒍 – 𝑥) ([𝐒𝐂𝐍 − ]𝒊𝒏𝒊𝒕𝒊𝒂𝒍 – 𝑥)
Question 6 Complete the following calculations for each solution A to D, and enter into Table 5 and 6.
Show all working.
Table 5 Initial concentrations of Fe3+ and SCN- in solutions A to D
Calculate the concentration of iron(III) ions ([Fe3+]initial) initially in each reaction flask using c1V1 = c2V2.
Calculate the concentration of thiocyanate ions ([SCN-]initial) initially in each reaction flask using c1V1 = c2V2.
Solution
Total
Volume
(mL)
V2
Fe(NO3)3
(mL)
V1
Fe(NO3)3 (M)
C1 =
Calculate C2
= [Fe3+] initial
KSCN
(mL)
V1
A
10.0
5.0
2.0
B
10.0
5.0
3.0
C
10.0
5.0
4.0
D
10.0
5.0
5.0
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
KSCN (M)
C1 =
Calculate C2
=[SCN-]initial
8
Equilibrium Constant Expression for formation of [Fe(SCN)]2+ Complex Ions
𝑲
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒆𝒒.
= [𝑭𝒆𝟑+]𝒆𝒒
. [𝑺𝑪𝑵− ]𝒆𝒒
=
[[𝑭𝒆(𝑺𝑪𝑵)]𝟐+ ]𝒏.
([𝐅𝐞𝟑+ ]𝒊𝒏𝒊𝒕𝒊𝒂𝒍 – 𝑥) ([𝐒𝐂𝐍 − ]𝒊𝒏𝒊𝒕𝒊𝒂𝒍 – 𝑥)
Table 6. Calculation of Equilibrium Constant for formation [Fe(SCN)]2+ Complex Ions
[[Fe(SCN)]2+] n
REACTION
FLASK
(from Table 4)
=x
[Fe3+] eq
= [Fe3+] initial ̶ [[Fe(SCN)]2+]n
= [Fe3+] initial ̶ x
[SCN–] eq
= [SCN–] initial ̶ [[Fe(SCN)]2+]n
= [SCN–] initial ̶ x
Equilibrium constant (Kc)
A
B
C
D
Average
Equilibrium
Constant
(Kc) =
Discussion:
(Answers to Questions and Discussions)
Question 7
Inspect the values for K obtained for reaction mixtures A to D.
(i)
Does the value of K depend on the initial concentrations of Fe3+ and SCN– ions?
(ii) From the average value of K obtained for this reaction, would you describe it as a product –
favoured or a reactant–favoured reaction?
Conclusion:
(What have you found out?)
2021 Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
9
REPORT COVER SHEET AND DECLARATION
School of Chemistry
The University of Melbourne
Laboratory Report Cover Sheet
Student Name:
Student Number:
Subject Name & Code:
Demonstrator:
Experiment Title:
Experiment F6: Analysis of Aspirin
Due Date: _____________________________
By submitting work for assessment, I hereby declare that I understand the University’s policy
on academic integrity and I declare that:
• This laboratory report is my own original work and does not involve plagiarism or
unauthorised collusion, except where due credit is given to the work of others. The report is
based on results and spectra obtained by me during my laboratory session.
• This laboratory report has not previously been submitted for assessment in this or any other
subject.
For the purposes of assessment, I give the assessor of this assignment the permission to:
• Reproduce this laboratory report and provide a copy to another member of staff; and
• Take steps to authenticate the assignment/laboratory report, including communicating a copy
of this assignment to a checking service (which may retain a copy of the assignment on its
database for future plagiarism checking).
Feedback on Report: Feedback on your report and the mark you received will be available on the
Online Practical Assignments page on Canvas.
Plagiarism:
Plagiarism is the act of representing as one’s own original work the creative works of another,
without appropriate acknowledgment of the author or source.
Collusion:
Collusion is the presentation by a student of an assignment as his or her own work, but which is in fact
the result in whole or in part of unauthorised collaboration with another person or persons. Collusion
involves the cooperation of two or more students in plagiarism or other forms of academic
misconduct.
Both collusion and plagiarism can even occur in group work. For examples of plagiarism, collusion and
academic misconduct in group work please see the University’s policy on Academic Honesty and
Plagiarism: https://academichonesty.unimelb.edu.au
Plagiarism and collusion constitute cheating. Disciplinary action will be taken against students who
engage in plagiarism and collusion as outlined in University policy. Proven involvement in plagiarism
or collusion may be recorded on your academic file in accordance with Statute 13.1.18.
1
Experiment F6: Analysis of Aspirin
Author(s):
Day/Time/Group number:
Abstract:
(Summary of what you did and what you found out)
Introduction and Aim:
(Why do we perform analysis? Why is it important to know content of tablets? How are
you planning to test samples and what will you learn? Are the tests being used here
enough to tell you the exact composition and identity of the sample being tested?)
