Hydrogen iodide is formed from its elements in the gas phase: H _2(g)+ I _2(g) 2 HI…

CHEMISTRY · Multi-part · A Level · Chemistry · Gaseous equilibria (Kp) · Gaseous equilibria (Kp)

Problem

Hydrogen iodide is formed from its elements in the gas phase:

\text{H}_2(g)+\text{I}_2(g)\rightleftharpoons2\text{HI}(g)

For this reaction

\Delta H^{\circ}= +51\,\text{kJ\,mol}^{-1}

(per mole of reaction as written). The equilibrium constant in terms of pressure is

K_p = 54.3

700\,\text{K}

. A mixture at

900\,\text{K}

contains the following partial pressures:

P_{\text{H}_2}=0.30\,\text{atm}

P_{\text{I}_2}=0.30\,\text{atm}

, and

P_{\text{HI}}=1.20\,\text{atm}

. The universal gas constant is

R = 8.314\,\text{J\,mol}^{-1}\text{K}^{-1}

. (a) Calculate the numerical value of

K_p

900\,\text{K}

. (b) Determine the value of the reaction quotient

Q_p

for the given gas mixture at

900\,\text{K}

. (c) Indicate whether the forward reaction or the reverse reaction will be favored as the system moves toward equilibrium at

900\,\text{K}

. Support your answer with appropriate comparisons between

Q_p

and

K_p