Texas Southern University Redox Reactions Lab Report

Fill in the lab sheet and write a report.

Exploring Redox Reactions
Oxidation and reduction reactions have been known for millennia but were not
understood until the 17th century. The terms come from metallurgy. Most metals do not
naturally exist in their metallic forms (except gold and silver), but were extracted from
rocks and minerals. As such the ores were “reduced” to a small amount of metal from a
large amount of ore. It was noted that most metals would react with oxygen and form a
new substance and hence were oxidized.
We now understand that redox (oxidation/reduction) reactions involve the transfer of
electrons. Oxidation is the loss of electrons (increasing charge/oxidation state) while
reduction is a gain of electrons (decreasing charge/oxidation state). Consider, for
instance, the reaction between copper(II) ions (Cu+2(aq)) and zinc metal (Zn(s)), which
occurs spontaneously.
Cu2+(aq) + Zn(s) ⟶ Cu(s) + Zn2+(aq)
As dissolved copper ions interact with the surface of the zinc metal, two electrons are
transferred from the neutral zinc to copper ions. The resulting zinc ions dissolve, while
copper metal deposits onto the surface of the zinc. Since electrons are exchanged in
this reaction, it is a redox reaction. To make the electron exchange more apparent, we
can break this reaction into “half reactions”:
Zn(s) ⟶ Zn2+(aq) + 2 e-
(Zinc metal gives up electrons)
Cu2+(aq) + 2 e- ⟶ Cu(s)
(Copper ion gains electrons)
Substances that gain electrons are said to be reduced and substances that give up
electrons are oxidized. Note the passive voice here – we’re describing what happens or
is being done to each substance. So in the above reaction, Zn(s) is oxidized and Cu2+ is
reduced. Another way of looking at the above reaction is to consider what the Cu 2+ ion is
doing to the Zn. Cu2+ is causing the Zn to be oxidized, so Cu2+ is acting as an oxidizing
agent. Conversely, Zn is causing Cu2+ to be reduced, so Zn is a reducing agent.
In the example above, Zn/Zn2+ and Cu/Cu2+ are sometimes referred to as redox
couples. This is somewhat analogous to conjugates in acid base chemistry, where the
each part of the pair has opposing roles. For the Zn/Zn2+ couple, Zn is a reducing agent
while the Zn2+ can act as an oxidizing agent. Similarly, in the Cu/Cu2+ couple. neutral Cu
can act as a reducing agent and Cu2+ can be an oxidizing agent.
Reactions such as that between Zn(s) and Cu+2(aq) only go in one direction. In other
words, we will not see the following reaction occur:
Cu(s) + Zn2+(aq) ⟶ Cu2+(aq) + Zn(s)
[DOES NOT OCCUR]
Just as acid-base reaction favors the side of the equation with the weaker acid/base
pair (i.e. more stable), redox reactions favor the side with the weaker oxidizing/reducing
agent. By observing which direction is spontaneous, we can say Zn is a stronger
reducing agent than Cu, and that Cu2+ is a stronger oxidizing agent than Zn2+.
Procedure.
Go to the Chem Collective virtual lab: http://chemcollective.org/vlab/106
Part I: The stockroom of the virtual lab contains solutions of Cu2+, Mg2+, Zn2+, Pb2+ and
Ag+ ions, and the correspond metals (Cu, Mg, Zn, Pb, and Ag). Your first task is to order
Cu, Mg, Zn, Pb and Ag from strongest to weakest reducing agent.
Hints: What experiment can you perform in the virtual lab to confirm that Cu is able to
reduce Zn2+? What experiment can you perform to confirm that Zn is not able to reduce
Cu2+?
Briefly describe the design of your experiment.
Write balanced net ionic equations for each reaction tested. Identify the oxidizing
agent and reducing agent for each reaction.
Include observations used to determine whether the reaction occurred or not.
Rank the metals in decreasing order of reducing strength (i.e. strongest to
weakest. Provide rationale for the order you chose.
Part II: The virtual lab stockroom also contains aqueous solutions of Cl2, Br2, and I2 as
well as Cl-, Br-, and I- solutions. Your task here is to order Cl2, Br2, and I2 from strongest
to weakest oxidizing agent.