Experimental:
(How did you perform your experiment? Full method is not required – refer to title of
source, year and publisher)
2
Results:
(Tables of data)
Solubility of Aspirin (This table refers to the solubility tests shown in the video for
this experiment)
Test
Observation
Aspirin in water
Aspirin in NaOH (aq)
PART A: Analysis of Commercial Aspirin
Concentration of NaOH solution: ……………………………………………
Aspirin Brand and stated content: ……………………………………………………………………………………………
Table 1. Titration of Aspirin Tablet with NaOH Solution (with extra entry boxes in case of error)
Titres used for
INITIAL BURETTE
FINAL BURETTE
VOLUME ADDED
calculation
TITRE
READING (mL)
READING (mL)
(mL) (2 decimal places)
(2 decimal places)
(2 decimal places)
1
2
PART B: Analysis of Student Aspirin (F2)
Table 2. Titration of Student Aspirin sample with NaOH Solution
(with extra entry boxes in case of error)
Mass of
INITIAL BURETTE
FINAL BURETTE
sample
Sample
READING (mL)
READING (mL)
used (g)
(2 decimal places)
(2 decimal places)
VOLUME ADDED (mL)
(2 decimal places)
1
2
3
3
PART C: TLC Analysis of Student Aspirin
Table 3. TLC Results – show your calculation of Rf values
Stationary Phase
Mobile Phase
TLC Results/Conditions
Spot______________
(identify by letter or number)
Spot______________
(identify by letter or number)
Spot______________
(identify by letter or number)
Spot______________
(identify by letter or number)
Rf =
Rf =
Rf =
Rf =
Sketch of TLC plate, with spots numbered:
4
Discussion:
(Answers to Questions and Discussions)
Question 1. (This question refers to the solubility tests shown in the video for this
experiment)
What is the effect of adding NaOH, a strong base, on the solubility of aspirin?
Provide a reason for this behaviour, including the intermolecular forces that would be
involved in the most soluble solution observed.
Question 2
Recall from Experiment F2, that aspirin can react with water to form salicylic acid.
As an acid, salicylic acid, can react with NaOH.
a) Calculate the Ka of aspirin and the Ka of the two acidic sites shown on salicylic acid in
the figure above, using the pKa values provided.
b) Which is the strongest acid site of the three circled sites shown? Explain your answer
(hint: what is the definition of a strong acid?).
5
Question 3
i. Complete the following table:
Calculate the mass of aspirin in the commercial aspirin tablet,
Calculate the mass of aspirin in the 0.100 g of student sample from F2.
Molar mass of aspirin, C9H8O4
Table 4. Titration results
………………………
CALCULATIONS FOR AVERAGE MASS OF
ASPIRIN
Commercial aspirin
Student sample of aspirin
a) Average titre of NaOH (mL)
b) Number of moles of NaOH reacted
c) Number of moles of aspirin that reacted
with NaOH
d) Mass of aspirin present (convert to mg)
6
ii. For 0.100 g sample of pure salicylic acid, calculate the volume of 0.100 M NaOH that
would be required for the titration (assume 1:1 reaction between salicylic acid and NaOH).
Molar mass of salicylic acid, C7H6O3 ………………………
mass
Molar mass
pure aspirin
pure salicylic acid
0.100 g
0.100 g
180.2 g.mol-1
138.1 g.mol-1
a) Number of mole, n
b) Number of mole NaOH required to react
(assume 1:1 stoichiometry)
c) volume of 0.10 M NaOH required to
completely react (mL)
Question 4
a. How accurate is your titration of the commercial tablet? Identify any errors that may
have affected your analysis.
b. How pure is your sample of recrystallized aspirin from your titration results? Identify
any factors that may have affected your results
(Hint: Think about answer to 3ii above. If your synthesised aspirin sample contains any
salicylic acid, would you expect your average titre to be larger than expected for pure
aspirin or lower than expected for pure aspirin?)
7
Question 5
With reference to the TLC results and to the polarity of the mobile and stationary phases,
explain:
a. What is the identity of your recrystallized product from Experiment F2?
b. How pure is the recrystallized product from Experiment F2?
c. What can you conclude from consideration of the results of both the analyses: a) your
titration results; and b) your TLC analysis? (Do they agree and point you to one
hypothesis? Is one method more sensitive than the other?)
Conclusion:
(What have you found out?)
8
EXPERIMENT
CHEMICAL EQUILIBRIUM
F5
AIMS OF THE EXPERIMENT
• To apply Le Châtelier’s Principle to explain the effects of changes in concentration and
temperature on the position of an equilibrium reaction for a chemical system.
• To determine the equilibrium constant for the formation of iron(III) thiocyanate
complex ion using a spectrophotometric technique.
NOTE: You will require a basic scientific calculator
READING
• Chemistry Human Activity, Chemical Reactivity, Mahaffy, Bucat, Tasker, Kotz, Treichel,
Weaver and McMurry 2nd ed. 2015:
The reaction quotient and the equilibrium constant: Section 13.3, pages 494 –
497.
Quantitative aspects of equilibrium constants: Section 13.4, pages 501 – 503.
Disturbing reaction mixtures at equilibrium: Section 13.6, pages 511 – 514.
