Chem 25 Chapter 11
Which pairs of species will spontaneously react? A) Cd and Sn²⁺ B) Cd with Sn C) Sn²⁺ with Cd²⁺ D) Sn with Cd²⁺ E) Sn²⁺ with H⁺ *See Image on Chem 101
A) Cd and Sn²⁺ You need to remember that a spontaneous redox reaction is one with a positive E°. Remember that E° is determined by E°(cathode) - E°(anode). Explanation: Choices "Cd and Sn²⁺" and "Sn with Cd²⁺" between the Cd and Sn half cells involve an oxidant reacting with a reductant. The half cell with the higher reduction potential will be reduced while the one with the lower oxidation potential will be oxidized. Therefore Sn²⁺, with a reduction potential of -0.15 will be reduced while Cd, with a reduction potential of -0.40 will be oxidized.
A galvanic cell is represented by the shorthand Cu | Cu²⁺ || Ag⁺ | Ag. Which reaction occurs at the anode? A) Cu(s) → Cu²⁺(aq) + 2e⁻ B) Ag⁺(aq) + e⁻ → Ag(s) C) Cu²⁺(aq) + 2e⁻ → Cu(s) D) Ag(s) → Ag⁺(aq) + e⁻
A) Cu(s) → Cu²⁺(aq) + 2e⁻ Explanation: A galvanic cell represented by the shorthand Cu | Cu²⁺ || Ag⁺ | Ag, is a representation for the two half-reactions: Cu(s) → Cu²⁺(aq) + 2e⁻, an oxidation occurring at the anode, and Ag⁺(aq) + e⁻ → Ag(s), a reduction, occuring at the cathode.
In the redox reaction 6 Fe²⁺ + Cr₂O₇²⁻ + 14 H⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 7 H₂O, what is the reducing agent? A) Fe²⁺ B) Cr₂O₇²⁻ C) H⁺ D) Cr³⁺ E) Fe³⁺
A) Fe²⁺ Explanation: Fe²⁺ increases in oxidation number from +2 to +3 and the Cr in Cr₂O₇²⁻ decreases in oxidation number from +6 to +3. Therefore Cr is being reduced, and Fe²⁺ is the reducing agent.
If E°(red) of a given half-cell is more negative than E°(red) for a standard hydrogen electrode, the half-cell will: A) Tend to oxidize B) Tend to reduce C) Be at equilibrium D) Not enough info
A) Tend to oxidize Explanation: The half cell with the higher E°(red) in a galvanic cell will be reduced, so if a half-cell has a more negative E°(red) than a standard hydrogen electrode, it will tend to oxidize while hydrogen ion is reduced.
During the electrolysis of molten NaI, what reaction occurs at the anode? A) I⁻(l) + e⁻ → I₂⁻(l) B) 2 I⁻(l) → I₂(g) + 2e⁻ C) NaI(l) → NaI₂(l) D) 2 I⁻(l) + e⁻ → I₂(g) E) Na⁺(l) + e⁻ → Na(l)
B) 2 I⁻(l) → I₂(g) + 2e⁻ Explanation: Oxidation occurs at the anode. 2 I⁻(l) → I₂(g) + 2e⁻ is the only oxidation reaction in the list of choices.
Which metal can be a sacrificial anode for an iron pipe? A) Pb B) Cr C) Co D) Cu E) Sn *See Image on Chem 101
B) Cr Explanation: A sacrificial anode is one that is more readily oxidized than the substance in question. To be a sacrificial anode for iron the metal must be more easily oxidized than iron which means it's ions must have a lower potential than Fe²⁺. Only Cr ions, with a reduction potential of -0.74 V, have a lower reduction potential than the -0.44 of Fe²⁺.
Consider the reaction in a lead storage battery below. Which of the following statements is correct? PbO₂(s) + Pb(s) + 2 H₂SO₄(aq) ⇌ 2 PbSO₄(s) + 2 H₂O(l) A) Pb is formed at the anode during use of the battery. B) Pb²⁺ is formed at the cathode during use of the battery. C) The mass of Pb decreases during recharging. D) The concentration of H₂SO₄ increases during use of the battery. E) The mass of PbSO₄ remains constant during use and recharging of the battery.
B) Pb²⁺ is formed at the cathode during use of the battery. Explanation: When the battery is being used, the reactants PbO₂, Pb, and H₂SO₄ are decreasing, while PbSO₄ is increasing, so statements A, C, D, and E must be false. During use Pb²⁺ is being formed at both the cathode and anode, so statement B is Correct!
Which of the following statements is incorrect about a concentration cell consisting of the same metal and metal ion solutions at different concentrations? A) Reduction occurs in the more concentrated solution. B) The cell containing the more concentrated solution is the anode. C) The standard electrode potential (E°) of the electrochemical cell is zero D) The greater the difference in concentrations of the half-cells, the greater the initial voltage is. E) A spontaneous redox reaction occurs until the concentrations become equal.
B) The cell containing the more concentrated solution is the anode. Explanation: The reaction will proceed spontaneously to equilibrium with the ions in the more concentrated solution being reduced to metal, and the metal being oxidized to form ions in the less concentrated solution. Oxidation occurs at the anode, therefore the statement that the more concentrated solution is the anode is an incorrect statement.
Why does aluminum (Al) metal not undergo corrosion like iron does? A) Al does not easily react with oxygen. B) Fe gives cathodic protection to Al. C) A protective layer of Al₂O₃ forms on the surface of Al. D) Al is more difficult to oxidize than Fe. E) An electrical circuit cannot be completed on a surface of Al.
