Chemistry 9701 · AS & A Level · Group 17

Group 17 — practice question

The Group VII elements chlorine, bromine and iodine each form a hydride.
(a)[3]

(i) Outline how the relative thermal stabilities of these hydrides alter from $\text{HCl}$ to $\text{HI}$. (ii) Explain the change you have outlined in (i).

(b)[2]

Hydrogen iodide may be prepared by heating hydrogen gas with iodine vapour. The reaction is not complete. $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$ Write an expression for $K_c$ and state the units.

(c)[4]

For this equilibrium, the numerical value of the equilibrium constant $K_c$ is $140$ at $500\text{ K}$ and $59$ at $650\text{ K}$. Use this information to state and explain the effect of the following changes on the equilibrium position. (i) increasing the pressure applied to the equilibrium (ii) decreasing the temperature of the equilibrium

(a(i))

Outline how the relative thermal stabilities of these hydrides alter from HCl to HI.

(a(ii))[3]

Explain the pattern you have outlined in (i).

(c(i))

For this equilibrium, the numerical value of the equilibrium constant $K_c$ is 140 at 500 K and 59 at 650 K. Use this information to state and explain the effect of increasing the pressure applied to the equilibrium on the equilibrium position.

(c(ii))[4]

For this equilibrium, the numerical value of the equilibrium constant $K_c$ is 140 at 500 K and 59 at 650 K. Use this information to state and explain the effect of decreasing the temperature of the equilibrium on the equilibrium position.

(d)[4]

A mixture containing $0.02$ mol of hydrogen and $0.02$ mol of iodine was put into a $1\,\text{dm}^3$ flask and left to reach equilibrium at $650\,\text{K}$. Calculate the amount, in moles, of each substance present in the equilibrium mixture at $650\,\text{K}$. $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$ Initial moles: $\text{H}_2 = 0.02$, $\text{I}_2 = 0.02$, $\text{HI} = 0$.

Worked solution & mark scheme

This 20-mark question has a full step-by-step worked solution and mark scheme.

  • Full mark scheme, point by point
  • Step-by-step worked solution
  • Write your answer & get it marked instantly by AI