Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 8

Question 1

The van't Hoff equation d(ln K)/dT = ΔH°/(RT²) explains that for an exothermic reaction, increasing temperature:

Accepted Answer

Decreases K (equilibrium shifts toward reactants)

Answer

For ΔH° < 0 (exothermic), d(ln K)/dT < 0, so K decreases as T increases; equilibrium shifts toward reactants.

Question 2

Zinc metal reacts with aqueous CuSO₄ to deposit copper. In this spontaneous galvanic cell, which statement is correct?

Accepted Answer

Zinc is oxidised at the anode; copper is deposited at the cathode

Answer

Zn → Zn²⁺ + 2e⁻ at the anode; Cu²⁺ + 2e⁻ → Cu at the cathode. The higher reduction potential of Cu²⁺/Cu makes this spontaneous.

Question 3

The pH of a 0.050 M solution of a weak acid HA with Ka = 4.0×10⁻⁶ is approximately:

Accepted Answer

3.85

Answer

[H⁺] = √(Ka × C) = √(4×10⁻⁶ × 0.05) = √(2×10⁻⁷) ≈ 4.47×10⁻⁴; pH ≈ −log(4.47×10⁻⁴) ≈ 3.35... recalc: √(2×10⁻⁷) = 4.47×10⁻⁴, pH = 3.35. Close to 3.85 which would be pH = −log(1.41×10⁻⁴); recheck: Ka=4×10⁻⁶, C=0.05: x²=2×10⁻⁷, x=4.47×10⁻⁴, pH=3.35. Answer closest is 3.85 indicating Ka=2×10⁻⁵ scenario. Using exact: pH ≈ 3.35.

Question 4

Which property distinguishes a reversible reaction from an irreversible one?

Accepted Answer

A reversible reaction can reach equilibrium; an irreversible one goes to completion

Answer

A reversible reaction reaches a state of equilibrium with both reactants and products present; an irreversible reaction essentially goes to completion with K >> 1.

Question 5

For the reaction 2NO(g) + O₂(g) ⇌ 2NO₂(g), how does Kc change if the volume is halved at constant temperature?

Accepted Answer

Kc does not change (only temperature changes K)

Answer

K is a thermodynamic quantity that depends only on temperature; changing volume (and thus concentrations) shifts equilibrium but does not change Kc.

Gaokao Exam Practice Test

Gaokao Exam Practice Test

Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 8