QCE Chemistry IA1 free practice questions with answers

Question 1 (3 marks) [Chemical equilibrium]

At equilibrium at a certain temperature the concentrations in the reaction H₂ + I₂

2HI are: [H₂] = 0.33 mol

L−¹, [I₂] = 0.43 mol L−¹, [HI] = 0.34 mol L−¹.

(a) Calculate the equilibrium constant, K_c, for this reaction.

(a) Answer: 0.815

• Write the expression from the coefficients: K_c = [HI]² ÷ ([H₂] × [I₂]).
• K_c = 0.34² ÷ (0.33 × 0.43)
• K_c = 0.815

(b) What is K_c for the REVERSE reaction at this temperature?

(b) Answer: 1.23

• Reversing a reaction inverts the equilibrium constant: K_reverse = 1 ÷ K_forward.
• K = 1 ÷ 0.815 = 1.23

K_c = [products]^coefficients ÷ [reactants]^coefficients (equilibrium concentrations only; pure solids and liquids are left out); reversing the reaction inverts K.