• Online guides – see LMS “Laboratory Information”:
• Chem 1000 Lab Manual – Techniques and Equipment:
Techniques and instrumentation: from page 149
CHEMCAL PRELAB MODULE
There is a ChemCAL Prelab module, which may be accessed from the LMS and will provide
some exercises related to this experiment. ChemCAL can be accessed via the LMS.
INTRODUCTION
Equilibrium and the Equilibrium Constant (Kc)
Chemical reactions, in practice, do not go to completion, but they usually approach an equilibrium
state. Using the collision theory model, as reactant molecules collide to form products and the product
concentrations increase, collisions between product molecules can reform the reactant molecules.
When the rate of the forward reaction equals the rate of the reverse reaction, the system is said to
have reached equilibrium.
At equilibrium, the concentrations of the reactants and products remain constant. This does not mean
that the reaction has stopped. In fact, the reaction continues to progress forward, as well as in reverse.
It is said to be in a “dynamic state of equilibrium”, where ‘dynamic’ points to the fact that reaction has
not stopped. As the rate of the forward reaction equals the rate of the reverse reaction, it means that
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
1
as quickly as we consume reactant molecules, they are reformed by the reverse reaction. Hence their
concentrations no longer change.
For the general reaction:
aA + bB ⥫⥬ cC + dD
this state of equilibrium is described by the equilibrium constant:
K” =
[&]( [)]*
[+], [-].
For a given temperature, a constant value for the above ratio of concentrations will be reached at
equilibrium.
The equilibrium constant, Kc measures the extent of a reaction for an equilibrium system at a given
temperature. The value may be determined from experimental data if the equilibrium concentrations
of the reactants ([A] and [B]) and the products ([C] and [D]) are known.
Le Chatelier’s Principle and Chemical Equilibrium Shifts
A reaction system at equilibrium will remain at equilibrium unless disturbed by a change to the system.
These changes may include:
• changing the concentration of one of the components of the system
• changing the pressure or volume
• changing the temperature at which the reaction is run.
The system will achieve equilibrium again, where the value of K will be equal to the previous ratio
(unless there is a change in temperature), however the actual concentrations of A, B, C and D will be
different. This is called a new equilibrium position.
Le Chatelier’s principle describes how the system will shift to establish a new equilibrium position,
where the rates of the forward and reverse reactions are equal again. According to Le Chatelier’s
Principle, in response to a disturbance to the equilibrium, the system will oppose the change in order to
achieve equilibrium again.
For instance, addition of more reactant A will disturb the equilibrium. The system will move to oppose
this change by reducing the concentration of A. This can only happen by favouring more forward
reaction to consume A, until the equilibrium condition is re-established.
Example: Re-establishing equilibrium when concentration is changed for cases i) and ii):
i) ADDING A REACTANT
Reaction shifts towards products – forward reaction favoured
A + B ⥫⥬ C + D
ii) ADDING A PRODUCT
Reaction shifts towards reactants – reverse reaction is favoured
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
2
Colour and the Absorbance of Light
The colour of a substance arises when white light strikes a substance (or solution of a substance) and
some wavelengths of light are absorbed. Those energies, or wavelengths, promote electrons in the
species to higher energy levels. The remaining wavelengths of light are reflected and transmitted
through the solution, and are recognised as the colour of the solution. For instance, if the wavelengths
of light absorbed are at the blue end of the visible spectrum, the solution will appear red to the eye.
Spectrophotometry and the Spectrophotometer
Spectrophotometry is the study of how different chemical species absorb light, looking at which
wavelengths have been absorbed. The amount of light absorbed by a solution can be related to the
amount of the species present in solution. This technique is used extensively in the analysis of nutrient
and contaminant levels, for example to determine nitrate and phosphate levels in water.
A spectrophotometer is an instrument designed to expose a sample to wavelengths of light in the
visible and UV region of the electromagnetic spectrum. An analysis of the amount of substance present
exploits the Beer-Lambert Law (F5.1) which relates the absorbance A (how much light is absorbed at a
given wavelength) to the concentration (c) of the absorbing species. One form of this law is:
A=εcl
(F 5.1)
where ε is the molar absorptivity or molar obsorption coefficient (cm .mol .L), c is the concentration
(mol.L-1) and l is the distance the light travels through the solution (cm) (usually the width of the sample
container which is called a cuvette). Both ε and l are constants for a given pure substance and
spectrophotometer (and cuvette) at a particular wavelength.
A calibration graph of A against c (that is a plot of A vs c) should give a straight line, with
gradient = ε.l
-1
-1
EXPERIMENT REPORT
There is a complete report template for students to complete and submit. All your results and
answers can be entered there and the report format is in the template – you do not need to copy or
write any supplementary report.
Print it and have it ready for entering data.
You can complete any parts of your report before your Lab session that you feel confident to do.
Practice the calculations. Prepare well and have questions ready to ask your demonstrator.
It is highly recommended that you aim to complete all your report during your lab session while you
are able to question your demonstrator.
SAFETY
!
Safety warning:
NaOH is a strong base. HNO3 is a strong acid. Avoid skin contact at all time.
If spillage occurs, use water to dilute and wash away.
CAUTION: The steam and hot surfaces of steam baths can cause burns.