C) A protective layer of Al₂O₃ forms on the surface of Al. Explanation: Aluminum is easily oxidized to Al₂O₃, but the surface layer of Al₂O₃ formed serves as a protective coating to the Al.
A voltaic cell is prepared where copper metal is oxidized to copper(II) and silver(I) is reduced to silver metal. Which of the following reactions occurs at the anode? A) 2 Cu(s) → 2 Cu²⁺(aq) + e⁻ B) Cu²⁺(aq) + 2e⁻ → Cu(s) C) Cu(s) → Cu²⁺(aq) + 2e⁻ D) Ag⁺(aq) + e⁻ → Ag(s) E) Ag(s) → Ag⁺(aq) + e⁻
C) Cu(s) → Cu²⁺(aq) + 2e⁻ Explanation: Oxidation, which is the loss of electrons, occurs at the anode. The correctly balanced half-reaction for the oxidation of Cu isCu(s) → Cu²⁺(aq) + 2e⁻
Which of the following species is the best reducing agent? A) Pb B) Ag⁺ C) Mg D) F⁻ E) Cu *See table image on Chem 101
C) Mg Explanation: The best reducing agent is the one most easily oxidized, which is the one with the lowest reduction potential. From the choices on the list, Mg, with a reduction potential of -2,36 V will be the best reducing agent.
Which of the following is the cell diagram for the reaction 3 Pb²⁺+(aq) + 2 Al(s) → 3 Pb(s) + 2 Al³⁺(aq)? A) Pb(s) | Pb²⁺(aq) || Al³⁺(aq) | Al(s) B) Pb²⁺(aq) | Pb(s) || Al³⁺(aq) | Al(s) C) Pb(s) | Pb²⁺(aq) || Al(s) | Al³⁺(aq) D) Al(s) | Al³⁺(aq) || Pb²⁺(aq) | Pb(s) E) Al³⁺(aq) | Al(s) || Pb(s) | Pb²⁺(aq)
D) Al(s) | Al³⁺(aq) || Pb²⁺(aq) | Pb(s) Explanation: 3 Pb²⁺+(aq) + 2 Al(s) → 3 Pb(s) + 2 Al³⁺(aq) Al is being oxidized at the anode and Pb²⁺ is being reduced at the cathode, so the cell diagram is Al(s) | Al³⁺(aq) || Pb²⁺(aq) | Pb(s)
What does a 'dead' battery mean chemically? A) Q > 1 B) Q < 1 C) Q = 1 D) Q = K E) Q/K = 0
D) Q = K When a battery is "dead" it means that the chemical components are at equilibrium. When that occurs, ∆G = 0, and Q = K.
Electrons always flow in a voltaic (galvanic) cell from _____ A) the salt bridge to the anode B) the oxidizing agent through the salt bridge to the reducing agent C) the reducing agent through the salt bridge to the oxidizing agent D) the anode to cathode E) the salt bridge to the cathode
D) the anode to cathode Explanation: Oxidation occurs at the anode and reduction at the cathode. This means electrons are removed from the anode and flow toward the cathode.
For the electrochemical cell represented by Cr²⁺ | Cr³⁺ || Li⁺ | Li, E°(red) = -2.63 V. This cell: A) Runs spontaneously B) Does not run spontaneously C) Is galvanic D) Is electrolytic E) Both (B) and (D)
E) Both (B) and (D) Explanation: An electrochemical cell with a negative voltage is not spontaneous, so it occurs only if electrical energy is applied. Therefore both does not run spontaneously and is electrolytic is correct.
Given the standard electrode reduction potentials, which reaction will occur spontaneously? A) V²⁺ + 2Cu⁺ → V + 2Cu²⁺ B) Mg²⁺ + V → V²⁺ + Mg C) Mg²⁺ + 2Cu⁺ → 2Cu²⁺ + Mg D) Mg²⁺ + V²⁺→ Mg + V E) V + 2Cu²⁺ → V²⁺ + 2Cu⁺ *See Image on Chem 101
E) V + 2Cu²⁺ → V²⁺ + 2Cu⁺ Explanation: The half cell with the higher reduction potential will be reduced while the one with the lower oxidation potential will be oxidized. Cu²⁺, with a reduction potential of +0.15 will be reduced while V, with a reduction potential of -1.18 will be oxidized, so the reaction will be V + 2Cu²⁺ → V²⁺ + 2Cu⁺
A galvanic cell Zn | Zn²⁺ || Ni²⁺ | Ni runs spontaneously. If a current is imposed to turn this into an electrolytic cell, which of the following will occur? A) Zn(s) still gets oxidized at the same rate B) Zn(s) gets oxidized at a faster rate C) Ni²⁺ gets oxidized D) Ni²⁺ gets reduced E) Zn²⁺ gets reduced
E) Zn²⁺ gets reduced Explanation: An electrolytic cell runs in the opposite direction of a galvanic cell. Applying a current to the galvanic cell Zn | Zn²⁺ || Ni²⁺ | Ni would convert the Zn anode to a cathode and cause Zn²⁺ to be reduced.
Given the standard electrode reduction potentials, which reaction will occur spontaneously?
Explanation: The half cell with the higher reduction potential will be reduced while the one with the lower oxidation potential will be oxidized. Cu²⁺, with a reduction potential of +0.15 will be reduced while V, with a reduction potential of -1.18 will be oxidized, so the reaction will be
