AP Chemistry
Unit 7 Practice Multiple Choice Key
Briefly explain why the answer is correct in the space provided.
1.
H2Se + 4 O2F2  SeF6 + 2 HF + 4 O2
Which is true regarding the reaction represented above?
(A) Oxidation number of O does not change.
(B) Oxidation number of H changes from -1 to +1.
(C) Oxidation number of F changes from +1 to -1.
(D) Oxidation number of Se changes from -2 to +6.
ox.# 1 -2
1 -1
6 -1
1 -1
0
D
H2Se + 4 O2F2  SeF6 + 2 HF + 4 O2
2.
6 I- + 2 MnO4- + 4 H2O  3 I2 + 2 MnO2 + OHWhich statement regarding the reaction is correct?
(A) Iodide ion is oxidized by hydroxide ion.
(B) MnO4- is oxidized by iodide ion.
(C) Manganese oxidation number changes from +7 to +2.
(D) Oxidation number of iodine changes from -1 to 0.
ox.# -1
7 -2
1 -2
0
4 -2 -2 1
D
6 I- + 2 MnO4- + 4 H2O  3 I2 + 2 MnO2 + OH3.
2 H2O + 4 MnO4- + 3 CIO2-  4 MnO2 + 3 CIO4- + 4 OHWhich species acts as an oxidizing agent in the reaction?
(A) MnO4- (B) CIO4(C) CIO2(D) MnO2
# 1 -2
7 -2
3 -2
4 -2
7 -2
-2 1
A
2 H2O + 4 MnO4- + 3 CIO2-  4 MnO2 + 3 CIO4- + 4 OH4.
Which species CANNOT function as an oxidizing agent?
(A) Cr2O72- (B) MnO4- (C) NO3(D) ID
5.
6.
7.
8.
9.
10.
11.
An oxidizing agent can go down in oxidation number. I- is as negative as iodine can go.
When acidified K2Cr2O7 solution is added to Na2S solution, green Cr3+ ions and free S are formed. Which is the best reducing
agent?
(A) K2Cr2O7 (B) Na2S
(C) Cr3+
(D) S
Cr2O72- + 14 H+ + 3 S2-  2 Cr3+ + 7 H2O + 3 S
B
S2- and Cr3+ are reducing agents, but  S2+
_Ag + _AsH3 + _OH-  _Ag + _H3AsO3 + _H2O
What is the coefficient for OH- in the balanced equation?
(A) 2
(B) 4
(C) 5
(D) 6
6(Ag+ + 1 e-  Ag) (add 6 OH- to make basic)
D
AsH3 + 3 H3O + 6 OH-  H3AsO3 + 6 H+ + 6 OH- + 6 e_Cr2O72- + _e- + _H+  _Cr3+ + _H2O
What is the coefficient for H+ in the balanced half-reaction?
(A) 2
(B) 6
(C) 7
(D) 14
ox.# 6 -2
1
3
1 -2
D
1 Cr2O72- + 6 e- + 14 H+  2 Cr3+ + 7 H2O
_Mg + _NO3– + _H+  _Mg2+ + _NH4+ + _H2O
What is the coefficient for H+ in the balanced equation?
(A) 1
(B) 3
(C) 5
(D) 10
5(Mg  Mg2+ + 2 e-)
D
NO3- + 10 H+ + 10 e-  NH4+ + 3 H2O
–
_CrO2 + _OH–  _CrO42– + _H2O + _e–
What is the ratio of the coefficients OH–/CrO2– in the balanced half-reaction
(A) 1:1
(B) 2:1
(C) 3:1
(D) 4:1
1 CrO2– + 2 H2O  1 CrO42– + 4 H+ + 3 e–
D
4 OH4 OH_Fe(OH)2 + _O2  _H2O _Fe(OH)3
If 1 mole of O2 oxidizes Fe(OH)2, how many moles of Fe(OH)3 can be formed?
(A) 2
(B) 3
(C) 4
(D) 5
O2 + 4 H+ + 4 e-  2 H2O
C
4(Fe(OH)2 + H2O  Fe(OH)3 + 1 H+ + 1 e-)
In which species does sulfur have the same oxidation number as it does in H 2SO4?
(A) H2SO3 (B) S2O32- (C) S2(D) SO2Cl2
D
H2SO4: 2+S–8=0 (S=6), H2SO3: 2+S–
2O3
2-:
2S–6=-2 (S=4), S2-: S=-2, SO2Cl2: S–4 –2=0 (S=6)
12.
13.
14.
15.
16.
17.
