When the temperature of an exothermic reaction at equilibrium is increased, the equilibrium shifts to favor the reverse, endothermic reaction, leading to a decrease in product formation and, crucially, a decrease in the value of the equilibrium constant (K).
Understanding Exothermic Reactions and Equilibrium
An exothermic reaction is a chemical process that releases energy, typically in the form of heat, into its surroundings. This means heat can be considered a "product" of the reaction. For example, the combustion of fuels or the formation of ammonia from nitrogen and hydrogen are exothermic.
Chemical equilibrium is a dynamic state in a reversible reaction where the rate of the forward reaction (reactants forming products) becomes equal to the rate of the reverse reaction (products forming reactants). At equilibrium, the concentrations of reactants and products remain constant, although the reactions continue to occur.
Le Chatelier's Principle and Temperature Changes
The behavior of a system at equilibrium when conditions change is governed by Le Chatelier's Principle. This principle states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that counteracts the change, attempting to re-establish a new equilibrium.
The Effect of Increased Temperature on Exothermic Equilibria
When the temperature of an exothermic system at equilibrium is increased, it's akin to adding more "heat" as a product. According to Le Chatelier's Principle, the system will try to relieve this stress by shifting in the direction that absorbs heat.
- For an exothermic forward reaction, the reverse reaction is always endothermic (it absorbs heat).
- Therefore, an increase in temperature will cause the equilibrium to shift towards the reactants (the left side of the equation). This shift reduces the concentration of products and increases the concentration of reactants.
- As a direct consequence of this shift, where the forward reaction is exothermic, increasing the temperature decreases the value of the equilibrium constant (K). This means that at the higher temperature, the ratio of products to reactants at equilibrium is smaller.
Consequences of the Shift
The shift in equilibrium due to increased temperature in an exothermic reaction has several key outcomes:
- Product Yield: The amount of desired product decreases significantly because the reaction is pushed back towards the reactants.
- Reactant Concentration: The concentrations of the reactants increase as products convert back into reactants.
- Equilibrium Constant (K): As established, the value of K decreases. This is a fundamental change, as K is temperature-dependent.
- Reaction Rate: While an increase in temperature generally increases the rates of both the forward and reverse reactions, the reverse (endothermic) reaction's rate increases more significantly, driving the net shift towards the reactants.
Practical Implications and Examples
Understanding the effect of temperature on exothermic equilibria is vital in industrial processes to optimize product yield. For instance, in the Haber-Bosch process for ammonia synthesis, which is an exothermic reaction:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + Heat
While high temperatures would increase the reaction rate (making the process faster), they would also drastically decrease the yield of ammonia (NH₃) due to the equilibrium shifting to the left. Industrial chemists must find a compromise, using moderately high temperatures to achieve a reasonable reaction rate while employing catalysts and high pressures to maximize yield. (Learn more about the Haber Process)
Summary Table of Effects
Here's a concise summary of what happens to an exothermic equilibrium when temperature is increased:
| Factor | Effect of Increased Temperature (Exothermic) |
|---|---|
| Equilibrium Shift | Towards Reactants (Left) |
| Product Yield | Decreases |
| Reactant Concentration | Increases |
| Equilibrium Constant (K) | Decreases |
| Net Reaction Rate at Equilibrium | Faster (both rates increase, but equilibrium composition changes) |
