Dinitrogen tetroxide decomposes according to the equilibrium 2 NO _2(g) N _2 O _4(g)…

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

Problem

Dinitrogen tetroxide decomposes according to the equilibrium 2NO2(g)⇌N2O4(g)2 \text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g) The standard enthalpy change for the reaction is ΔH∘=−58 kJ mol−1\Delta H^{\circ}= -58\,\text{kJ\,mol}^{-1} (per mole of N2O4\text{N}_2\text{O}_4 formed). At 298 K298\,\text{K} the equilibrium constant is Kc=4.5K_c = 4.5. A sealed vessel initially at equilibrium is quickly heated to 350 K350\,\text{K}; the volume of the vessel remains constant and no heat is lost during the rapid temperature change. (a) Describe qualitatively how the numerical value of the equilibrium constant is expected to change when the temperature is raised from 298 K298\,\text{K} to 350 K350\,\text{K}. Base your reasoning on thermodynamic principles. (b) Immediately after the temperature increase—before any further reaction occurs—state whether the reaction quotient QcQ_c is greater than, less than, or equal to the new value of KcK_c. Justify your answer. (c) Predict the direction in which the net reaction will proceed to re-establish equilibrium at 350 K350\,\text{K} and explain your reasoning in terms of Le Châtelier’s principle.

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