Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 2 — Questions and Answers
Question 1: In the Haber process N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol), which conditions maximise yield of NH₃?
- High pressure, low temperature (Correct answer)
- High pressure, high temperature
- Low pressure, low temperature
- Low pressure, high temperature
Correct answer: High pressure, low temperature
High pressure favours fewer moles of gas (products); low temperature favours the exothermic forward reaction thermodynamically (though kinetics require compromise).
Question 2: The common ion effect refers to:
- The decrease in solubility or dissociation when an ion in common with the equilibrium is added (Correct answer)
- The increase in reaction rate due to common ions
- Formation of a precipitate with common ions
- Equal concentrations of ions in solution
Correct answer: The decrease in solubility or dissociation when an ion in common with the equilibrium is added
Adding a common ion shifts the equilibrium toward the undissociated or insoluble form, decreasing dissociation or solubility.
Question 3: For a reaction with K >> 1, the equilibrium position lies:
- Far to the right, heavily favouring products (Correct answer)
- Far to the left, heavily favouring reactants
- Exactly at the midpoint
- Depends only on temperature
Correct answer: Far to the right, heavily favouring products
K >> 1 means [products] >> [reactants] at equilibrium; the reaction essentially goes to completion.
Question 4: The pH of a 0.01 M HCl solution at 25°C is:
- 2 (Correct answer)
- 7
- 12
- 1
Correct answer: 2
HCl is a strong acid, fully dissociated: [H⁺] = 0.01 M; pH = −log(0.01) = 2.
Question 5: In a buffer solution made from CH₃COOH and CH₃COO⁻, adding a small amount of strong acid causes:
- The base component (CH₃COO⁻) to neutralise the acid, minimising pH change (Correct answer)
- A large drop in pH
- The acid component to react with H⁺
- No change at all
Correct answer: The base component (CH₃COO⁻) to neutralise the acid, minimising pH change
Buffer action: added H⁺ reacts with the conjugate base CH₃COO⁻ → CH₃COOH, consuming the extra acid and stabilising pH.
Question 6: Which statement correctly describes Le Chatelier's principle?
- A system at equilibrium shifts to partially offset any imposed stress (Correct answer)
- A system shifts to fully eliminate any imposed stress
- Equilibrium constants change with all applied stresses
- Catalysts shift the equilibrium position
Correct answer: A system at equilibrium shifts to partially offset any imposed stress
Le Chatelier's principle states the system partially counteracts the imposed change; it does not fully reverse it, and K only changes with temperature.
Question 7: The hydrolysis of the salt CH₃COONa in water produces a solution that is:
- Basic (pH > 7) (Correct answer)
- Acidic (pH < 7)
- Neutral (pH = 7)
- pH depends on concentration only
Correct answer: Basic (pH > 7)
CH₃COO⁻ hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻; the weak acid anion produces OH⁻, making the solution basic.
Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 2
In the Haber process N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol), which conditions maximise yield of NH₃?