Risk Assessment
Before you undertake this experiment, you must read through the experimental procedure, including
the Risk Assessment sheet. OnCampus students: Please sign the declaration at the end of these
notes to acknowledge that you have read and understood the information on the Risk Assessment
sheet. Your demonstrator will check and COLLECT this declaration at the start of class.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
3
EXPERIMENTAL PROCEDURE
Part A: Chemical Equilibrium Shifts for the Iron(III) Thiocyanate (FeSCN)2+ Complex Ion System
Iron(III) (Fe3+) ions react with thiocyanate (SCN–) ions to form iron(III) thiocyanate [Fe(SCN)]2+
complexes. It is an exothermic reaction and the ionic equation for this equilibrium reaction is given in
equation F5.2.
Fe3+(aq)
+
SCN –(aq) ⥫⥬
[Fe(SCN)] 2+(aq)
D H = -ve
(F 5.2)
1. Preparation of iron(III) thiocyanate complex ion
a) With an auto-zippette, deliver 1 mL of the 0.100 M iron(III) nitrate (Fe(NO3)3) solution to a
50 mL conical flask.
• Record the colour of iron(III) nitrate solution in Results Sheet
b) With a second auto-zippette add 1 mL of 0.100 M potassium thiocyanate (KSCN) solution to the
above iron(III) solution in the conical flask.
• Record the colour of potassium thiocyanate solution in your report.
• Record the colour of the mixture in your report.
a) Add 30 mL of distilled water using a 100 mL measuring cylinder to the mixture. Mix it thoroughly
by swirling.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
4
2. Test tube reactions
a) Place 0.5 mL of iron(III) thiocyanate complex ion solution (prepared above in Step 1) using a
plastic dropping pipette into nine micro test tubes.
b) Label the micro test tubes from 1 to 9:
i. Test tube 1 will serve as reference for results obtained in following steps (ii) to (ix).
ii. Add 4 – 5 drops of 0.200 M Fe(NO3)3 to test tube 2.
iii. Add 4 – 5 drops of 0.200 M KSCN to test tube 3.
iv. Add 4 – 5 drops of 0.1 M AgNO3 to test tube 4.
v. Add 4 – 5 drops of 0.2 M KCl to test tube 5.
vi. Add 6 – 7 drops of 1.0 M NH3 to test tube 6.
vii. Add 6 – 7 drops of 2.0 M NaOH to test tube 7.
viii. Place test tube 8 in an ice bath for 15 minutes.
ix. Place test tube 9 in a boiling water bath for 15 minutes.
c) Allow the mixture in each test tube from (ii) to (vii) to react undisturbed for 15 minutes.
• Record your observations of each test tube reaction in Results Sheet.
Is there any colour change? Is there any precipitation?
Record observations into Table in your report and answer the following questions:
Question 1
In each test, the equilibrium has been disturbed. As the system returns to equilibrium, using the colours
in the table above, for each test tube reaction 2 – 9, identify which reaction is favoured, forward or
reverse. Provide an explanation (see example in table).
(Tips: Fe(OH)3, AgSCN are insoluble in water,
NH3 is good base – what reaction does it undergo with water?)
Question 2
Write an ionic equation for any precipitation reaction you have observed.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
5
Part B: Determination of the Equilibrium Constant for Iron(III) Thiocyanate Complex Ion
Absorbance (A) measurements using spectrophotometer
a) Your demonstrator will explain the use of the spectrophotometer to make absorbance
measurements at a wavelength of 460 nm.
b) Always use the reference solution to zero the spectrophotometer at a given wavelength before
taking any absorbance measurement.
c) A set of cuvettes, each with an optical path length of 1.00 cm, is provided for measurement.
d) Use of spectrophotometer cuvette:
i. The cuvettes should only be handled by top edge of the two ribbed sides; fingers must not
touch the optically clear sides.
ii. Rinse cuvette three times with distilled water from a wash bottle and drain on a tissue.
iii. ALWAYS rinse cuvette twice with the solution to be measured.
iv. Fill the cuvette (only to two-third full) with the solution to be measured, away from the
spectrophotometer and over a waste container.
v. Carefully wipe the clear sides with a tissue to remove dust or liquid droplets.
vi. Ensure there are no air bubbles on the inside walls of the cuvette by gently tapping the
cuvette on a hard surface.
vii. Check all sides are properly clean before inserting the cuvette in the spectrophotometer
cuvette holder. Always position the cuvette so the light passes through the clear sides.
e) When the experiment is concluded, rinse with distilled water and drain on a tissue.
NOTE: Cuvettes must never be placed in or removed from the cuvette holder when the holder is in the
spectrophotometer. Notify your demonstrator if any liquid is spilt inside the spectrophotometer.
