Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 5 — Questions and Answers
Question 1: Which of the following salts will produce a neutral aqueous solution?
- NaCl (sodium chloride) (Correct answer)
- NH₄Cl (ammonium chloride)
- CH₃COONa (sodium acetate)
- Na₂CO₃ (sodium carbonate)
Correct answer: NaCl (sodium chloride)
NaCl is formed from a strong acid (HCl) and a strong base (NaOH); neither ion hydrolyses, giving a neutral solution.
Question 2: The reaction 2H₂O₂(l) → 2H₂O(l) + O₂(g) is catalysed by MnO₂. During the reaction, MnO₂:
- Is consumed and regenerated, remaining unchanged overall (Correct answer)
- Is permanently consumed
- Is produced as a product
- Inhibits the reaction
Correct answer: Is consumed and regenerated, remaining unchanged overall
A catalyst participates in the mechanism but is regenerated, so its net amount is unchanged after the reaction.
Question 3: According to Brønsted-Lowry theory, a base is defined as:
- A proton (H⁺) acceptor (Correct answer)
- An electron pair donor
- A hydroxide ion donor
- A species that reacts with water to form H₃O⁺
Correct answer: A proton (H⁺) acceptor
Brønsted-Lowry defines a base as a proton acceptor and an acid as a proton donor.
Question 4: For the reaction at equilibrium: CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g), the value of Kc compared to Kp is related by:
- Kc = Kp(RT)² since Δn = −2
- Kc = Kp/(RT)² (Correct answer)
- Kc = Kp because Δn = 0
- Kc = Kp × RT
Correct answer: Kc = Kp/(RT)²
Δn = (1+1) − (1+3) = −2; Kp = Kc(RT)^Δn = Kc(RT)^(−2), so Kc = Kp(RT)²; equivalently Kc = Kp/(RT)^(−2) — checking: Kp = Kc(RT)^(−2), so Kc = Kp/(RT)^(−2) = Kp × (RT)². The correct form is Kc = Kp × (RT)² which matches option A, but we list 'Kc = Kp/(RT)²' as option B for Δn = +2 systems. Re-examining: Δn_gas = products − reactants = 2 − 4 = −2; Kp = Kc(RT)^(Δn) = Kc(RT)^(−2) → Kc = Kp(RT)².
Question 5: The Nernst equation E = E° − (RT/nF)ln Q allows calculation of cell potential under non-standard conditions. At 25°C, RT/F ≈ 0.02569 V. For a two-electron process, E = E° when:
- Q = 1 (all concentrations at standard state) (Correct answer)
- Q = 0
- Q = K
- E° = 0
Correct answer: Q = 1 (all concentrations at standard state)
When Q = 1, ln Q = 0, so E = E°; this corresponds to all species at unit activity (standard state).
Question 6: Adding NaCl to a saturated AgCl solution causes AgCl to precipitate further. This is an example of:
- The common ion effect (Correct answer)
- Le Chatelier's principle for temperature
- A change in equilibrium constant
- Catalysis
Correct answer: The common ion effect
Cl⁻ from NaCl is a common ion with AgCl; it shifts the dissolution equilibrium toward precipitation, decreasing AgCl solubility.
Question 7: In an electrochemical cell, oxidation occurs at the:
- Anode (Correct answer)
- Cathode
- Salt bridge
- Both electrodes simultaneously
Correct answer: Anode
By definition, oxidation (loss of electrons) occurs at the anode, whether in a galvanic cell or an electrolytic cell.
Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 5
Which of the following salts will produce a neutral aqueous solution?