2 HClO + 3 O2  2 HClO4
As the reaction proceeds to the right, the oxidation number of chlorine changes from
(A) -1 to +3 (B) -1 to +5 (C) +1 to +7 (D) +3 to +7
ox.# 1 1 -2
0
1 7 -2
C
2 HClO + 3 O2  2 HClO4
Which will generate H2(g) when added to 1 M HCl?
(A) CuS
(B) Zn
(C) CaCO3 (D) Mg(OH)
Zn + 2 H+  Zn2+ H2(g)
B
(CuS  H2S, CaCO3  CO2, Mg(OH)2  H2O)
_Cr2O72- + _H2S + _H+  _Cr3+ + _S + _H2O
What is the coefficient for H+ in the balanced equation?
(A) 2
(B) 4
(C) 6
(D) 8
3(H2S  S + 2 H+ + 2 e-)
D
Cr2O72- + 14 H+ + 6 e-  2 Cr3+ + 7 H2O
3 Cu + 8 H+ + 2 NO3-  3 Cu2+ + 2 NO + 4 H2O
Which statements about the reaction are true?
I. Cu acts as an oxidizing agent.
II. Nitrogen's oxidation state changes from +5 to +2.
III. Hydrogen ions are oxidized to form H2O.
(A) I only
(B) II only (C) III only (D) I and II
ox # 0
1
5 -2
2
2 -2
1 -2
B
3 Cu + 8 H+ + 2 NO3-  3 Cu2+ + 2 NO + 4 H2O
In which reaction does the same element undergo both oxidation and reduction?
(A) S8(s) + 8 O2(g)  8 SO2(g)
(B) 3 Br2(aq) + 6 OH-  5 Br- + BrO3- + 3 H2O
(C) Ca2+ + SO42-  CaSO4(s)
(D) PtCI4(s) + 2 CI-  PtCI62Br (ox.# = 0)  BrO3- (ox.# = +5)  oxidation
B
Br (ox.# = 0)  Br- (ox.# = -1)  reduction
Which reaction is an oxidation-reduction reaction?
(A) HC2H3O2(aq) + NH3(aq)  C2H3O2- + NH4+
(B) Ba2+ + SO42-  BaSO4(s)
(C) Zn(OH)2(s) + 2 OH-  [Zn(OH)4]2(D) 2 K(s) + Br2(l)  2 KBr(s)
K (ox.# = 0)  K+(ox.# = +1)  oxidation
D
Br2 (ox.# = 0)  Br- (ox.# = -1)  reduction
10 HI + 2 KMnO4 + 3 H2SO4  5 I2 + 2 MnSO4 + K2SO4 + 8 H2O
How many moles of HI are used to produce 2.5 mol of I2?
(A) 5.0
(B) 8.0
(C) 10.
(D) 12
2.5 mol I2 x 10 mol HI = 5 mol HI
A
5 mol I2
19. 2 H2O + 4 MnO4- + 3 CIO2-  4 MnO2 + 3 CIO4- + 4 OHHow many moles of ClO2- react with 0.20 L of 0.20 M MnO4-?
(A) 0.030 (B) 0.053 (C) 0.075 (D) 0.13
0.20 L x 0.20 mol MnO4- x 3 mol ClO2- = 0.030 mol
A
1L
4 mol MnO420. Use the reduction potentials to determine which one of the reactions below is spontaneous.
-0.5 V
0.5 V
Cd2+ + 2 e-  Cd
Cu+ + 1 e-  Cu
2+
-1.2
V
0.7 V
Mn + 2 e  Mn
Fe3+ + 1 e-  Fe2+
2+
(A) Cd + 2 Cu  Cd + 2 Cu
(B) Mn2+ + 2 Cu  Mn + 2 Cu+
(C) Cd2+ + Mn  Cd + Mn2+
(D) Cu+ + Fe3+  Cu + Fe2+
Etot = -0.5 + 1.2 = 0.7 V  C is spontaneous.
C
A and D no reducing agent, B nonspontaneous
21. According to the information below, what is the standard reduction potential for the half-reaction: M3+ + 3 e-  M?
M + 3 Ag+  3 Ag + M3+
Ecello = + 2.5 V
+
Ag + e  Ag
Eredo = + 0.8 V
(A) -1.7 V (B) -0.1 V (C) 0.1 V
(D) 1.7 V
Etot = EAg + EM
A
2.5 V = 0.8 V + EM  EM = 1.7 V (ox) Ered = -1.7 V
18.
Questions 22-23 refer to an electrolytic cell that involves the following half-reaction. AIF63- + 3 e-  Al + 6 F22. Which of the following occurs in the reaction?
(A) AIF63- is reduced at the cathode.
(B) Al is oxidized at the anode.
(C) Aluminum is converted from the -3 oxidation state to the 0 oxidation state.
(D) F- acts as a reducing agent.
A
Al in the complex ion, AlF63-, is reduced to Al (3 to 0). Reduction takes place at the cathode.
23.
A steady current of 10 A is passed through an aluminum-production cell for 15 minutes. Which of the following is the correct
expression for calculating the number of grams of aluminum produced? (1 faraday = 96,500 coulombs)
(A) (10)(15)(96,500)/(27)(60)
(B) (10)(15)(27)/(60)(96,500)
(C) (10)(15)(60)(27)/(96,500)(3)
(D) (96,500)(27)/(10)(15)(60)(3)
15 min x 60 s x 10 C x 1 mol e- x 1 mol Al3+ x 27 g Al =
C
1 min 1 s
96,500
3 mol e- 1 mol Al
24. Which of the following expressions is correct for the maximum mass of copper, in grams, that could be plated out by electrolyzing
aqueous CuCl2 for 16 hours at a constant current of 3.0 A? (1 faraday = 96,500 coulombs)
(A) (16)(3,600)(3.0)(63.55)(2)/(96,500)
(B) (16)(3,600)(3.0)(63.55)/(96,500)(2)
(C) (16)(3,600)(3.0)(63.55)/(96,500)
(D) (16)(60)(3.0)(96,500)(2)/(63.55)
16 hr x 3600 s x 3.0 C x 1 mol e- x 1 mol Cu2+ x 63.55 g =
B
1 hr
1s
96,500
2 mol e- 1 mol Cu
25. If 0.060 moles of e- is passed through an electrolytic cell containing a solution of In3+ ions, the maximum number of moles of In
that could be deposited at the cathode is
(A) 0.010 (B) 0.020 (C) 0.030 (D) 0.060
B
It requires 3 moles e- to plate 1 mole of metal (In3+)  0.06 F will plate 0.020 mol In3+.
26.
A power supply has lost the markings that indicate the positive and negative. A chemist suggests that the terminals be connected
to a pair of platinum electrodes that dip into 0.1 M KI solution. Which correctly identifies the polarities of the terminals?
(A) A gas will be evolved only at the positive electrode.
(B) A gas will be evolved only at the negative electrode.
(C) An amber color will appear near the negative electrode.
(D) A metal will be deposited on the positive electrode.
negative (cathode): 2 H2O + 2 e-  H2(g) + 2 OHB
positive (anode): 2 I-  I2 + 2 e- (amber color)
27. In the electroplating of nickel, 0.200 faraday (same as 1mole of e- flowing) of electrical charge is passed through a solution of
NiSO4. What mass of nickel is deposited?
(A) 2.94 g (B) 5.87 g (C) 11.7 g (D) 58.7 g
B
1 faraday = 1 mol e-. 0.2 F will plate 0.100 mol Ni2+ 0.100 mol Ni2+ x 58.7 g/mol = 5.87 g
Practice Free Response
A power supply is connected to two platinum electrodes immersed in a beaker containing 1.0 M CuSO4(aq) at 25oC. As the cell
operates, copper metal is deposited on the left electrode and O2(g) is produced at the right electrode. The reduction halfreactions that occur are:
O2(g) + 4 H+(aq) + 4 e-  2 H2O(l) Eo = +1.23 V
Cu2+(aq) + 2 e-  Cu(s)
Eo = +0.34 V
a. Is the direction of electron flow in the wire from left to right or from right to left?
The electrons flow from right to left.
b. Write a balanced net ionic equation for the electrolysis reaction that occurs.
2(Cu2+(aq) + 2 e-  Cu(s))
2 H2O(l)  O2(g) + 4 H+(aq) + 4 e2+
2 Cu (aq) + 2 H2O(l)  2 Cu(s) + O2(g) + 4 H+(aq)
A current of 1.50 A passes through the cell for 40.0 minutes.
c. Calculate the mass of the Cu(s) that is deposited.
40.0 min x 60 s x 1.50 C x 1 mol e- x 1 mol Cu x 63.5 g =
1 min
1s
96,500 C 2 mol e- 1 mol Cu
1.18 g Cu
d. Calculate the dry volume, in liters, measured at 25oC and 1.16 atm, of the O2(g) that is produced.
1.19 g Cu x 1 mol Cu x 1 mol O2 = 0.00937 mol O2
63.5 g Cu 2 mol Cu
V = nRT/P = (0.00937)(0.0821)(298)/1.16 = 0.198 L
1.
2.
