Chemical Equilibrium Summary Table

The qualitative aspect of Chemical Equilibrium can be summarized into the table below.

1. Temperature: The Key Factor in Chemical Equilibrium

Among the conditions that can affect Kc​, temperature is unique in its impact. The value of Kc​ depends on the enthalpy change (ΔH) of the reaction, meaning that whether the reaction is exothermic (releases heat) or endothermic (absorbs heat) will determine how Kc​ responds to temperature changes.

• For an Exothermic Reaction (ΔH<0): Increasing the temperature causes Kc​ to decrease. This happens because, according to Le Chatelier’s Principle, adding heat shifts the equilibrium position toward the endothermic reaction to counteract the effect of increase in temperature, thereby decreasing the concentration of products relative to reactants.
• For an Endothermic Reaction (ΔH>0): When we increase the temperature, Kc​ increases. The system shifts towards the endothermic reaction to absorb the additional heat, resulting in a higher concentration of products relative to the reactants at equilibrium.

If the enthalpy change for the reaction is zero, temperature will have no effect on the chemical equilibrium. Similarly, if the reaction has the same number of moles of gaseous particles on both sides, pressure will have no effect on it.

2. Pressure and Volume Changes: Limited to Gaseous Reactions

For gaseous reactions, changing the pressure or volume can affect the position of equilibrium, but it does not alter Kc​. Increasing pressure will cause the system to shift towards the side with fewer moles of gas. While decreasing pressure will cause the position of equilibrium to shift towards the side with more moles if pressure decreases. However, the equilibrium constant Kc​ is only affected by temperature, so it remains unchanged under pressure or volume modifications.

3. Concentration Changes: No Direct Effect on Kc

Changing the concentrations of reactants or products can shift the equilibrium position, it does not alter the value of Kc. Instead, the system adjusts to re-establish chemical equilibrium at the same Kc value. For instance, if we add more reactant, the equilibrium will shift towards the products, but Kc​ remains constant.

4. Catalysts: Increase reaction speed without changing Kc

Catalysts increase the rate at which equilibrium is reached without changing the equilibrium position or the value of Kc​. They provide an alternative pathway with a lower activation energy, helping both the forward and reverse reactions proceed more quickly. However, since catalysts do not favor either side of the reaction, they do not alter Kc​; they simply allow the system to reach equilibrium faster.

An interesting point to note is catalysts have no effect on position of chemical equilibrium and equilibrium constant. The only effect it has is to increase the rate of both forward and backward reactions to the same extent.

Key Takeaways for Chemical Equilibrium

To sum up, temperature is the only factor that directly affects Kc. Changes in concentration, pressure, and the presence of catalysts only influence how quickly or in what direction equilibrium shifts, not the value of Kc​ itself. By understanding these principles, you’ll have a clearer grasp of chemical equilibrium and how systems respond to various changes.

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