Consider the equilibrium 2 , NOBr (g) 2 , NO (g)+ Br _ 2 (g) which has ( H^ =+30 ; kJ…

CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

Consider the equilibrium

2\,\text{NOBr}(g)\rightleftharpoons 2\,\text{NO}(g)+\text{Br}_{2}(g)

which has

\Delta H^{\circ}=+30\;\text{kJ mol}^{-1}

. The equilibrium constant is

K_{c}=1.0\times10^{-2}

at 25 °C and

K_{c}=2.5\times10^{-1}

at 100 °C. A 1.00-L flask is filled at 25 °C with the following initial concentrations: [NOBr]=0.30 M, [NO]=0.10 M, [Br

_2

]=0.050 M. (a) Compute the value of

Q_{c}

at 25 °C and determine the direction in which the net reaction will proceed to reach equilibrium at this temperature. (b) The contents of the flask are then rapidly heated to 100 °C, and the concentrations have not yet had time to change. Compute the value of

Q_{c}

relative to the new value of

K_{c}

and predict the direction of the net reaction at 100 °C. (c) Explain how the temperature dependence of

K_{c}

is consistent with the sign of

\Delta H^{\circ}

for this reaction. (d) Describe qualitatively how the equilibrium concentrations of all three species after re-equilibration at 100 °C compare with their original equilibrium concentrations at 25 °C.

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