1. Preparation of standard solution for iron(III) thiocyanate complex ion
a) With an auto-zippette, dispense 18 mL of the 0.200 M iron(III) nitrate, Fe(NO3)3, in 1 M HNO3
solution into a DRY 50 mL conical flask.
b) Collect 10 mL of 0.002 M potassium thiocyanate (KSCN) solution, via an auto-zippette, into a DRY
50 mL conical flask.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
6
Pipette
c) Rinse a 2.00 mL pipette twice with distilled water and twice with the 0.002 M potassium
thiocyanate (KSCN) solution.
d) Pipette a 2.00 mL aliquot of the 0.002 M KSCN solution into the above conical flask containing the
Fe(NO3)3 in 1 M HNO3 solution. Mix it thoroughly by swirling the contents gently.
e) Allow the mixture to equilibrate for 1 minute.
f) Fill the cuvette carefully (only to two-third full) with the reaction mixture, away from the
spectrophotometer and over a waste container.
g) Measure the absorbance (Astandard) of this standard solution at a wavelength of 460 nm.
Use distilled water as the reference.
• Record the absorbance (Astandard) in your report.
h) Calculate the concentration of iron(III) thiocyanate complex ion, [FeSCN2+]standard, in the 20 mL
standard solution. Use equation F5.3 as guide (NOTES: consider the dilution factor of 0.002 M
KSCN in standard solution). Record all working in your report.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
7
2. Determination of equilibrium constant for iron(III) thiocyanate complex ion
a)
b)
Label four DRY 25.0 mL conical flasks “A” to “D”.
Table F5.1 shows the amount of each following solution required to provide a total of 10 mL in
each reaction conical flask:
i. Add 0.002 M iron(III) nitrate (Fe(NO3)3) in 1 M HNO3, via an auto-zippette.
ii. Add 0.002 M potassium thiocyanate (KSCN) solution using a DRY 10 mL measuring
cylinder.
iii. Add distilled water using a 10 mL measuring cylinder.
Table F5.1 Reaction mixture in each flask.
Conical Flask
Fe(NO3)3
(mL)
KSCN
(mL)
Distilled Water
(mL)
Total Volume
(mL)
A
B
C
D
5.0
5.0
5.0
5.0
2.0
3.0
4.0
5.0
3.0
2.0
1.0
0.0
10.0
10.0
10.0
10.0
c) Mix each reaction mixture thoroughly by swirling the contents gently.
d) Allow the mixture to equilibrate for 1 minute.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
8
e) Fill the cuvette carefully (only to two-third full) with the reaction mixture, away from the
spectrophotometer and over a waste container.
f) Measure the absorbance (A) of each reaction mixture at the wavelength 460 nm.
Use distilled water as the reference.
• Record the absorbance (A) of each reaction mixture in your report.
Proceed to complete all sections in your Report Template including discussion questions.
Please review the Risk Assessment on the last page – these are compulsory for all laboratory work
and as you progress in your studies in second and third year you will be asked to prepare these
assessments.
Submitting your report
When you have completed your report (using report template), proceed to create ONE pdf document.
If you have entered your answers and calculations directly into the electronic version of the report,
simply save the pdf.
If you have handwritten part or all of your report, either:
i. scan the pages using a mobile telephone app (recommended Scannable (for iOS) or Genius Scan
(Android)) and compile into ONE pdf, or
ii. take photographs of the pages and merge and compress into ONE pdf.
Submit your report to the appropriate assignment tab under PRACTICAL ASSIGNMENT in Assignments
on the LMS.
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
9
RISK ASSESSMENT
Nature of Chemical Hazard (check as appropriate)
¨ Corrosive
þ Irritant
¨ Pungent
¨ Stench
þ Toxic
¨ Carcinogenic
¨ Mutagenic
¨ Teratogenic
¨ Oxidising
¨ Pyrophoric
¨ Highly flammable
¨ Cytotoxic
¨ Non-commercial compounds where high risk
is assumed based on personal experience
(no data available)
¨ Non-commercial compounds where low risk
is assumed based on personal experience
(no data available)
¨ Reacts violently with water
þ Minimal risk
Procedural hazards
¨ Large scale reactions, particularly involving
solvent distillation
¨ High pressure reactions
¨ Reactions in sealed tubes
¨ Radioactivity above the specified OHS levels?
¨ Potentially explosive reactions
¨ Reactions in glass or other containers under
high vacuum
¨ Other
Special Precautions
þ Special eye protection
¨ Safety shield
¨ Face mask
þ Special clothing/gloves
¨ Is help necessary during the process?
¨ Any other
Special Location
¨ Fume Cupboard
¨ Schlenk line
¨ Biohazard laboratory
¨ Sharps
¨ Biowaste
¨ Cytotoxic waste
¨ Filter aid
¨ Silica
¨ Other
¨ Other
Waste Disposal
¨ Filter papers
Category of Risk (tick one)
þ 3 Minimal risk
¨ 2a Low risk (Fume hood recommended)
¨ 2b Low risk (Fume hood/Schlenk line essential)
¨ 1a Significant risk (Chemical hazard)
¨ 1b Significant risk (Special location or facility)
Risk Assessed by:
Director:
Date:
Sonia Horvat
6th January 2022
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
10
RISK ASSESSMENT DECLARATION: CHEM1000___
Name: _____________________________
Student Number: _____________
Day: M / T / W / T / F Session: AM / PM / EVE
Group Number: _________________
Demonstrator: ______________________
Experiment Number: ____ Experiment Name:__________________________________
Risk Assessment: I have read and understood the safety information, including the Risk
Assessment, for this experiment.