In separate flasks mix Cl2 with Br- and Cl2 with I-. Similarly react Br2 with the Cl- a
nd I- solution, and I2 with the Cl- and Br- solutions.
You should observe that these reactions are equilibria rather than one-way
reactions of Part I. What evidence supports this?
Write balanced net ionic equations for each reaction tested. Identify which
substance is the oxidizing agent and reducing agent in each case.
For each equilibrium observed, indicate which side is favored. What
evidence/observations support your conclusions?
Rank the halogens in order of decreasing oxidizing strength (i.e. strongest
oxidizer to weakest).
LAB REPORT OUTLINE
Abstract
An executive summary of the entire report. A single paragraph capturing the essence of each
section below. State the objective(s) of the experiment, the methods used (not a step-by-step
procedure, summarize), and the key results.
Introduction/Background
A description of the theoretical concepts the experiment is testing or demonstrating and how the
experiment will achieve the objectives.
Any new terms, concepts, phenomena should be defined or described in your words. Assume the
reader has knowledge of CHEM 1411 topics.
Avoid step-by-step descriptions of the experiment. Similarly, avoid detailed descriptions of
calculations – assume the reader has knowledge of algebra). The description of the experiment
should focus on what was measured/observed and how this raw data was used to achieve the
objectives.
Results
Depending on the nature of the experiment, the results section may consist of one or more tables,
graphs or a combination. Raw data written in your notebook or lab manual should be
processed/summarized in typed tables, not scans of the lab manual.
Charts or graphs should be produced in Excel or a similar program.
A major goal of the results section is to transform unwieldy or cluttered raw data to a form that
makes trends or general findings apparent. For tables, remove unnecessary items when possible –
try to streamline data to the key inputs and outputs. Sample calculations should be included in
scans of the lab manual.
In some cases, a graph may take the place of a huge table of numbers, and if so, there’s no need
to include the table at all. Don’t assume every report requires a graph, but if a picture can tell the
story, it should.
Discussion
This is a discussion of results. State any trends and findings in the data. Were the observations
expected? Why or why not?
Compare results to known or literature values whenever possible. Evaluate the magnitude of
error and suggest likely causes/sources of error. Support your statements! Was the source of
error based on observation? If not, why do you think it was likely? How would that source of
error affect the result? A single source of error that is well-supported is much better than a
laundry list that isn’t.
“Human errors” and “miscalculations” should never be given as sources of error. “Human error”
is far too vague. While errors in calculation are common, you can and should ask for help if you
suspect this is an issue. Otherwise, assume the calculations are correct.
Conclusion
Briefly assess the effectiveness of the experiment. Were the objectives achieved? Why or why
not? Suggest ways to improve the results or the design of the experiment.
LAB REPORT RUBRIC
Criterion and Point Allocation 15 total
Abstract summarizes the experiment effectively. (2 pts)
The abstract is a concise (one-paragraph) summary of the report and contains each of
the following:
Statement of objective(s) (0.5 pt)
How objective(s) were achieved; (0.5 pt)
Key results; (0.5 pt)
Whether objectives were achieved. (0.5 pt)
Introduction provides background on processes used in lab (3 pts)
Defines key terms and presents any relevant equations (1 pt)
Discusses theory behind relevant concetpts/phenomena. (1 pt)
Describes how the experiment is designed to achieve the objectives, outlining what data
is being collected and how it is used (1 pt)
Results section shows all data and calculations in readable format. (3 pt)
The results section summarizes all data collected with text, tables, and/or figures/charts.
(1pt)
Calculations are correct and a percent error is provided, where applicable (2 pts)
Discussion discusses and analyzes results (4 pt)
Describes the patterns, principles, relationships indicated by data (2 pts)
Sources of error are considered and supported (2 pts)
Conclusion: Effectiveness of experiment is assessed and supported. Suggestions
for improvement are provided. (1 pt)
References: In-text citations are provided (1pt)
Report avoids 1st person (e.g. “I, we, our, etc “) and is relatively free of
grammatical errors (1pt)
Score