Question 15.12: Consider the equilibrium N2O4(g) ⇌ 2 NO2(g) ΔH° = 58.0 kJ In...
Consider the equilibrium
N_{2}O_{4} (g) ⇌ 2 NO_{2} (g) ΔH° = 58.0 kJ
In which direction will the equilibrium shift when (a)N_{2}O_{4} is added, (b) NO_{2} is removed, (c) the pressure is increased by addition of N_{2}(g), (d) the volume is increased, (e) the temperature is decreased?
Analyze We are given a series of changes to be made to a system at equilibrium and are asked to predict what effect each change will have on the position of the equilibrium.
Plan Le Châtelier’s principle can be used to determine the effects of each of these changes.
Solve
(a) The system will adjust to decrease the concentration of the added N_{2}O_{4}, so the equilibrium shifts to the right, in the direction of the product.
(b) The system will adjust to the removal of NO_{2} by shifting to the side that produces more NO_{2}; thus, the equilibrium shifts to the right.
(c) Adding N_{2} will increase the total pressure of the system, but N_{2} is not involved in the reaction. The partial pressures of NO_{2} and N_{2}O_{4} are therefore unchanged, and there is no shift in the position of the equilibrium.
(d) If the volume is increased, the system will shift in the direction that occupies a larger volume (more gas molecules); thus, the equilibrium shifts to the right.
(e) The reaction is endothermic, so we can imagine heat as a reagent on the reactant side of the equation. Decreasing the temperature will shift the equilibrium in the direction that produces heat, so the equilibrium shifts to the left, toward the formation of more N_{2}O_{4}. Note that only this last change also affects the value of the equilibrium constant, K.