3.2.3 Chemical Equilibrium

Question 1

What does dynamic equilibrium mean?

Answer 1

• It occurs when the rate of forward reaction equals the rate of reverse reaction
• The concentration of reactants and products remains constant in a closed system

Question 2

Changing what factors can alter the position of equilibrium?

Answer 2

• Concentration of reactants or products
• Pressure
• Temperature

Question 3

Explain Le Chatelier’s principle.

Answer 3

• If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

Question 4

What effect would increasing the temperature have on the position of equilibrium in this equation: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g); ΔH⁰ = +210 kJ mol⁻¹?

Answer 4

• The equilibrium position shifts to the right.
• This is because the forward reaction is endothermic.
• Yield of hydrogen increases.

Question 5

What effect would increasing the pressure have on the position of equilibrium in the equation: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g); ΔH⁰ = +210 kJ mol⁻¹?

Answer 5

• The equilibrium position shifts to the left.
• This is because the forward reaction produces more moles of gas (4 moles of product vs 2 moles of reactant).
• Therefore, the yield of hydrogen decreases.

Question 6

Suggest and explain why an industrial chemist may use high pressure for the production of hydrogen in the reaction: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g); ΔH⁰ = +210 kJ mol⁻¹.

Answer 6

• High pressure increases the collision frequency, thus increasing the rate of reaction.
• It is a compromise pressure between an economically viable rate of reaction and a slightly lower yield of hydrogen.

Question 7

What effect does a catalyst have on the position of equilibrium?

Answer 7

• No effect.
• A catalyst affects the rate of the forward and reverse reactions equally.

Question 8

What condition affects the value of Kc?

Answer 8

• Temperature
  (Note: Concentration, catalyst, and pressure do not affect Kc.)

Question 9

For the reaction: 2[A] + 3[B] ⇌ [D] + 4[E], deduce an expression for Kc.

Answer 9

Kc = ([D][E]⁴) / ([A]²[B]³)

Question 10

Deduce units for Kc in the above reaction.

Answer 10

• Units: mol⁻¹ dm³

Question 11

What type of system is Kc relevant for?

Answer 11

• Homogeneous systems in equilibrium

Question 12

What does Kc being greater or less than 1 suggest for the position of equilibrium?

Answer 12

• Greater than 1 = Equilibrium lies to the right.
• Less than 1 = Equilibrium lies to the left.

Question 13

What effect does decreasing the temperature of an endothermic reaction have on Kc?

Answer 13

• Kc decreases.

Question 14

What effect does increasing the temperature of an endothermic reaction have on Kc?

Answer 14

• Kc increases.

Question 15

What effect does decreasing the temperature of an exothermic reaction have on Kc?

Answer 15

• Kc increases.

Question 16

What effect does increasing the temperature of an exothermic reaction have on Kc?

Answer 16

• Kc decreases.