A voltaic cell is constructed with a strip of Sn in a solution of Sn(NO3)2 in one container and a strip of an unknown metal, X, in a
solution of X(NO3)3 in another container. The two containers are connected by a salt bridge and the two metal strips are
connected with a metal wire. The mass of the Sn electrode increases. The half-reactions are:
Sn2+(aq) + 2 e-  Sn(s)
Eo = -0.14 V
X3+(aq) + 3 e-  X(s)
Eo = ?
a. Which electrode is the cathode? Justify your answer.
Sn is the cathode because Sn gained mass.
b. What directions do electrons flow in terms of the Sn and unknown metal strips.
Electrons flow from metal X to Sn.
c. If the cell potential, Eocell, is + 0.60 V, what is the standard reduction potential for X3+(aq) + 3 e-  X(s)?
Eored + Eoox = Etot
-0.14 V + Eoox = 0.60 V  Eoox = 0.74 V and Eored = -0.74 V
d. Identify metal X from the chart of standard potentials.
Chromium
e. Write a balanced net-ionic equation for the overall chemical reaction occurring in the cell.
3 Sn2+ + 2 Cr  3 Sn + 2 Cr3+
3.
An electrolytic cell contains a solution of Cr(NO3)3. Assume that chromium metal plates out at one electrode and oxygen gas is
evolved at the other electrode.
a. Write the anode half-reaction, the cathode half-reaction, and the overall reaction for the cell.
cathode: 4(Cr3+ + 3 e-  Cr)
anode:
3(2 H2O  O2 + 4 H+ + 4 e-)
3+
4 Cr + 6 H2O  4 Cr + 3 O2 + 12 H+
b. How long will it take to deposit 15.0 g of chromium metal, using a current of 4.50 A?
15.0 g Cr x 1 mol x 3 mol e- x 96,500 C x 1 s = 1.86E4 s
52.0 g 1 mol Cr 1 mol e- 4.50 C
c. A current of 4.50 A is passed through the cell for 30.0 min. If we start out with 250 mL of 1.00 M Cr(NO3)3, what is the
concentration of Cr3+ after electrolysis?
0.250 L x 1 mol/L = 0.250 mol Cr3+
30.0 min x 60 s x 4.50 C x 1 mol e- x 1 mol Cr3+ = 0.0280
1 min
1s
96,500 C 3 mol e0.250 mol – 0.0280 mol = 0.222 mol/0.250 L = 0.888 M
A voltaic cell consists of two half-cells, one of which contains a Pt electrode surrounded by Cr3+ and Cr2O72- ions. The other
half-cell contains a Pt electrode surrounded by Mn2+ ions and MnO2(s). Assume the cell reactions, which produce a positive
voltage, involves both Cr3+ and Mn2+ ions.
Cr2O72- + 14 H+ + 6 e-  2 Cr3+ + 7 H2O
Eored = 1.33 V
+
2+
MnO2(s) + 4 H + 2 e  Mn + 2 H2O
Eored = 1.229 V
d. Write the anode half-reaction, the cathode half-reaction, and the overall reaction for the cell.
cathode: Cr2O72- + 14 H+ + 6 e-  2 Cr3+ + 7 H2O
anode:
3(Mn2+ + 2 H2O MnO2(s) + 4 H+ + 2 e-)
2Cr2O7 + 2 H+ + 3 Mn2+  2 Cr3+ + H2O + 3 MnO2(s)
e. Calculate Eotot for the cell.
4.
Eotot = Eored + Eoox = 1.33 V – 1.229 V = 0.10 V
5.
An electrochemical cell is constructed with an open switch, as shown in the diagram above. A strip of Sn and a strip of an
unknown metal, X, are used as electrodes. When the switch is closed, the mass of the Sn electrode increases.
The half-reactions are shown below.
Sn2+ + 2 e-  Sn(s)
Eo = -0.14 V
3+
X + 3 e  X(s)
Eo = ?
a. In the diagram above, label the electrode that is the cathode. Justify your answer.
Sn is the cathode because Sn gained mass  it is the site of reduction (Sn2+ + 2 e-  Sn).
b.
In the diagram above, draw an arrow indicating the direction of the electron flow in the external circuit when the switch is
closed.
Electrons flow from right to left.
c. If the standard cell potential, Eocell, is + 0.60 V, what is the standard reduction potential, in volts, for the X3+/X electrode?
Eored + Eoox = Etot
-0.14 V + Eoox = 0.60 V
Eoox = 0.74 V  Eored = -0.74 V
d. Identify metal X.
Chromium (Eored for Cr = -0.74 V)
e.
Write a balanced net-ionic equation for the overall chemical reaction occurring in the cell.
3 Sn2+ + 2 Cr  3 Sn + 2 Cr3+