Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 3 — Questions and Answers
Question 1: In oxidation-reduction reactions, the oxidising agent is the species that:
- Gains electrons and is itself reduced (Correct answer)
- Loses electrons and is itself oxidised
- Acts as both oxidiser and reducer
- Does not change oxidation state
Correct answer: Gains electrons and is itself reduced
An oxidising agent causes oxidation in another species by accepting electrons, and is itself reduced in the process.
Question 2: In the standard hydrogen electrode, the half-reaction is H⁺(aq) + e⁻ → ½H₂(g). The standard reduction potential is defined as:
- 0.00 V by definition (Correct answer)
- +1.23 V
- −0.76 V
- +0.34 V
Correct answer: 0.00 V by definition
The standard hydrogen electrode (SHE) is assigned a reduction potential of exactly 0.00 V by convention.
Question 3: The reaction rate of A + B → products is found experimentally to double when [A] doubles (B constant) and quadruple when [B] doubles (A constant). The rate law is:
- rate = k[A][B]² (Correct answer)
- rate = k[A]²[B]
- rate = k[A][B]
- rate = k[A]²[B]²
Correct answer: rate = k[A][B]²
Rate doubles with [A] → first order in A; rate quadruples with [B] → second order in B. Rate = k[A][B]².
Question 4: The activation energy of a reaction can be determined graphically from the slope of a plot of:
- ln k vs 1/T (Arrhenius plot) (Correct answer)
- rate vs temperature
- ln[A] vs time
- rate vs [A]
Correct answer: ln k vs 1/T (Arrhenius plot)
The Arrhenius equation: ln k = ln A − Ea/(RT); a plot of ln k vs 1/T gives slope = −Ea/R.
Question 5: At 700 K, the equilibrium constant for CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g) is K = 2.0. If [CO] = [H₂O] = 1.0 M and [CO₂] = [H₂] = 0.5 M, the system is:
- Not at equilibrium; Q < K, reaction goes forward (Correct answer)
- At equilibrium
- Not at equilibrium; Q > K, reaction goes backward
- At equilibrium only if temperature changes
Correct answer: Not at equilibrium; Q < K, reaction goes forward
Q = (0.5 × 0.5)/(1.0 × 1.0) = 0.25 < K = 2.0, so the reaction proceeds in the forward direction to reach equilibrium.
Question 6: A galvanic cell uses the half-reactions: Zn²⁺/Zn (E° = −0.76 V) and Cu²⁺/Cu (E° = +0.34 V). The standard cell potential is:
- 1.10 V (Correct answer)
- 0.42 V
- −1.10 V
- 0.76 V
Correct answer: 1.10 V
E°cell = E°cathode − E°anode = 0.34 − (−0.76) = 1.10 V.
Question 7: The relationship between Gibbs free energy change ΔG° and the equilibrium constant K at temperature T is:
- ΔG° = −RT ln K (Correct answer)
- ΔG° = RT ln K
- ΔG° = −nFE°
- ΔG° = K/RT
Correct answer: ΔG° = −RT ln K
The standard thermodynamic relationship is ΔG° = −RT ln K; negative ΔG° means K > 1 (products favoured).
Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 3
In oxidation-reduction reactions, the oxidising agent is the species that: