Chemical reactions rarely go to completion, instead reaching a state known as chemical equilibrium. This is a dynamic state where the rate at which reactants form products is exactly equal to the rate at which products revert back to reactants. Once this balance is achieved, the concentrations of all substances remain constant. Temperature is a unique factor because it can fundamentally change the ultimate balance point of a reaction. Unlike a catalyst, which only speeds up how quickly equilibrium is reached, a change in temperature shifts the ratio of reactants to products.
Understanding Heat in Chemical Processes
To understand how temperature shifts equilibrium, one must consider how a reaction handles energy. Chemical reactions are categorized based on whether they absorb or release heat energy during the process.
An exothermic reaction releases heat into its surroundings, meaning heat is considered a product of the reaction. Examples include lighting a gas stove or using a chemical hand warmer.
Conversely, an endothermic reaction absorbs heat energy from its environment. For these reactions, heat is considered a necessary reactant to drive the process forward, such as when a salt in an instant cold pack dissolves in water and absorbs heat.
Predicting the Direction of Change
The response of a chemical system to a change in temperature is governed by Le Chatelier’s Principle. This principle states that if a system at equilibrium is disturbed, it will shift in a direction that counteracts the change. When temperature is the disturbance, the system favors the reaction that either consumes or produces heat to restore the original thermal condition.
If the temperature is raised (heat added), the equilibrium shifts toward the endothermic reaction to absorb the excess energy. If the temperature is lowered, the system favors the exothermic reaction, which generates heat to offset the cooling.
The Haber process, which synthesizes ammonia (\(\text{NH}_3\)), provides an industrial example. The forward reaction creating ammonia is exothermic, meaning it releases heat. Therefore, increasing the temperature shifts the equilibrium backward, favoring the original reactants and resulting in a lower yield of ammonia.
To maximize ammonia yield, the temperature should theoretically be low to favor the exothermic forward reaction. However, lowering the temperature too much slows the reaction rate considerably. A compromise temperature, often around 450°C, is used to balance yield and speed.
Temperature also affects solubility, as seen in how sugar dissolves in hot versus cold water. If the process of a solid dissolving is endothermic, increasing the temperature shifts the equilibrium toward the products, increasing solubility. Conversely, when a gas dissolves in a liquid, the process is typically exothermic. An increase in temperature shifts this equilibrium backward, causing the gas to become less soluble, which is why carbon dioxide escapes a warm soda bottle.
How Temperature Alters the Equilibrium Constant
Temperature is the only factor that changes the numerical value of the equilibrium constant, represented as \(K\). The equilibrium constant measures the ratio of product concentrations to reactant concentrations once equilibrium is established. Changes in concentration or pressure shift the position of the equilibrium, but the ratio \(K\) remains the same.
Since a change in temperature forces the system to find a completely new balance point, the numerical value of \(K\) must also change. For an endothermic reaction, increasing the temperature drives the reaction toward the products, increasing the product concentration. This shift results in a larger numerical value for \(K\).
For an exothermic reaction, an increase in temperature favors the reactants, reducing the product concentration. This shift leads to a decrease in the numerical value of \(K\). This change reflects a fundamental alteration in the inherent balance of the system.