Chem 6 practice exam

1

PRACTICE EXAM (not all questions pertain to a specific mid-term). This is taken from an earlier final exam. 1. (4 pts) Permanganate ion oxidizes sulfide ion to elemental sulfur (which you should represent as a single
S atom) and is reduced to MnO2 in basic solution. In the balanced equation for this reaction, which you should determine in the work area, how many water molecules and hydroxide ions are needed in the final balanced equation? Work area:

# of H2O ____4______

# of OH- ___8_______

MnO4- + S2- ---> S + MnO2 x3 Oxidation:
S2- -----> S + 2eReduction:
4H+ + MnO4- + 3e- -----> MnO2 + 2H2O x2 +
2acidic:
8H + 2MnO4 + 3S ------> 3S + 2MnO2 + 4H2O
But the question asks to balance in basic, thus add the same no. of OH- as H+ to both sides, combine two species to become H2O and balance no. of H2O from both sides
8OH- + 8H+ + 2MnO4- + 3S2- ------> 3S + 2MnO2 + 4H2O + 8OH8H2O + 2MnO4- + 3S2- ------> 3S + 2MnO2 + 4H2O + 8OH4H2O + 2MnO4- + 3S2- ------> 3S + 2MnO2 + 8OHNote My technique is to balance in acidic solution every time and take care of OH- if basic solution is required. If you are familiar with the technique in the textbook (pp 488-489) for basic solution, please follow those steps.
Finally, we will end up with the same answer after all.
2.

(5 pts) Data: ClO4- (aq) + 2 H+ (aq) + 2eAg+ (aq) + e-

Ag(s)

ClO3- (aq) + H2O (l)

E° = +1.23 V
E° = +0.80 V

Calculate the reaction quotient Q for a spontaneous reaction using these two half reactions if the potential for the cell is measured to be 0.40 V.
Q = ___________________
Work area:
Ecell = E0cell - RT/nF lnQ
Keys: Calculate E0 and know how many e- involve in the reaction
Spontaneous reaction need positive Ecell, as given (0.40 V). Thus know need to know which reaction should be reduction or oxidation by comparing the given E0 half cell. The more positive E0 is, the more likely to undergo reduction. In this problem the first reaction is reduction (more