How temperature changes affect chemical equilibrium
The rate and extent of chemical change • Reversible reactions and equilibrium
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Key concepts
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Definition of equilibrium and temperature as a perturbation
Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction so that macroscopic concentrations remain constant. Temperature acts as an external perturbation that changes the energy distribution of particles and therefore the relative rates of the forward and reverse reactions. A change in temperature removes the initial balance of rates and causes a shift in the composition of the equilibrium mixture until a new equilibrium establishes.
Le Chatelier’s principle applied to temperature
Le Chatelier’s principle predicts that a system at equilibrium counteracts a change. An increase in temperature favours the endothermic direction because that direction absorbs the added heat. A decrease in temperature favours the exothermic direction because that direction releases heat to oppose the cooling. The prediction uses only the sign of the enthalpy change for the forward reaction: positive ΔH (endothermic forward) or negative ΔH (exothermic forward).
Effect on equilibrium position and concentrations
When temperature increases and the forward reaction is endothermic, the equilibrium shifts to the right so product concentrations increase and reactant concentrations decrease. When temperature increases and the forward reaction is exothermic, the equilibrium shifts to the left so product concentrations decrease and reactant concentrations increase. The opposite concentration changes occur when temperature decreases. The shift continues until forward and reverse rates equalise again at the new temperature.
Equilibrium constant (K) and temperature dependence
The equilibrium constant K changes with temperature. For a given reaction, an increase in temperature causes K to increase if the forward reaction is endothermic and K to decrease if the forward reaction is exothermic. K provides a quantitative measure: K = [products]^p / [reactants]^r (for concentrations). A measured change in K between two temperatures identifies whether the forward reaction is endothermic or exothermic.
Interpreting data to predict temperature effects (HT skill)
Given experimental equilibrium concentrations or a change in the equilibrium constant at two temperatures, the sign of ΔK indicates the heat flow of the forward reaction. If K increases with temperature, the forward reaction absorbs heat (endothermic). If K decreases with temperature, the forward reaction releases heat (exothermic). When concentration data at two temperatures are available, comparison of product/reactant ratios also shows the direction of the shift: higher product ratio at higher temperature indicates an endothermic forward reaction.
Limiting factors and common misconceptions
Catalysts change the rate at which equilibrium is reached but do not change the equilibrium position or the value of K. Pressure changes affect equilibria only when gases are involved and when the total number of moles of gas differs between products and reactants. Concentration changes shift equilibrium according to Le Chatelier’s principle but do not change K at constant temperature. Temperature change is the only perturbation among these that changes the value of K itself.
Key notes
Important points to keep in mind