Le Chatelier's Principle

Concentration Changes: Increasing the concentration of a reactant causes the system to consume the excess by shifting the equilibrium to the right (product side). Conversely, removing a product will also shift the equilibrium to the right to replace what was lost.

Pressure Changes: This factor only affects reactions involving gases. An increase in total pressure shifts the equilibrium toward the side with the fewer number of gas molecules to reduce the pressure, while a decrease in pressure shifts it toward the side with more gas molecules.

If a reaction has an equal number of gas moles on both sides (e.g., $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$), changing the pressure has no effect on the position of equilibrium because neither side can effectively counteract the pressure change.

Temperature Changes: The direction of the shift depends on whether the forward reaction is exothermic (releases heat) or endothermic (absorbs heat). Increasing temperature favors the endothermic direction to absorb the added thermal energy.

The Unique Role of Temperature: Temperature is the only factor that changes the numerical value of the equilibrium constant ($K_c$). While concentration and pressure shifts change the relative amounts of substances, they do so in a way that keeps the ratio defined by $K_c$ constant; temperature actually alters the ratio itself.

For an exothermic reaction ($ΔH < 0$), increasing temperature shifts the equilibrium to the left, which decreases the value of $K_c$. For an endothermic reaction ($ΔH > 0$), increasing temperature shifts it to the right, increasing $K_c$.

Always check the states: When analyzing pressure changes, ignore solids ($s$) and liquids ($l$); only count the coefficients of species marked as gas ($g$).

The 'Compromise' Argument: In industrial contexts like the Haber Process, exams often ask why 'ideal' conditions aren't used. High pressure increases yield but is expensive and dangerous; low temperature increases yield for exothermic reactions but makes the reaction too slow. Therefore, a compromise temperature and pressure are chosen.

Explain, don't just state: When asked to explain a shift, always use the phrasing 'to counteract the change' or 'to oppose the increase in [factor]'. This demonstrates a conceptual understanding of Le Chatelier's Principle rather than just memorized rules.