Ch8 Equilibrium

In an industrial process, two gases X and Y react together to form a single gaseous product Z.

X (g) + Y (g) ⇌ Z(g)

The percentage yield of product Z varies according to the pressure and the temperature as shown in the grap

Which statement about this equilibrium reaction is correct?

• Decreasing the temperature decreases the value of the equilibrium constant.

• Decreasing the temperature increases the rate of this reaction.

• Increasing the pressure increases the value of the equilibrium constant.

• The reaction is exothermic in the forward direction.

The reaction between sulfur dioxide and oxygen is reversible.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) K_c = 280 mol^–1 dm³ at 1000 K

In an equilibrium mixture at 1000 K the sulfur dioxide concentration is 0.200 mol dm^–3 and the oxygen concentration is 0.100 mol dm^–3.

What is the sulfur trioxide concentration?

• 1.058 mol dm^–3

• 1.120 mol dm^–3

• 2.366 mol dm^–3

• 5.600 mol dm^–3

Two compounds X and Y react to produce compound Z. The reaction is reversible.

X + Y ⇌ Z

When X and Y are mixed together in a closed system a dynamic equilibrium is gradually established.

Which graph could represent the change in the rates of the forward and reverse reactions over time?

• A

• B

• C

• D

X and Y react together to form Z in a reversible reaction.

The equilibrium yield of Z is lower at higher temperature.

The equilibrium yield of Z is lower at lower pressure.

Which equation could represent this reaction?

• X(g) + Y(g) ⇌ Z(g) ∆H = –100 kJ mol^–1

• X(g) + Y(g) ⇌ Z(g) ∆H = +100 kJ mol^–1

• X(s) + Y(g) ⇌ 2Z(g) ∆H = –100 kJ mol^–1

• X(s) + Y(g) ⇌ 2Z(g) ∆H = +100 kJ mol^–1

The equation represents an equilibrium.

4NH₃(g) + 5O₂(g) ⇌ 4NO(g) + 6H₂O(g) ∆H = –900 kJ mol^–1

What would increase the concentration of NO at equilibrium?

• a reduction in the reaction temperature

• the use of a suitable catalyst

• an increase in the total pressure

The diagram shows the reaction pathway for a reversible reaction.

Which statements are correct?

• The enthalpy change for the backward reaction is +90 kJ mol^–1.

• The forward reaction is exothermic.

• The enthalpy change for the forward reaction is –30 kJ mol^–1.

Graphs can be drawn to show the percentage of ammonia at equilibrium when nitrogen and hydrogen are mixed at different temperatures and pressures.

Which diagram correctly represents these two graphs?

• A

• B

• C

• D

What are necessary properties of a dynamic equilibrium?

• Equal amounts of reactants and products are present.

• Concentrations of reactants and products remain constant.

• The rate of the forward reaction is the same as the rate of the reverse reaction.

Phosphorus pentachloride is introduced into an empty gas syringe which has a movable, tightlyfitting plunger. The gas is allowed to expand until equilibrium is reached at a temperature at which the phosphorus pentachloride partially dissociates.

PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

Which statements are correct?

• The equilibrium pressure inside the syringe will be greater than atmospheric pressure.

• When the plunger is pushed in the equilibrium adjusts to produce more PCl₅(g).

• The volume of gas in the syringe at equilibrium will be greater than if no dissociation had

  occurred.

In this question you should assume that all gases behave ideally.

Hydrogen and iodine react reversibly in the following reaction. The system reaches dynamic equilibrium.

H₂(g) + I₂(g) ⇌ 2HI(g) ∆H = –9.5 kJ mol^–1

Which statement must be true for the K_p of this equilibrium to be constant?

• The partial pressures of H₂, I₂ and HI are equal.

• The external pressure is constant

• The forward and reverse reactions have stopped.

• The temperature is constant