Download Answers to post-lab questions

Recommendation to Teachers:
It’s always a good idea to “road test” any student lab that you have not previously used with your
students, as tweaks are often needed to make the experiment flow more smoothly and to provide
meaningful results. In addition, performing the experiment (using the same materials and
procedures that will be used by the students) in advance will allow you to be aware of common
pitfalls and errors made by students and alert your students before they begin the activity.
Note: This is a micro-scale activity, which by design allows students to perform the experiment
without consuming substantial quantities of the salt solutions, thereby saving money (especially
significant given the relatively high cost of 1.0M silver nitrate solutions). The metal samples can
be rinsed with distilled water and reused many times.
Answers to post-lab questions
Question 1:
The NaNO3 solution on filter paper serves as the salt bridge to maintain charge balance between
the two cell “compartments”. To serve as a salt bridge, it must be ionically conductive and
electrically insulating.
Question 2:
Electrochemical cells will require a special electrode such as platinum when one or both
compartments contain species that are gases and/or aqueous solutions in both oxidized and
reduced states, i.e., no non-water soluble solids are present. The electrode must be electrically
conductive and non-reactive in this environment.
Question 3:
We want the salt bridge to be electrically insulating, and since graphite is electrically conductive,
we do not want to use pencil here. Most ball-point pen inks are at least partially water soluble,
and therefore they would tend to bleed when exposed to an aqueous environment.
General Conclusions 1:
From table of standard reduction potentials, Ered for the cadmium half-reaction = - 0.40 V and
Ered for the chromium half-reaction = 1.33 V. The more positive value represents the reduction
half-reaction. Therefore, chromium is being reduced, cadmium is being oxidized, and the left
side of the reaction equation is favored.
The unbalanced half-reactions are:
Cd  Cd2+ and Cr2O72-  Cr3+
Cr2O72- is being reduced (gaining e-) to become Cr3+, and therefore is found at the cathode. Since
both states exist in aqueous solution, a platinum electrode will be required.
Cd metal is being oxidized (losing e-) to become Cd2+, and therefore is found at the anode.
Electrons will flow through the external circuit from the cadmium compartment toward the
chromium compartment.
Anions (spectator ions) will flow in the reverse direction, from the chromium into the cadmium
compartment.
E = (Ered ) at cathode – (Ered) at anode, so E = 1.33 V + 0.40 V = 1.73 V
The balanced equation for the reaction in acidic media is:
3Cd + Cr2O72- + 14H+  3Cd2+ + 2Cr3+ + 7H2O
General Conclusions 2:
Answers will vary somewhat, but typically will in mention the following:
PEMFC
Hydrogen is the fuel
At anode: H2  2H+ + 2eAt cathode: ½ O2 + 2H+ + 2e-  H2O
The carrier ion is H+ (i.e., protons)
The electrolyte is an extremely thin semi-permeable polymer membrane (sulfonated fluoropolymer) that is conductive to protons
Fairly low operating temperature, typically ranging from 50 C – 100 C
Strengths – short startup times and relatively lightweight, making them well-suited for mobile
applications
Weaknesses – requires platinum catalysts ($$$), can only use hydrogen gas as fuel, so fuel storage
is a major issue, relatively modest operating efficiency (~30% for distributed power generation,
but up to 60% for mobile power generation, i.e., transportation)
SOFC
Fuel can be hydrogen, but can also use simple hydrocarbons such as methane
At anode (based on hydrogen as fuel): H2 + O2-  H2O + 2eAt cathode: ½ O2 + 2e-  O2The carrier ion is usually oxide (O2-)
The electrolyte is a solid ceramic material that is conductive to oxide ions
Fairly high operating temperatures, usually in excess of 600 C and typically 800 C - 1000 C
Strengths – can use a variety of hydrocarbons as fuel, uses cheap metals as catalysts (e.g., nickel),
relatively good power generation efficiencies (50% - 85%, depending on how waste heat is
utilized)
Weaknesses – long startup times to reach operating temperature, prone to thermal degradation
after a couple of years in operation
General Conclusions 3:
Methane combustion primarily produces heat, whereas a methane/oxygen electrochemical reaction
primarily produces electrical energy. So if the goal is to generate electrical power, a fuel cell
can produce the same power with less methane than can a conventional power plant. Since less
methane is consumed per kWatt generated, less CO2 is produced as a waste gas.