Le Chatelier's Principle and Keq

Video Notes

Objectives

Define reversible reactionDefine chemical equilibriumCalulate Keq

State Le Chatelier’s PrincipleUse Le Chatelier’s Principle to predict shifts in equilibrium

Reversible reactions

A reaction in which the products can react to form the reactants is called a reversible reaction.This is shown with arrows that point in both directions Examples

Sunglasses that darken in sunlightWeather indicators – blue when weather is dry and turn to pink when it is supposed to rain

Chemical Equilibrium

Chemical equilibrium occurs when the reactants in a reversible reaction form the products at the same RATE that the products form the reactants.The concentration of the reactants and products does not changeThe concentration of the reactants can be equal to, greater than or less than the products.The point at which the forward reaction is equal to the reverse reaction

Calculating Keq

The Equilibrium Constant is calculated through this formula

aA + bB cC + dDA,B,C,D represent the substancesa,b,c,d represent the coefficient that show the mole ratio of the reactants to products

Calculating Keq

Keq = [C]c [D]d

[A]a [B]b

The product of the products raised to the power of their coefficients divided by the product of the reactants raised to the power of their coefficients

Calculating Keq

Known Keq values can be used to determine which direction is favored in a reactionKeq = 1 forward and reverse equally favoredKeq < 1 reverse reaction is favoredKeq > 1 forward reaction is favored

Calculating Keq

N2O4 2 NO2

[N2O4] = 0.23M [ NO2] = 0.037M

Keq = [NO2]2 = [0.037]2 = 6.0 x 10-3

[N2O4] [0.23]

Since Keq < 1, the reverse reaction is favored

Calculating Keq

N2(g) + 3H2(g) 2 NH3(g)

[N2] = 0.602 M [ H2] = 0.420 M

[NH3] = 0.113 M

Keq = [N2] [H2]3 = [0.602]2 [0.420]3 = 2.10

[NH3]2 [0.113]2

Since Keq > 1, the forward reaction is favored

Le Chatelier’s PrincipleWhen a stress is applied to a system in a state of equilibrium, the system reacts in a way to reduce the stress.In order to reduce the stress added to a system, the system will shift away from the added chemical.When a chemical is removed, the system will shift toward the removed chemical.When the temperature is increased, the system will shift away from the source of energy.When the temperature decreases, the system shifts toward the source of energy.

Le Chatelier’s Principle

If the system is a gas, an increase of pressure will shift the system toward the side with the smallest number of gas particles.If the pressure is decreased, the system will shift toward the side with the greater number of gas particles.Addition of a catalyst, will result in no shift in equilibrium, while the rate of reaction may increase.

Le Chatelier’s Principle

When the equilibrium shifts to the

Products Reactants

RIGHT Increase Decrease

LEFT Decrease Increase