CH302H - Principles of Chemistry II: Honors
Spring
2014, Unique 51880

Homework, Week 12


Homework, week 12
Answer Key

Homework week 12 (electrochemistry, batteries, corrosion and balancing equations)

In some cases, there are “hints” at the bottom of the problem set. In these cases the word Hint is shown after the question. Try struggling with the question for a while before jumping to the hint which, by the way, is often not a “big hint”.

1.    Given the electrochemical cell used for the determination of Cu in the test solution: SCE || Cu2+ (?? M) | Cu where SCE is a saturated calomel reference electrode, EoSCE = +0.241 V.
a) What is the cell potential if the [Cu2+] = 1x10-3 M?
b) Make a plot of Ecell vs some function of [Cu2+] such that the plot results in a straight line. (What “function of would you use? What is the slope of the line?) Hint

2.    Consider an automobile’s lead-acid storage battery consisting of 6 cells, i.e., assume EBatt = 12.0 V.
a) How many amp-hours of energy can be generated per kg of Pb(s)? Hint
b) Show that units of amp-hours are units of energy, e.g., show how to convert from amp-hours to Joules.(Any assumptions being made?)

3.    a) Use the Eo tables to argue that a 1.0 M solution of Cu+ will or will not disproportionate into Cu(s) and Cu2+. Hint
b) Estimate the ratio of [Cu2+]/[Cu+] if CuNO3 were initially dissolved in solution? Hint

4.    What prevents a typical dry cell battery from being recharged?

5.    Draw a typical H2/O2 fuel cell and explain how electricity is generated.

6.    A Zn plate is often bolted to the hull of metal ships and serves as a “sacrificial anode”. a) Explain chemically how this functions to protect the ships iron hull from rusting. Hint
b) Try transforming this into an electrochemical cell notation to illustrate what is taking place with and without the Zn plate presence. (Assume the O2(aq) is the oxidizing agent.) Hint

7.    So... you feel pretty comfortable balancing equations, eh? Want to try a couple of them? (All are done in aqueous solution so you have H2O, H+ and OH- that you can include in the reaction as needed to balance it.)
Remember: If you have been successful, the final equation will be mass and charged balanced, i.e., number of moles of particular elements is the same on left and right side of equation and the total charge on the left equals the charge on the right. Hint

FeS + HCl --> FeCl2 + H2S
Cu2S + HNO3 --> Cu(NO3)2 + NO2 + S
NO2- --> NO3- + NO
FeSO4 + KClO3 + H2SO4 --> Fe2(SO4)3 + K2SO4 + Cl2
U(SO4)2 + KMnO4 --> UO2SO4 + MnSO4 + KHSO4
(For this reaction, write it to imply that occurs in a neutral solution but results in an acidic solution. Then write it to imply that it is run in an alkaline solution.)
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SCE: Hg2Cl2 +2e --> 2Hg+2Cl- (sat’dKCl)    Eo = +0.241V


Hints

1b Write out the Nernst equation for this cell and transpose it into a y = mx + b format where x = f([Cu2+]).

2a Consider what basic SI units that amp-hours could represent.

3a Write the balanced equation and then break it into 2 half-cell reactions. Determine Eocell to make a conclusion. ________
b Consider the full Nernst equation where Q=K and Ecell=0.

6a You may have to go to the internet or text to explore this topic.
b Your cell will take on the general form of X|Y, etc. ||Z, etc. |W

7    If you’re having trouble, the following may assist you in approaching the problems:


General rules for balancing equations

• Dissociate all soluble salts (strong electrolytes), e.g., Na+, K+, Cl-, etc.
• Do not include these in the equation to be balanced unless they are involved in an electron transfer, a precipitation or complexation reaction (i.e., exclude “spectator ions”)
• Final equation must balance in mass and charge

When no electron transfer occurs...
• Most can be done by inspection
• Balance mass
• Check to insure that charge is also balanced

With electron transfer (oxidation-reduction reactions)
1.    Identify species being reduced
2.    Write half reaction with that species
3.    Mass balance (except for H and O)
4.    Add O as H2O and balance H on other side (if needed) by adding H+.
5.    Balance charge with e- addition
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6.    Repeat steps 2-5 above for the species being oxidized
7.    See if number of electrons consumed or generated in the balanced half reactions are the same. If not,
multiply equations by appropriate so free electrons will balance.
8.    Add the two half reactions
9.    Add spectator ions as needed. (must be added to both sides of reaction equation)