Signature: ___________________
Date: ____________________________________
2022
Experiment F5: Chemical equilibrium
Copyright: School of Chemistry, The University of Melbourne.
11
EXPERIMENT
ANALYTICAL ANALYSIS OF AN ASPIRIN TABLET:
F6
THE DETERMINATION OF ACETYLSALICYLIC ACID IN AN ASPIRIN TABLET
AIMS OF THE EXPERIMENT
•
•
•
•
To gain experience in the use of balances and important volumetric glassware.
To perform an acid-base titration and become proficient with titrimetric techniques.
To use a volumetric method to determine the amount of acetylsalicylic acid in a commercial
aspirin tablet.
To establish the purity of an aspirin tablet using the technique of thin layer chromatography.
For this experiment:
• Students work individually.
• You will require a basic scientific calculator.
READING
•
Chemistry Human Activity, Chemical Reactivity, Mahaffy, Bucat, Tasker, Kotz, Treichel,
Weaver and McMurry 2nd ed. 2015:
Solution concentration: Section 6.8, pages 200 – 203.
Solutions and solubility: Section 12.2, pages 459 – 460.
Acid-base titrations: Section 14.10, pages 573 – 574 and 576 – 578.
•
Online guides – see LMS “Laboratory Information”:
• Chem 1000 Lab Manual – Techniques and Equipment:
Techniques and instrumentation: from page 149
CHEMCAL PRELAB MODULE
There is a ChemCAL Prelab module, which may be accessed from the LMS and will provide some
exercises related to this experiment. ChemCAL can be accessed via the LMS.
INTRODUCTION
Aspirin has been widely used as a pain killer for over 100 years to relieve aches and pains such as
headaches and joint pains. It has also been shown useful in reducing fever and lowering the risk of
heart attacks and strokes by preventing blood platelets in our body from aggregating into clots.
The acid catalysed acetylation of salicylic acid with acetic anhydride is shown in Figure F6.1 as the
final step in the industrial synthesis of aspirin. Aspirin (acetylsalicylic acid) contains an aromatic
group (benzene ring), a carboxylic acid ( ̶ COOH) functional group and an ester ( ̶ OCOCH3) functional
group. It is slightly soluble in water but soluble in ethanol and acetone.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
1
Figure F6.1 Reaction scheme for synthesis of Aspirin
The active ingredient of a commercially available aspirin tablet is acetylsalicylic acid and each tablet
typically contains 100 mg, 300 mg or 500 mg of the compound. A binding agent (such as corn starch)
and a lubricant (such as hydrogenated vegetable oil or talc) are commonly added to assist in holding
the tablet together in shape. In this experiment, the amount of aspirin (acetylsalicylic acid) per tablet
is determined for various aspirin tablets ranging from the well-established brands to those much
cheaper generic brands from supermarkets.
Acid-Base Titration of Aspirin
Aspirin (acetylsalicylic acid) is a weak acid and the amount present in an aspirin tablet can be
accurately determined from a direct titration with a strong base, for example, NaOH. The method to
be used in this determination involves the neutralisation reaction of aspirin (C8H7O2COOH) with
sodium hydroxide (NaOH) of a known concentration as given in the following equation F6.1.
–
C8H7O2COOH(aq) + NaOH(aq) ® C8H7O2COO Na+(aq) + H2O(l)
aspirin
base
(F6.1)
In this neutralisation reaction, the number of moles of acid (C8H7O2COOH) is reacted with an equal
number of moles of base (NaOH):
number of moles of NaOH = concentration(NaOH) x volume added (NaOH) (F6.2)
and,
number of moles of aspirin = number of moles (NaOH)
(F6.3)
The amount of aspirin (acetylsalicylic acid) per aspirin tablet can be determined using the following
equation,
mass of aspirin = number of moles of aspirin x molar mass (aspirin)
2022
(F6.4)
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
2
Thin Layer Chromatography
Thin layer chromatography (TLC) is a very useful technique for establishing the purity of the product
of a chemical synthesis, which may be a mixture of the compound of interest and one or more
impurities. It can also be used to identify whether two samples contain the same compound.
In the thin layer chromatography, the sample to be analysed is first adsorbed onto an inert
substance called the stationary phase (in this experiment a thin layer of silica gel – SiO2·xH2O – on an
aluminium backing) which has some polar groups. The stationary phase is then brought into contact
with a less polar organic solvent (mobile phase). As the solvent moves past the point on the TLC
plate where the sample has been applied, the components of the sample move different distances
depending on their polarity.
Polar organic molecules, having a higher proportion of oxygen- and nitrogen-containing functional
groups, are attracted more to the polar stationary phase and do not shift far from the point at which
they were applied (i.e. the origin). Less polar compounds containing a higher proportion of carbon
and hydrogen are attracted less to the polar stationary phase and move further from the point of
origin.
SAFETY
!
Safety warning:
NaOH is a strong base. Concentrated solution of base is corrosive and gives off
irritating vapours. Avoid skin contact at all time. If spillage occurs, use water to dilute
and wash away.
