The equilibrium is exothermic in the forward direction

CHEMISTRY · Single correct · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

The equilibrium

2\,\mathrm{A}(g)\;\rightleftharpoons\;\mathrm{B}(g)

is exothermic in the forward direction. At

500\,\text{K}

the system has

K_c = 3.5

and is at equilibrium with

[\mathrm{A}] = 0.60\,\text{M}

and

[\mathrm{B}] = 1.26\,\text{M}

. The following sequence of stresses is applied: 1. The temperature is raised to

550\,\text{K}

, at which

K_c

becomes 1.0. 2. Three minutes later, an additional 0.20 mol of A is injected into the same

1.0\,\text{L}

vessel. Immediately after the second stress, the concentrations are

[\mathrm{A}] = 0.80\,\text{M}

and

[\mathrm{B}] = 1.05\,\text{M}

. Which statement correctly describes the value of

Q_c

after the second stress and the direction of the net shift that will follow? (a)

Q_c < 1.0

; the reaction will shift toward

\mathrm{B}

. (b)

Q_c > 1.0

; the reaction will shift toward

\mathrm{B}

. (c)

Q_c > 1.0

; the reaction will shift toward

\mathrm{A}

. (d)

Q_c = 1.0

; the mixture is already at equilibrium.

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