What Happens To Equilibrium When Temperature Is Increased

When we talk about equilibrium, we're often referring to a state of balance in a reversible chemical reaction. Temperature, as a key factor in chemical reactions, plays a significant role in shifting this equilibrium. Understanding how temperature affects equilibrium is crucial in various fields, from industrial chemistry to environmental science.

Understanding Chemical Equilibrium

Chemical equilibrium is a dynamic state where the rate of the forward reaction equals the rate of the reverse reaction. In simpler terms, it's when the formation of products and the reformation of reactants occur at the same pace. This doesn't mean the reaction has stopped; it means that the concentrations of reactants and products remain constant over time.

Key Concepts:

• Reversible Reactions: Reactions that can proceed in both forward and reverse directions.
• Equilibrium Constant (K): A value that represents the ratio of products to reactants at equilibrium. It indicates the extent to which a reaction will proceed.
• Le Chatelier's Principle: A principle stating that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

The Impact of Temperature on Equilibrium

Temperature affects the equilibrium position by altering the rates of the forward and reverse reactions differently. This change is governed by Le Chatelier's Principle. When the temperature of a system at equilibrium is changed, the system will adjust to counteract the change and establish a new equilibrium.

Exothermic and Endothermic Reactions

The way temperature affects equilibrium depends on whether the reaction is exothermic or endothermic.

• Exothermic Reactions: Reactions that release heat into the surroundings. In these reactions, heat can be considered a product.

  • Example: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) + Heat

• Endothermic Reactions: Reactions that absorb heat from the surroundings. In these reactions, heat can be considered a reactant.

  • Example: N₂O₄ (g) + Heat ⇌ 2NO₂ (g)

Le Chatelier's Principle and Temperature Changes

According to Le Chatelier's Principle, if you increase the temperature of a system at equilibrium:

• The equilibrium will shift in the direction that absorbs heat (endothermic direction) to counteract the increase in temperature.
• Conversely, if you decrease the temperature, the equilibrium will shift in the direction that releases heat (exothermic direction) to counteract the decrease in temperature.

How Increased Temperature Affects Equilibrium

Exothermic Reactions

When the temperature is increased in an exothermic reaction:

• The equilibrium shifts towards the reactants. The system tries to reduce the heat by favoring the reverse reaction, which absorbs heat.
• The equilibrium constant (K) decreases. As the reverse reaction is favored, the concentration of reactants increases, and the concentration of products decreases, leading to a smaller K value.
• The yield of products decreases. Higher temperatures will result in less product formation as the reaction shifts backward.

Example: Consider the Haber-Bosch process, an exothermic reaction used for synthesizing ammonia:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) + Heat

Increasing the temperature will shift the equilibrium to the left, favoring the decomposition of ammonia back into nitrogen and hydrogen.

Endothermic Reactions

When the temperature is increased in an endothermic reaction:

• The equilibrium shifts towards the products. The system tries to absorb the additional heat by favoring the forward reaction, which requires heat.
• The equilibrium constant (K) increases. As the forward reaction is favored, the concentration of products increases, and the concentration of reactants decreases, leading to a larger K value.
• The yield of products increases. Higher temperatures will result in more product formation as the reaction shifts forward.

Example: Consider the reaction where nitrogen dioxide (N₂O₄) decomposes into nitrogen dioxide (NO₂):

N₂O₄ (g) + Heat ⇌ 2NO₂ (g)

Increasing the temperature will shift the equilibrium to the right, favoring the formation of NO₂.

The Van't Hoff Equation

The Van't Hoff equation provides a quantitative relationship between the change in temperature and the change in the equilibrium constant. It's particularly useful for calculating the change in K with temperature if the enthalpy change (ΔH) of the reaction is known.

