Chemical Equilibrium A Balancing Act

Equilibrium two opposing processes occurring at the same rate a system at equilibrium is in balance

During a game, players enter and leave. Always the same number of players on field.

H2O(l) H2O(g)

Photochromic sunglasses AgCl + light Ag + Cl (transparent) (dark)

Reversible reaction a chemical reaction that can occur in both the forward and the reverse directions N2(g) + 3H2(g) 2NH3(g)

Chemical equilibrium a state in which the forward and reverse reactions balance each other because they take place at equal rates dynamic state; no net change

Law of chemical equilibrium At a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

Equilibrium constant expression ratio of molar concentrations of products to reactants; each raised to a power equal to coefficient in balanced equation

aA + bB cC + dD

Equilibrium constant (Keq) numerical value of the ratio of product to reactant concentrations constant only at a specific temperature

Value of equilibrium constant (Keq) shows the extent to which reactants are converted into products.

Keq > 1: Products are favored at equilibrium Keq < 1: Reactants are favored at equilibrium

Homogeneous equilibrium all reactants and products in same phase

Write an equilibrium constant expression for: N2(g) + 3H2(g) 2NH3(g) [NH3]2 Keq = ___________ [N2] [H2]3

Heterogeneous equilibrium reactants and products present in more than one physical state H2O(l) H2O(g)

Heterogeneous equilibrium Since concentrations of pure liquids and solids remain constant, these substances are omitted from the equilibrium constant expression.

Write an equilibrium constant expression for: H2O(l) H2O(g) Keq = [H2O(g)]

Equilibrium constant expression Keq = . . . Products over reactants raised to power of coefficient; leave out pure solids and liquids.

Equilibrium position Equilibrium concentrations can vary from trial to trial.

H2(g) +I2(g) 2HI(g)

Each set of equilibrium concentrations represent an equilibrium position. A system has only one value for Keq at a specific temperature, however there are unlimited number of equilibrium positions.

A system at equilibrium must: take place in closed system temperature remain constant all reactants and products are present (both reactions can occur)

Write the equilibrium constant expression for N2(g) + O2(g) 2NO(g)

Calculate the value of Keq if [N2] = 0.20 mol/L, [O2] = 0.15 mol/L, and [NO] = 0.0035 mol/L. Keq = 4.1 x 10-4 What does the value of Keq tell you about the equilibrium?

What happens when a system is at equilibrium and you upset the balance?

Le Chatelier’s Principle If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

Le Chatelier’s Principle Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.

Changes in Concentration Add reactant 1. Forward rxn speeds up. 2. Over time, forward rxn slows down and reverse rxn speeds up.

3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. Equilibrium has shifted right, value of Keq unchanged.

Changes in Concentration Remove reactant 1. Reverse rxn speeds up. 2. Over time, reverse rxn slows down and forward rxn speeds up.

3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. Equilibrium has shifted left; value of Keq unchanged.

Changes in Pressure (Volume) For a gas, decreasing volume of container increases pressure; particles have less space, collide more frequently.

2. System will respond by trying to relieve stress (decrease pressure.) 3. Shifts to the side with fewer moles of gas.

4. Forward rxn speeds up. 5. Over time, forward rxn slows down and reverse rxn speeds up.

6. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. 7. Equilibrium has shifted right, value of Keq unchanged.

Changes in Pressure (Volume) If volume is decreased concentrations of all gaseous substances increases. Note: Side with fewer moles of gas increases more.

Changes in Temperature Review: If DH is negative rxn is exothermic heat written as product If DH is positive rxn is endothermic heat written as reactant

Changes in Temperature N2O4(g) 2NO2(g) DH = 55.3 kJ 55.3 kJ + N2O4(g) 2NO2(g) light dark

55.3 kJ + N2O4(g) 2NO2(g)

Le Chatelier’s Principle Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature. Chemists can change conditions to make rxn more productive.