Overview
This lecture covers the concept of chemical equilibrium, Le Chatelier’s Principle, and the equilibrium constant (Kc) for reversible reactions in A-level Chemistry.
Reversible Reactions and Dynamic Equilibrium
- Reversible reactions can proceed in both forward and backward directions, represented by a double arrow.
- In each reversible reaction, one direction is endothermic (absorbs heat), the other is exothermic (releases heat).
- Dynamic equilibrium occurs when the rates of the forward and backward reactions are equal and constant.
- At equilibrium, concentrations of reactants and products remain constant.
- Dynamic equilibrium can only be established in a closed system, where no substances enter or leave.
Le Chatelier’s Principle
- Le Chatelier’s Principle predicts how equilibrium shifts when conditions change: a system will adjust to counteract the change.
- This principle helps maximize product yield in industrial processes.
Effect of Temperature
- Increasing temperature favors the endothermic direction (+ΔH), increasing yield of endothermic products.
- Decreasing temperature favors the exothermic direction (−ΔH), increasing yield of exothermic products.
Effect of Pressure
- Increasing pressure favors the side with fewer gas moles, increasing product yield on that side.
- Decreasing pressure favors the side with more gas moles, increasing product yield there.
Effect of Concentration
- Increasing reactant concentration favors product formation and increases product yield.
- Decreasing product concentration favors the reverse reaction, reforming reactants.
Effect of Catalysts
- Catalysts do not change the equilibrium position; they speed up both forward and backward reactions equally.
- Catalysts allow equilibrium to be reached faster but do not affect yield.
Equilibrium Constant (Kc)
- Kc indicates the position of equilibrium for a reversible reaction at a certain temperature.
- Kc is calculated using concentrations of products over reactants, with each raised to the power of its mole number.
- The units of Kc vary and are determined by substituting units into the Kc expression and simplifying.
- Changing temperature affects the value of Kc, but changes in concentration or use of a catalyst do not.
Key Terms & Definitions
- Dynamic Equilibrium — State where forward and backward reaction rates are equal, and concentrations remain constant.
- Le Chatelier’s Principle — When a system at equilibrium is disturbed, it adjusts to minimize that disturbance.
- Kc (Equilibrium Constant) — Value representing the ratio of product to reactant concentrations at equilibrium, specific to a reaction and temperature.
Action Items / Next Steps
- Practice applying Le Chatelier’s Principle to different reactions.
- Calculate Kc values and their units for given equilibrium problems.
- Review how changes in temperature, pressure, and concentration affect equilibrium.