Electrochemistry sample question 1

E falls toward zero, while E° stays at +0.62 V.

Electrochemistry sample question 3

H₂O₂ is the oxidizing agent, and its oxygen changes from −1 to −2.

Electrochemistry sample question 4

the cathode, where Li⁺ ions are reduced and inserted into the graphite.

Electrochemistry sample question 5

The silver electrode gains mass, while the platinum electrode's mass does not change.

MCAT · Chemical and Physical Foundations · General Chemistry

Electrochemistry

Electrochemistry is part of General Chemistry in the MCAT Chemical and Physical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include galvanic cell electrodes and flow, nernst equation and concentration cells, electrolysis and Faraday's law, delta G and E and K.

Common traps: subtracting potentials with the wrong sign; multiplying E by stoichiometric coefficients; calling the anode positive in a galvanic cell.

Sample questions

Question 1
A galvanic cell with a standard potential of +0.62 V is allowed to discharge through a resistor at constant temperature. How do E and E° change as the reaction proceeds?
- A.
  E and E° both fall toward zero.
- B.
  E stays at +0.62 V until the reactants run out, then drops to zero.
- C.
  E rises as products accumulate, while E° stays at +0.62 V.
- D.
  E falls toward zero, while E° stays at +0.62 V.
Answer: D. E falls toward zero, while E° stays at +0.62 V.
As products build up, Q grows and the Nernst term lowers E until Q = K and E = 0; E° is a standard-state constant that does not change (D). A lets E° depend on concentration, B ignores the Nernst dependence on Q, and C has the effect of Q backward.
Question 2
For a galvanic (voltaic) cell operating spontaneously under standard conditions, which combination is correct?
- A.
  E°cell < 0 and ΔG° < 0
- B.
  E°cell > 0 and ΔG° < 0
- C.
  E°cell > 0 and ΔG° > 0
- D.
  E°cell < 0 and ΔG° > 0
Answer: B. E°cell > 0 and ΔG° < 0
ΔG° = −nFE°, so a positive cell potential corresponds to a negative free-energy change, the condition for spontaneity. The pairing E°cell < 0 with ΔG° > 0 describes an electrolytic cell, which requires an external power source.
Question 3
In tissue, hydrogen peroxide can react with iron: H₂O₂ + 2Fe²⁺ + 2H⁺ → 2Fe³⁺ + 2H₂O. Which statement is correct?
- A.
  H₂O₂ is the oxidizing agent, and its oxygen changes from −1 to −2.
- B.
  Fe²⁺ is the oxidizing agent, and oxygen changes from −2 to −1.
- C.
  H⁺ is the oxidizing agent, and oxygen stays at −2 throughout.
- D.
  H₂O₂ is the reducing agent, and its oxygen changes from −1 to 0.
Answer: A. H₂O₂ is the oxidizing agent, and its oxygen changes from −1 to −2.
In a peroxide each oxygen is −1; it becomes −2 in water, so H₂O₂ is reduced and acts as the oxidizing agent while Fe²⁺ is oxidized to Fe³⁺ (A). B calls the species that is oxidized the oxidizing agent, C misassigns peroxide oxygen as −2, and D describes oxidation to O₂, which does not occur here.
Question 4
In a phone's lithium-ion cell, the graphite electrode is the anode during discharge. While the phone is charging, the graphite electrode is:
- A.
  still the anode, because an electrode's label is fixed by its material.
- B.
  the cathode, where lithium is oxidized and released as Li⁺.
- C.
  the cathode, where Li⁺ ions are reduced and inserted into the graphite.
- D.
  still the anode, where Li⁺ ions are reduced and stored.
Answer: C. the cathode, where Li⁺ ions are reduced and inserted into the graphite.
Charging reverses the reactions, so graphite now hosts reduction (Li⁺ + e⁻ → Li in graphite) and is therefore the cathode (C). A and D keep the discharge label, but anode and cathode are defined by whether oxidation or reduction occurs; B calls the electrode a cathode but describes an oxidation, which is the discharge process.
Question 5
A galvanic cell is written Pt(s) | Fe²⁺(aq), Fe³⁺(aq) || Ag⁺(aq) | Ag(s). Which statement about the electrodes is correct as the cell operates?
- A.
  The silver electrode gains mass, while the platinum electrode's mass does not change.
- B.
  The platinum electrode loses mass as it is oxidized at the anode.
- C.
  The silver electrode is the anode and dissolves into the solution.
- D.
  Fe³⁺ is reduced to Fe²⁺ at the platinum electrode.
Answer: A. The silver electrode gains mass, while the platinum electrode's mass does not change.
The left side is the anode, where Fe²⁺ is oxidized to Fe³⁺ on inert platinum; on the right, Ag⁺ is reduced and plates onto the silver cathode, which gains mass (A). B ignores that platinum is an inert electrode, while C and D read the notation backward.

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