Equation

The Van't Hoff equation is expressed as:

ln(K₂/K₁) = -ΔH/R (1/T₂ - 1/T₁)

Where:

• K₁ is the equilibrium constant at temperature T₁
• K₂ is the equilibrium constant at temperature T₂
• ΔH is the standard enthalpy change of the reaction
• R is the ideal gas constant (8.314 J/mol·K)
• T₁ and T₂ are the absolute temperatures in Kelvin

Applications

Using the Van't Hoff equation, we can predict how the equilibrium constant changes with temperature:

• For exothermic reactions (ΔH < 0), increasing the temperature will decrease K, confirming that the equilibrium shifts towards the reactants.
• For endothermic reactions (ΔH > 0), increasing the temperature will increase K, confirming that the equilibrium shifts towards the products.

Practical Examples and Applications

Haber-Bosch Process

As mentioned earlier, the Haber-Bosch process (N₂ + 3H₂ ⇌ 2NH₃) is exothermic. Industrially, a moderate temperature (around 400-450°C) is used. Although lower temperatures favor ammonia formation, the reaction rate is too slow. Higher temperatures increase the reaction rate but reduce the equilibrium yield of ammonia. Therefore, a compromise temperature is chosen to balance yield and rate.

Water-Gas Shift Reaction

The water-gas shift reaction (CO + H₂O ⇌ CO₂ + H₂) is another industrial process. Depending on the specific catalysts used, the reaction can be either exothermic or endothermic. Adjusting the temperature allows for optimized production of hydrogen or carbon dioxide, depending on the desired application.

Environmental Chemistry

In environmental science, temperature affects the solubility of gases in water. For example, the dissolution of oxygen in water is exothermic. As water temperature increases, the solubility of oxygen decreases, which can have significant impacts on aquatic life.

Biochemical Reactions

Many biochemical reactions in living organisms are temperature-sensitive. Enzymes have optimal temperature ranges within which they function most efficiently. Changes in body temperature (fever or hypothermia) can significantly alter the rates of biochemical reactions, affecting overall health.

Factors Affecting Equilibrium Besides Temperature

While temperature is a critical factor, other conditions also influence the equilibrium position.

Pressure

Changes in pressure primarily affect reactions involving gases. According to Le Chatelier's Principle:

• Increasing the pressure will shift the equilibrium towards the side with fewer moles of gas.
• Decreasing the pressure will shift the equilibrium towards the side with more moles of gas.

Example: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

Increasing the pressure will favor the formation of ammonia because the product side has fewer moles of gas (2 moles) compared to the reactant side (4 moles).

Concentration

Changes in the concentration of reactants or products can also shift the equilibrium:

• Increasing the concentration of reactants will shift the equilibrium towards the products.
• Increasing the concentration of products will shift the equilibrium towards the reactants.

Example: Fe³⁺ (aq) + SCN⁻ (aq) ⇌ [FeSCN]²⁺ (aq)

Adding more Fe³⁺ ions will shift the equilibrium to the right, increasing the concentration of [FeSCN]²⁺.

Catalysts

Catalysts increase the rate of both the forward and reverse reactions equally. They do not change the equilibrium position but help the system reach equilibrium faster.

Common Misconceptions

• Equilibrium Means Equal Concentrations: It's a common misconception that at equilibrium, the concentrations of reactants and products are equal. Equilibrium means the rates of the forward and reverse reactions are equal, leading to constant concentrations, but these concentrations do not necessarily have to be the same.
• Temperature Always Favors the Forward Reaction: The effect of temperature depends on whether the reaction is exothermic or endothermic. Increasing the temperature favors the forward reaction only for endothermic reactions.
• Catalysts Shift the Equilibrium: Catalysts only speed up the rate at which equilibrium is reached. They do not alter the equilibrium position.

Conclusion

The impact of temperature on equilibrium is governed by Le Chatelier's Principle and quantified by the Van't Hoff equation. Understanding how temperature affects equilibrium is crucial in various applications, from optimizing industrial processes to predicting environmental changes. By considering whether a reaction is exothermic or endothermic, we can predict how changes in temperature will shift the equilibrium position and affect the yield of products. Additionally, it's important to remember that while temperature is a key factor, other variables like pressure and concentration also play a significant role in determining the equilibrium state.