Gaokao Chemistry: Chemical Equilibrium and Reactions Flashcards

In the Haber process N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol), which conditions maximise yield of NH₃?

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).

The common ion effect refers to:

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.

For a reaction with K >> 1, the equilibrium position lies:

Answer: Far to the right, heavily favouring products

K >> 1 means [products] >> [reactants] at equilibrium; the reaction essentially goes to completion.

The pH of a 0.01 M HCl solution at 25°C is:

Answer: 2

HCl is a strong acid, fully dissociated: [H⁺] = 0.01 M; pH = −log(0.01) = 2.

In a buffer solution made from CH₃COOH and CH₃COO⁻, adding a small amount of strong acid causes:

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.

Which statement correctly describes Le Chatelier's principle?

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.

The hydrolysis of the salt CH₃COONa in water produces a solution that is:

Answer: Basic (pH > 7)

CH₃COO⁻ hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻; the weak acid anion produces OH⁻, making the solution basic.