Many organic solvents are toxic by skin absorption or breathing of vapours. Avoid
breathing the vapour and avoid contact with skin.
Risk Assessment
Before you undertake this experiment, you must read through the experimental procedure,
including the Risk Assessment sheet. OnCampus students: Please sign the declaration at the end of
these notes to acknowledge that you have read and understood the information on the Risk
Assessment sheet. Your demonstrator will check and COLLECT this declaration at the start of class.
EXPERIMENT REPORT
There is a complete report template for students to complete and submit. All your results and
answers can be entered there and the report format is in the template – you do not need to copy
or write any supplementary report.
Print it and have it ready for entering data.
You can complete any parts of your report before your Lab session that you feel confident to do.
Practice the calculations. Prepare well and have questions ready to ask your demonstrator.
It is highly recommended that you aim to complete all your report during your lab session while
you are able to question your demonstrator.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
3
EXPERIMENTAL PROCEDURE
Part A: Analysis of the Aspirin Tablet
I. Preparation of Burette for Titration
1. Dispense, via zippette, the solution of sodium hydroxide (NaOH) into a clean DRY 100 mL
beaker.
• Record the concentration of the NaOH solution.
2. Place a funnel into the top of burette, rinse the burette twice with 10 mL of
distilled water using a wash bottle.
3. Rinse twice and fill the burette with the NaOH solution, then release 1 to 2
mL of solution to remove any air bubbles in burette tip.
4. Remember to remove the funnel.
Volumetric
techniques
A sample box is provided containing two batches of commercially available aspirin tablets.
Batch 1 contains 100 mg aspirin tablets; Batch 2 contains 300 mg aspirin tablets
CHOOSE ONE SET (either the 100 mg or the 300 mg)
• Record the brand of the aspirin tablet and the aspirin content specified on the packet.
NOTES: It is important that you return the empty sample box back to your demonstrator at the end of
practical session.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
4
II. Titration of Commercial Aspirin Tablet:
1. For aspirin tablet 1 and tablet 2:
a) Rinse two x 200 mL conical flasks with small amount of distilled water and dry with paper
towel before the start of the experimental preparation.
b) Remove each aspirin tablet from its silver foil packaging.
c) Add 1 aspirin tablet to each of the CLEAN and DRY conical flask.
d) Add 10 mL of ethanol from the zippette to each conical flask. Swirl the flask gently at
room temperature for about 2 to 3 minutes. Should the aspirin tablet not dissolve
completely, mix it thoroughly using a glass rod. (Note: The binding agent and lubricant of
the tablet are not expected to dissolve readily.)
Add 20 mL distilled water, via a zippette, into each conical flask and thoroughly mix the
mixture by stirring using the glass rod.
e) The aspirin in each tablet is determined titrimetrically with NaOH solution:
(i) Record the initial burette reading to two decimal places in the Table in your report.
(ii) Add three drops of phenolphthalein indicator to the mixture in conical flask and
swirl to mix.
(iii) Titrate to the endpoint, which occurs when the colourless solution changes to pale
pink.
(iv) Record the final burette reading to two decimal places once the endpoint is
reached.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
5
Part B: Titration of Student Aspirin:
1. Rinse three x 200 mL conical flasks with small amount of distilled water and dry with paper
towel before the start of the experimental preparation.
2. Using the analytical balance, weigh 0.100 g
(100 mg) of aspirin into each flask. If you do
not have sufficient product from your
synthesis (F2), please ask your demonstrator
for a sample to use.
Analytical balance
3. The mass of aspirin in your sample is determined titrimetrically with NaOH solution following
the same procedure used above in Steps 1(d) and (e).
II. Calculations
1. Determination of aspirin in tablet 1 and tablet 2 of each batch (100 mg OR 300 mg aspirin
tablets) and in your sample of aspirin using titration:
a) Calculate the average titre of sodium hydroxide (NaOH) for tablets 1 and 2.
b) Use the average titre and the known concentration of NaOH to calculate the number of
moles of NaOH which reacted with aspirin (equation F6.2).
c) Use the mole ratio from the chemical equation in equation F6.1 to calculate the number
of moles of aspirin that have reacted with NaOH (equation F6.3).
d) Use the number of moles of aspirin and its molar mass to calculate the average mass of
aspirin present in the tablet (equation F6.4).
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
6
Part C: Thin Layer Chromatography
1. Take the TLC plate provided and, being careful not to touch the surface with fingers, lightly
draw a pencil line 1 − 2 cm from the bottom edge.
2. Prepare a solution of your aspirin product by dissolving a few crystals of your recrystallized
material in a few drops of ethanol.
3. Use separate capillary pipettes to carefully place one small spot of each
following solutions at the sample positions indicated in Figure 6.2:
A. the solution of salicylic acid (supplied)
B. the solution of commercially available aspirin tablet (supplied)
C. the solution of your previously prepared aspirin (from Experiment F2)
TLC setup
(Ensure that the spots are placed higher than the solvent level in the tank).
