Le Châtelier’s Principle CHEMICAL EQUILIBRIUM Le Châtelier’s Principle

CHEMICAL EQUILIBRIUM A state when the rate of the forward reaction is equal to the rate of the reverse reaction. A + B  C + D A + B  C + D A + B ⇄ C + D

CHEMICAL EQUILIBRIUM Dynamic process. Both forward and reverse reactions continue, but there is NO NET CHANGE in the amount of reactants and products.

CHEMICAL EQUILIBRIUM M Time Products Reactants  Equilibrium

CHEMICAL EQUILIBRIUM The equilibrium point is actually a point in a reaction where a specific mixture of reactants and products has obtained a minimum energy state – the most stable state. This is a temperature dependent condition.

LE CHATELIER’S PRINCIPLE When a system at equilibrium experiences a stress, the system responds to relieve that stress by changing the ratio of reactants and products. It shifts right or left!

STRESSORS Concentration Changes of R or P Temperature Changes Pressure/Volume Changes

Concentration Changes Concentration changes only affect reactants or products in the aqueous (aq) or gaseous (g) state – pure solids (s) or liquids (l) don’t have concentrations. System will consume what is added or replace what is removed.

Example 1: Consider the following reaction: A (g) + B (g) ↔ C (s) + D (g) Adding more A will cause the reaction to shift to the right, the direction that consumes A.

Example 2: Consider the following reaction: A (g) + B (g) ↔ C (s) + D (g) Removing some B will cause the reaction to shift to the left, the direction that replaces B.

Example 3: Consider the following reaction: A (g) + B (g) ↔ C (s) + D (g) Adding or removing C has no effect on the equilibrium position, since it is in the solid state.

Example 4: Consider the following reaction: A (g) + B (g) ↔ C (s) + D (g) Which direction do you think it will shift if some D is removed? Yes, that’s right! To the Right!!

Temperature Changes Effect depends on whether the reaction is endothermic or exothermic. Increasing the temperature is like adding energy. Decreasing the temperature is like removing energy.

Temperature Changes In endothermic reactions, energy is written on the reactant side: Energy + A + B  C + D In exothermic reactions, energy is written on the product side: A + B  C + D + Energy

Example 5: Consider the following reaction: A + B ↔ C + D + Energy Is this endothermic or exothermic? Which way will it shift if the temperature is increased? Yes, that’s right! To the Left!!

Example 6: Consider the following reaction: 25 kJ + A + B ↔ C + D Is this endothermic or exothermic? Which way will it shift if the temperature is decreased? Yes, that’s right! To the Left!!

Pressure/Volume Changes Pressure/Volume changes only affect reactants or products in the gaseous (g) state. Systems that differ in the number of moles of gaseous reactants and products respond by shifting to a more or less compact state.

Pressure/Volume Changes Consider the following reaction: A (g) + B (g) ↔ C (s) + D (g) How many moles of gaseous reactants/products are represented on each side of the arrow? 2 moles on left; 1 mol on right

Example 7: A (g) + B (g) ↔ C (s) + D (g) Which way will it shift if the volume is decreased? (Pressure increased) Yes, that’s right! To the Right!!

Factors That DO NOT Affect Equilibrium Positions Adding a Catalyst Adding an Inert substance, like a noble gas.

Affect of Catalyst Speeds up the rate of establishing equilibrium, but will NOT affect the equilibrium position.

Affect of Inert Substance Does not change any reactant or product concentration, does not change temperature, does NOT affect any individual gas pressure – thus, NO EFFECT on equilibrium position.

EQUILIBRIUM EXPRESSIONS An equilibrium expression computes an equilibrium constant, Keq. This constant is the ratio of products to reactants raised to their stoichiometric coefficients once equilibrium has been established. Again, pure solids (s) and liquids (l) are OMITTED from the expression.

EQUILIBRIUM EXPRESSIONS For the following reaction: aA (g) + bB (g) ⇄ cC (s) + dD (g) [A]a [D]d [B]b Keq =

EQUILIBRIUM EXPRESSIONS Meaning of K: If K > 1 then it’s products favored. If K = 1 then it’s neither. If K < 1 then it’s reactants favored.