AP Chemistry Chapter 20 Exam
Multiple Guess:
Name ___________________________
Questions 1-5: The set of lettered choices is a list of ions that participate in oxidation reduction
reactions. Select the one lettered choice that best fits each numbered description. A choice may be used
once, more than once or not at all.
A) C2O42B) CrO42C) MnO4D) NO3E) Fe2+
1. undergoes oxidation such that its oxidation number changes from +3 to +4 ___A: carbon goes from
3+ to 4+ in carbon dioxide_________________
2. includes the species with a +7 oxidation number ______C: Mn_______________
3. reacts with a reducing agent in acid solution such that the oxidation number of the metal becomes
+3 _____B: Chromate becomes Cr3+__________________
4. when oxidized, forms a cation with a +3 oxidation number __E: iron can be further oxidized to
3+___________________
5. reacts with a reducing agent in acid solution to form a brown gas __D: nitrate becomes NO2 a
brown gas______________
6. Mg + 2Ag+  Mg2+ + Ag
Eo = 3.17 V
For the reaction above in one experiment, the observed value for E is +3.00 V. Which statement(s)
apply to this cell?
I. Addition of more oxidizing agent will increase the value of E.
II. Addition of products will decrease the value of Eo.
III. As this reaction proceeds, the value of E will decrease and the value of Eo will remain the
same.
A) I only
B) II only
C) I and II only
D) I and III only
E) I, II, and III
D: Ag+ is the oxidizer, and since it is a reactant, increasing its concentration increases the value of E.
Eo is always the same value as it is at standard conditions. Adding products reduces E.
Questions 7 and 8: Balance the following equation using redox techniques.
…H+ + …Br- + …MnO2  …Mn2+ + …Br2 + …H2O
7. What is the sum of the coefficients in this balanced equation?
A) 12
B) 11
C) 10
D) 8
E) 6
B: the coefficients are 4, 2, 1, 1, 1, 2
8. What is the reducing agent in this reaction?
A) H+
B) BrC) MnO2
D) Mn2+
E) Br2
B: Br- is the only thing capable of losing an electron in this list
Questions 9-11:
A 1.0 M solution of KI is undergoing electrolysis. A few drops of phenolphthalein solution are added.
The current is supplied at the rate of 2.5 amperes.
9. Which describes the appearance of the solution after the reaction has proceeded for a few minutes?
At the anode
At the cathode
A)
colorless
pink
B)
brown
pink
C)
brown
colorless
D)
pink
colorless
E)
colorless
colorless
B: Water is reduced at the cathode producing hydrogen and hydroxide (pink), iodide is oxidized at the
anode making brown iodine
10. Which describes the behavior of the K+ ions in this system?
A) K+ ions migrate, as spectator ions, toward the cathode.
B) K+ ions migrate toward the anode where they become oxidized to K(s).
C) K+ ions migrate toward the anode where they become reduced to K(s).
D) K+ ions migrate toward the cathode where they become oxidized to K(s).
E) K+ ions migrate toward the cathode where they become reduced to K(s).
A: All the other options involve producing solid K in water, which will never happen
11. Which best describes the behavior in the chamber containing the electrode at which bubbles of gas
are observed?
A) O2 is produced as the pH of the solution increases.
B) O2 is produced as the pH of the solution decreases.
C) H2 is produced as the pH of the solution increases.
D) H2 is produced as the pH of the solution decreases.
E) O2 is produced as the pH of the solution remains the same.
C: water is reduced to hydrogen gas and hydroxide, which increases pH
12. In the reaction:
2KMnO4 + 3H2SO4 + 5H2S  5S + 2MnSO4 K2SO4 + 8H2O
the oxidation number of Mn changes from
A) 0 to -2
B) +7 to -8
C) -2 to 0
D) -2 to +2
E) +7 to +2
E: I hope I don’t have to explain this one
13. The order of chemical activity of three metals is L>M>N. Which describes the behavior of these
metals?
