Reaction Conditions & The Equilibrium Constant

Temperature is the only variable that changes the value of $K_c$ or $K_p$ because it alters the energy distribution of the molecules and the thermodynamics of the reaction.

For endothermic reactions ($ΔH > 0$), increasing the temperature provides energy that favors the forward reaction, leading to a higher concentration of products and thus an increase in $K$.

For exothermic reactions ($ΔH < 0$), increasing the temperature favors the reverse reaction to absorb the excess heat, resulting in a lower concentration of products and a decrease in $K$.

Conversely, decreasing the temperature has the opposite effect: it increases $K$ for exothermic reactions and decreases $K$ for endothermic reactions.

Concentration Changes: If a reactant is added, the system shifts to the right to consume it, but the ratio of products to reactants eventually returns to the original $K$ value.

Pressure Changes: In gaseous systems, changing the total pressure shifts the position of equilibrium toward the side with fewer or more moles of gas, yet the $K_p$ value remains constant.

Catalyst Addition: A catalyst increases the rate of both the forward and reverse reactions equally, meaning the system reaches equilibrium faster but the final ratio (the constant) is unaffected.

Always check the Enthalpy Change ($ΔH$): Before predicting the effect of temperature on $K$, identify if the forward reaction is exothermic or endothermic.

The 'Only Temperature' Rule: In multiple-choice questions, if asked what changes the value of $K_c$, look for temperature immediately; ignore options involving pressure, volume, or concentration.

Units Check: Remember that while the value of $K$ changes with temperature, the units of $K$ (if any) remain determined solely by the stoichiometry of the balanced equation.

Sanity Check: If a reaction is exothermic, an increase in temperature must result in a smaller $K$. If your calculation shows otherwise, re-evaluate your logic.

The Catalyst Trap: Students often assume a catalyst increases the yield or the constant because it 'improves' the reaction; in reality, it only improves the speed, not the equilibrium state.

Pressure and $K_p$: There is a common misconception that because $K_p$ uses partial pressures, changing the total pressure must change $K_p$. However, the individual partial pressures adjust to keep the ratio constant.

Le Chatelier Confusion: While Le Chatelier's Principle explains why the position shifts, it does not explain the change in the constant itself, which is rooted in the Van't Hoff relationship.