Hydrogen gas and iodine vapor react according to At 400 K the equilibrium constant is

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

Problem

Hydrogen gas and iodine vapor react according to

\mathrm{H_2(g)} + \mathrm{I_2(g)} \rightleftharpoons 2\,\mathrm{HI(g)}

At 400 K the equilibrium constant is

K_{c,400}= 50.0

. For this temperature the equilibrium mixture in a 2.00 L vessel has

[\mathrm{H_2}]=[\mathrm{I_2}]=0.0400\,\text{M}

and

[\mathrm{HI}]=0.283\,\text{M}

. The forward reaction is endothermic with

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

(assume

\Delta H^\circ

is temperature-independent over the range considered). The vessel is then rapidly heated to 450 K and held at that temperature; volumes remain unchanged. (1) Using the information provided, calculate the new value of

K_c

at 450 K. Express your answer to three significant figures. (The universal gas constant

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

.) (2) Immediately after the temperature rise, the concentrations of all species are unchanged. Calculate the reaction quotient

Q_c

at 450 K and predict the direction in which the reaction will proceed to establish equilibrium at the higher temperature. (3) Develop the necessary algebraic expressions and determine the equilibrium concentrations of

\mathrm{H_2}

\mathrm{I_2}

, and

\mathrm{HI}

at 450 K. (4) Determine the amount of heat (in kJ) absorbed or released by the system as it moves from the original equilibrium at 400 K to the new equilibrium at 450 K. State whether the process is endothermic or exothermic for the system as written.

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