I. Atoms of L can reduce atoms of N.
II. Atoms of L can reduce cations of N.
III. Atoms of M can reduce cations of L.
A) I only
B) II only
C) I and II only
D) II and III only
E) I, II, and III only
B: The most reactive (chemical activity) metal is Lithium, which will lose its electron (from an atom)
to become lithium ion, to any other cation. You did this in honor for the reactivity of metals lab. By
putting a solid metal (atom) with an ion of a different metal, you can see which is more reactive (the
one that stays the ion or becomes the ion is the most reactive). No atoms of a metal will gain electrons,
this would make an anion of a metal, which won’t happen.
14. When the value of Eo for a standard galvanic cell is greater than zero, which ranges apply to Go
and Keq for the cell reaction?
Go
Keq
A)
<0
>1
B)
<0
<0
C)
>0
>1
D)
<0
>0 but <1
E)
>0
>0 but <1
o
A: If E is positive it is spontaneous, therefore G is negative. When K>1 it is product favored.
15. Which describes the oxidation number of nitrogen in each ion of NH4NO2?
A) Both nitrogen atoms have the same oxidation number with the same sign.
B) Both nitrogen atoms have the same oxidation number with the opposite sign.
C) The oxidation numbers of the two nitrogen atoms are +4 and -5, respectively.
D) The oxidation numbers of the two nitrogen atoms are -3 and +4, respectively.
E) The oxidation numbers of the two nitrogen atoms are -4 and +5, respectively.
B: -3+4=+1, +3-4=-1
Free Response
1. The compound NaI dissolves in pure water according to the equation
NaI (s)  Na+ (aq) + I- (aq).
An electric current is applied to a 1.0 M NaI solution.
a) Write the balanced oxidation half-reaction for the reaction that takes place.
2I-  I2 + 2eb) Write the balanced reduction half-reaction for the reaction that takes place.
2H2O + 2e-  H2 + 2OH(sodium ion won’t gain an electron better than water)
c) Which reaction takes place at the anode, the oxidation reaction or the
reduction reaction?
Oxidation (always takes place at the anode)
d) All electrolysis reactions have the same sign for Go. Is the sign positive or negative?
Justify your answer.
Positive – it is not a spontaneous reaction, energy must be suppied
2.
Answer the following questions about the setup and operation of a galvanic cell.
a) One item is missing from this electrochemical cell that prevents it from functioning. Identify that
item by drawing it onto the diagram. Label the missing item.
The wire is missing, connect the 2 electrodes with it (green and blue). It completes the circuit.
b) Once this cell is operating, a redox reaction occurs.
i) Which electrode, Cu or Zn, is the cathode? Explain.
Cu is the cathode. Copper ion gets reduced to solid copper at the cathode.
ii) Write the reaction for the half-cell containing zinc.
Zn  Zn2+ + 2eiii) Calculate the voltage Eo for this cell.
0.34 + 0.76 = 1.10V (don’t forget to reverse one reaction to get the most positive voltage in a galvanic
cell)
c) Set up the equation to calculate E in volts for the cell when the molarity of Cu2+ in the left half-cell
is reduced to 0.10 M. Assume standard temperature. Label each factor in the equation with proper
units.
You don’t need to do this, Nernst equation was removed from the curriculum
d) Suppose that the temperature of the original galvanic cell is decreased (from standard temperature)
to 273 K. Would the voltage decrease, stay the same, or increase? Explain.
You don’t need to do this, Nernst equation was removed from the curriculum
e) What would be the effect on the voltage if the metal electrodes were doubled in mass? Explain.
None – as long as some solid is present, the size of the electrode does not impact the voltage
f) What would be the effect on the voltage if a solution of NaOH were poured into the Zn/Zn2+ halfcell? Explain.
Increases voltage – hydroxide combines with zinc ion making a precipitate and removing zinc ion from
solution. Zinc ion is a product, so removing some shifts the reaction toward greater formation of
products.
g) What would be the effect on the voltage if a solution of 10.0 M Zn(NO3)2 were poured into the
Zn/Zn2+ half-cell? Explain.
Decreases voltage – adding more zinc ion increases the concentration of products in the system,
reducing the forward drive of the reaction