Gaokao Chemistry: Chemical Equilibrium and Reactions Flashcards

Which of the following changes will increase the rate of the reaction A(g) + B(g) → C(g) + D(g)?

Answer: Increasing the partial pressure of A

Increasing the pressure of A increases its concentration, thereby increasing the collision frequency and the reaction rate.

The relationship ΔG = ΔH − TΔS determines reaction spontaneity. A reaction is spontaneous at all temperatures when:

Answer: ΔH 0

When ΔH 0, ΔG = ΔH − TΔS is always negative (for T > 0 K), so the reaction is spontaneous at all temperatures.

In an acidic potassium permanganate titration with iron(II) sulfate, the end point is detected by:

Answer: The first permanent faint pink colour (excess KMnO₄)

KMnO₄ (purple) is reduced to Mn²⁺ (colourless) by Fe²⁺; at the endpoint, one extra drop of KMnO₄ gives a permanent faint pink, indicating all Fe²⁺ is consumed.

The ion product of water Kw increases from 1×10⁻¹⁴ at 25°C to a larger value at higher temperatures. This implies that the autoionisation of water is:

Answer: Endothermic

Kw increases with temperature, meaning the forward reaction is favoured at higher T; by Le Chatelier/van't Hoff, this indicates the reaction is endothermic.

For the half-reaction Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V), the Cr changes from oxidation state:

Answer: +6 to +3

In Cr₂O₇²⁻, Cr is +6 (the −2 from O: 2x + 7(−2) = −2 → x = +6); in Cr³⁺ it is +3. Each Cr gains 3 electrons; 2 Cr atoms × 3e⁻ = 6e⁻ total.

A reaction mixture initially contains only reactants. The system will reach equilibrium when:

Answer: The forward and reverse reaction rates become equal

Equilibrium is a dynamic state where the forward and reverse reaction rates are equal, resulting in constant concentrations.

Electroplating copper onto a metal object requires the object to be connected as the:

Answer: Cathode (reduction of Cu²⁺ onto the object)

In electroplating, the object to be plated is the cathode; Cu²⁺(aq) + 2e⁻ → Cu(s) deposits copper metal onto it.