The Effect of a Change in Temperature
In a chemical equilibrium there is almost always a difference in energy, and hence in enthalpy, between the reactants and the products. The thermochemical equation for dissociationThe breaking apart of one species into two or more smaller species; often applied to ions in a crystal lattice, which dissociate when the ionic solid dissolves in water. Dissociation refers to separation of particles that already exist; ionization refers to the formation of ions from neutral species, as in the ionization of a weak acid in aqueous solutoin. of N2O4 for example, is
N2O4(g) 2NO2(g) ΔHm = 54.8 kJ mol–1
Because of this enthalpy difference, any shift in the equilibrium toward further dissociation will result in the absorptionPermeation of a solid by a gas or liquid, or permeation of a liquid by a gas. Absorption differs from adsorption in that the substance absorbed is found throughout the absorbent. of heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. energy and a momentary decrease in temperature. Conversely, a shift in the reverse direction will cause a small rise in temperature. If we increase the temperature of a mixtureA combination of two or more substances in which the substances retain their chemical identity. of N2O4 and NO2, the mixture should respond in such a way as to oppose the rise in temperature. This can happen if some N2O4 in the mixture dissociates, since the resulting absorption of energy will produce a cooling effect. We would therefore expect that by raising the temperature of the equilibrium mixture, we would shift the equilibrium in favor of dissociation. Indeed we see in the result of Example 3 from Calculating the Extent of a Reaction that raising the temperature from 200 to 600 K changes an equilibrium mixture which is almost pure N2O4 into an equilibrium mixture which is almost pure NO2.
In the general case, if we raise the temperature of any mixture of species which are in chemical equilibrium with each other, Le Chatelier's principle tells us that we will shift the equilibrium in the direction of those species with the higher energy. Thus, if the reaction is endothermicIn chemical thermodynamics, describes a process in which energy is transferred from the surroundings to the system as a result of a temperature difference., as in the dissociation just discussed, raising the temperature will swing the equilibrium toward the products, and the value of the equilibrium constantThe value of the equilibrium constant expression when equilibrium concentrations are substituted; a value greater than one indicates the position of equilibrium lies toward products (product-favored), and a value less than one indicates the position of equilibrium lies toward reactants (reactant-favored). Kc will increase with temperature. Conversely, if the reaction is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference., a rise in temperature will favor the reactants, and Kc will get smaller as the temperature increases. We can also turn the argument around. If we find a reaction for which Kc increases with temperature, we know immediately that the reaction must be endothermic. Conversely, if Kc decreases as temperature increases, the reaction must be exothermic.