Le Chatelier's Principle

Pressure changes primarily affect systems involving gaseous components. An increase in total pressure (or a decrease in volume) shifts the equilibrium toward the side with fewer moles of gas to reduce the pressure.

Conversely, a decrease in pressure (or increase in volume) shifts the equilibrium toward the side with more moles of gas to increase the pressure back toward the original state.

If the number of moles of gas is identical on both sides of the chemical equation, changes in pressure or volume will have no effect on the position of equilibrium, although they may change the reaction rate.

To predict temperature effects, the reaction must be identified as either exothermic (releases heat, $\Delta H < 0$) or endothermic (absorbs heat, $\Delta H > 0$).

Increasing temperature is equivalent to adding heat energy. The system shifts in the endothermic direction to absorb the added heat and lower the temperature.

Decreasing temperature is equivalent to removing heat. The system shifts in the exothermic direction to release heat and counteract the cooling effect.

Unlike concentration or pressure changes, temperature changes actually alter the value of the equilibrium constant ($K$).

Step 1: Identify the Stress: Clearly state what has been changed (e.g., 'Temperature increased').

Step 2: Apply the Principle: State that the system will move to oppose this change (e.g., 'The system will shift to decrease the temperature').

Step 3: Determine the Direction: Use the specific rules (e.g., 'Since the forward reaction is exothermic, the equilibrium shifts left in the endothermic direction').

Step 4: State the Result: Describe the final outcome (e.g., 'The concentration of reactants increases, and the yield of product decreases').

Always check the states of matter: Only $(g)$ and $(aq)$ species are typically included in concentration/pressure considerations; solids $(s)$ and pure liquids $(l)$ do not affect the equilibrium position when added or removed.

Misconception: Catalysts increase yield: Students often think catalysts produce more product. In reality, they only reduce the time taken to reach the same equilibrium position.

Ignoring Moles of Gas: When analyzing pressure, students sometimes count all species. You must only count the coefficients of substances in the gaseous state.

Confusing Heat and Temperature: Remember that 'heat' is treated like a reactant in endothermic reactions ($Heat + A \rightleftharpoons B$) and a product in exothermic reactions ($A \rightleftharpoons B + Heat$). This visualization helps in predicting shifts correctly.