How far? The extent of chemical change (equilibrium) practice questions.

For N2(g) + 3H2(g) <-> 2NH3(g), delta H = -92 kJ/mol, which change increases the equilibrium yield of ammonia?

• Increasing temperature
• Decreasing pressure
• Increasing pressure
• Adding a catalyst

Answer: Increasing pressure

There are 4 mol of gas on the left and 2 mol on the right, so increasing pressure shifts equilibrium toward the side with fewer gas moles (products), raising the ammonia yield. Higher temperature favours the endothermic reverse reaction; a catalyst only speeds attainment of equilibrium without shifting it.

If the reaction quotient Q is greater than the equilibrium constant Kc, in which direction does the reaction proceed?

• To the right (forming more products)
• To the left (forming more reactants)
• It is already at equilibrium
• It stops completely

Answer: To the left (forming more reactants)

When Q > Kc there are too many products relative to equilibrium, so the reaction shifts to the left (reverse direction) to form more reactants until Q = Kc. If Q < Kc it shifts right; if Q = Kc it is at equilibrium.

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at constant temperature, increasing the total pressure by reducing the volume shifts the equilibrium position

• towards the reactants
• towards the products
• it has no effect
• so far that the reaction stops

Answer: towards the products

There are 4 mol of gas on the left and 2 mol on the right. Increasing pressure shifts equilibrium to the side with fewer gas molecules (the products) to reduce the pressure.

State which statement correctly describes a system at dynamic equilibrium in a closed container.

• The forward reaction has completely stopped and now only the reverse reaction continues to occur
• The concentrations of reactants and products are equal
• The forward and reverse reactions occur at equal rates and concentrations remain constant
• No reactions occur once equilibrium is reached

Answer: The forward and reverse reactions occur at equal rates and concentrations remain constant

At dynamic equilibrium the forward and reverse reactions continue at equal rates, so macroscopic concentrations stay constant (but are not necessarily equal).

For the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH < 0, determine the change that increases the equilibrium yield of SO₃.

• Increasing the temperature
• Decreasing the total pressure
• Removing SO₃ as it forms
• Adding a catalyst

Answer: Removing SO₃ as it forms

Removing a product shifts the position of equilibrium to the right (Le Châtelier), increasing yield; a catalyst does not change yield.

For the reaction A(g) + B(g) ⇌ C(g), a large value of Kc (Kc >> 1) indicates that:

• The reaction reaches equilibrium very quickly
• At equilibrium the mixture contains mostly products
• At equilibrium the mixture contains mostly reactants
• The reaction is exothermic

Answer: At equilibrium the mixture contains mostly products

Kc >> 1 means the numerator (products) dominates, so the equilibrium mixture is rich in products; Kc says nothing about rate or enthalpy sign.