Try not to pierce or scratch the surface of the silica gel as you spot your sample. Ensure that the
spots are kept as small as possible. Spotting can be done more than once to ensure enough
material is applied. (consult your demonstrator)
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
7
Figure 6.2 Thin Layer Chromatogram
4. Solvent tanks (in FUMEHOOD) containing a 1:1 (v/v) mixture of ethyl acetate and hexane are
used to develop the TLC plates. Lower the plate into the solvent making sure that the spots
are above the level of the solvent in the tank. Replace the tank lid and allow the plate to
develop undisturbed (do not walk around with the tank in your hands!) until the solvent is
1 cm or less below the top of the silica gel. This will normally take about 3 to 4 minutes.
5. Remove the plate from the tank and immediately mark with pencil the position of the
solvent front when the plate was removed from the tank. Allow the solvents to evaporate
from the surface of the plate by leaving it face-up in the fume hood for about 30 seconds.
6. The next step in the TLC process involves visualisation of the plate. Place
the dry plate underneath the ultra-violet (U.V.) lamp provided.
7. Single dark circles should be seen (against a green background) in the
upper section of the plate above each point at which each solution
containing the aspirin samples or salicylic acid had been spotted.
These dark spots which “show up” under U.V. light because the molecules
contain a chromophoric benzoyl (C6H5CO–) unit.
Developing TLC
plates
8. Lightly outline the dark circle with a pencil.
9. Calculation of Rf values:
If the preparation of the TLC plate and its development has been carried out under strictly
constant conditions, the ratio of the distance travelled by the spot to the distance travelled
by the solvent front should be constant for a given compound. This ratio is known as the Rf
value for that compound and will always be in the range 0 – 1.
R” =
distance travelled by spot from spotting position
distance travelled by solvent front from spotting position
Make the measurement from the centre of spot.
• Calculate the Rf values for each of the spots on your plate.
10. Write your name on the back of the plate using marker from your demonstrator and place
the TLC plate in the sample bag indicated by your demonstrator.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
8
When the preparation of the chromatogram and its development has been carried out under
strictly constant conditions, the spots that belong to different samples will travel at equal
distances from the spotting positions if they are the same compound (such as the presence of
aspirin in commercially available tablets and in your product from experiment F2).
How pure is your recrystallised aspirin compound, looking at your TLC results? Is there any
indication of the presence of salicylic acid (the starting material)? Answer in your report.
Proceed to complete all sections in your Report Template including discussion questions.
Submitting your report
When you have completed your report (using report template), proceed to create ONE pdf
document.
If you have entered your answers and calculations directly into the electronic version of the
report, simply save the pdf.
If you have handwritten part or all of your report, either:
i. scan the pages using a mobile telephone app (recommended Scannable (for iOS) or Genius Scan
(Android)) and compile into ONE pdf, or
ii. take photographs of the pages and merge and compress into ONE pdf.
Submit your report to the appropriate assignment tab under PRACTICAL ASSIGNMENT in
Assignments on the LMS.
2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
9
RISK ASSESSMENT
Nature of Chemical Hazard (check as appropriate)
Procedural hazards
þ Corrosive
þ Irritant
¨ Pungent
¨ Stench
¨ Toxic
¨ Carcinogenic
¨ Mutagenic
¨ Teratogenic
¨ Oxidising
¨ Pyrophoric
¨ Highly flammable
¨ Cytotoxic
¨ Non-commercial compounds where high
risk is assumed based on personal
experience (no data available)
¨ Non-commercial compounds where low
risk is assumed based on personal
experience (no data available)
¨ Reacts violently with water
¨ Minimal risk
¨ Large scale reactions, particularly involving
solvent distillation
¨ High pressure reactions
¨ Reactions in sealed tubes
¨ Radioactivity above the specified OHS
levels?
¨ Potentially explosive reactions
¨ Reactions in glass or other containers under
high vacuum
¨ Other: Exposure to hot surfaces (hotplate)
Special Precautions
þ Special eye protection
¨ Safety shield
¨ Face mask
þ Special clothing/gloves
¨ Is help necessary during the process?
¨ Any other
Special Location
þ Fume Cupboard
¨ Schlenk line
¨ Biohazard laboratory
¨ Other
Waste Disposal
¨ Sharps
¨ Biowaste
¨ Cytotoxic waste
¨ Filter aid
¨ Silica
¨ Other
¨ Filter papers
Category of Risk (tick one)
þ 3 Minimal risk
¨ 2a Low risk (Fume hood recommended)
¨ 2b Low risk (Fume hood/Schlenk line essential)
¨ 1a Significant risk (Chemical hazard)
¨ 1b Significant risk (Special location or facility)
Risk Assessed by:
2022
Director:
Sonia Horvat
Date:
6th January 2022
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
10
RISK ASSESSMENT DECLARATION: CHEM1000___
Name: ________________________________
Student Number: ________________
Day: M / T / W / T / F
Group Number: _________________
Session: AM / PM / EVE
Demonstrator: _________________________
Experiment Number: ______ Experiment Name: ______________________________________
Risk Assessment: I have read and understood the safety information, including the Risk
Assessment, for this experiment.
Signature: ______________________
2022
Date: ________________________________________
Experiment F6: Analytical analysis of an aspirin tablet
Copyright: School of Chemistry, The University of Melbourne.
11