Chemical Equilibrium What is a reversible reaction? What is LeChatlier’s Principle? Predicting Equilibrium Shifts

Reaction Completion Some may be reversible – a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously Forward: 2 SO 2(g) + O 2(g) → 2 SO 3(g) Reverse: 2 SO 2(g) + O 2(g) ← 2 SO 3(g) 2 SO 2(g) + O 2(g) → 2 SO 3(g) ←

What is Chemical Equilibrium? 2 SO 2(g) + O 2(g) ↔ 2 SO 3(g) Chemical equilibrium occurs when there is no net change in the actual amounts of reactants (being used up) and products (being formed). All [reactants] ↔ All [products]

LeChatlier’s Principle French chemist, Henri Le Chatelier studied how the equilibrium shifts as a result of changing conditions Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way to relieve the stress

LeChatlier’s Principle … When a system in equilibrium is disturbed by a change, the system will SHIFT to counteract the change System can be disturbed by stresses: – Δ Concentration – Δ Temperature – Δ Pressure (for gases)

Factors Causing Stress Concentration Pressure Temperature

Concentration Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product [Reactants] – shifts RIGHT to favor products [Products] – shifts LEFT to favor reactants   Ex. N H 2 2 NH 3 Which way would equilibrium shift if more [H 2 ] is added? shifts RIGHT

Temperature Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat Ex. Which way will equilibrium shift if the heat is added? 2 SO 2 + O 2 2 SO 3 + heat shifts LEFT

Pressure Pressure – changes in pressure will only effect gases ONLY Increasing the pressure will usually favor the direction that has fewer molecules EX. Which way will the equilibrium shift if the pressure increases? PCl 3 + Cl 2 PCl shifts RIGHT