EQUILIBRIUM

What Is Equilibrium? Consider mixing the reactants nitrogen gas (N2) and hydrogen gas (H2). When reactants are put together the forward reaction starts. Since there are no products there is no reverse reaction.

What Is Equilibrium? As the forward reaction proceeds the reactants are used up so the forward reaction slows. The products build up, and the reverse reaction speeds up. 3

Equilibrium Eventually you reach a point where the reverse reaction is going as fast as the forward reaction. This is dynamic equilibrium.

Equilibrium The double arrow implies the process is dynamic. 5

Equilibrium The rate of the forward reaction is equal to the rate of the reverse reaction. The concentration of products and reactants stays the same, but the reaction is still running. 6

Equilibrium Catalysts speed up both the forward and reverse reactions.

Equilibrium where K is the equilibrium constant 8

Equilibrium Note that the equilibrium constant expression has products over reactants. Coefficients become exponents. Square brackets [ ] mean concentration/molarity (moles/liter).

Equilibrium Pure liquids and pure solids are omitted from the equation! Pure liquids and solids have a fixed concentration which will always be constant, so there is no need to include that in the equilibrium constant expressions. 10

2C (aq) + D (aq) ↔ 3A (aq) + 2B (s) Equilibrium For example the equilibrium constant expression is 2C (aq) + D (aq) ↔ 3A (aq) + 2B (s) 11

Writing Equilibrium Expressions Write the equilibrium expression for the following reaction. a) 3H2(g) + N2(g) 2NH3(g) [NH3]2 K = [H2]3 [N2]

Writing Equilibrium Expressions Write the equilibrium expression for the following reaction. b) NiO (s) + CO (g) ↔ Ni (s) + CO2 (g) [CO2] K = [CO] 13

Writing Equilibrium Expressions Write the equilibrium expression for: c) 2NO (g) ↔ N2 (g) + O2 (g) K = [N2] [O2] [NO]2

Writing Equilibrium Expressions Write the equilibrium expression for: d) 2O3 (g) ↔ 3O2 (g) K = [ O ]3 [ O ]2 3 2

Writing Equilibrium Expressions Write the equilibrium expression for: e) NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq) K = [NH4+] [OH-] [NH3]

Writing Equilibrium Expressions Write the equilibrium expression for: f) 2H2 (g) + O2 (g) ↔ 2H2O (l) K = 1 [H2]2 [O2]

Le Châtelier’s Principle Regaining Equilibrium

Le Châtelier’s Principle If something is changed in a system at equilibrium, the system will respond to relieve the stress until a new equilibrium is established. Three types of stress can be applied.

Changing Concentration Adding a reactant or product shifts the equilibrium away from the increase. Removing a reactant or product shifts the equilibrium towards the decrease.

Changing Concentration 1) Consider the reaction X + Y ↔ A + B If the concentration of X is increased, the reaction will shift to the _____. right

Changing Concentration 2) Consider the reaction X + Y ↔ A + B If the concentration of Y is decreased, the reaction will shift to the _____. left 22

Changing Concentration 3) Consider the reaction X + Y ↔ A + B If the concentration of A is increased, the reaction will shift to the _____. left 23

Changing Concentration 4) Consider the reaction X + Y ↔ A + B If the concentration of B is decreased, the reaction will shift to the _____. right 24

Changing Temperature Reactions either require or release heat. For an endothermic reaction, H > 0 and heat can be considered as a reactant. For an exothermic reaction, H < 0 and heat can be considered as a product. 25

Changing Temperature Adding heat (i.e. increasing the temperature) favors away from the increase. Removing heat (i.e. decreasing the temperature) favors toward the increase.

Changing Temperature 5) Consider the reaction 2 SO2 + O2 ↔ 2 SO3 + heat If the temperature is decreased, the reaction will shift to the _____. right 27

Changing Temperature 6) Consider the reaction N2 + O2 + heat ↔ 2 NO If the temperature is decreased, the reaction will shift to the _____. left 28

Changing Temperature 7) Consider the reaction 2 SO2 + O2 ↔ 2 SO3 ΔH is negative If the temperature is increased, the reaction will shift to the _____. left 29

Changing Temperature 8) Consider the reaction N2 + O2 ↔ 2 NO ΔH is positive If the temperature is increased, the reaction will shift to the _____. right 30

Changes in Pressure An increase in pressure favors the direction that has fewer moles of gas. A decrease in pressure favors the direction that has the greatest moles of gas. In a reaction with the same number of product and reactant moles of gas, pressure has no effect. 31

Changing Pressure 9) Consider the reaction If the pressure is increased, the reaction will shift to the _____. left 32

Changing Pressure 10) Consider the reaction 2H2(g) + O2(g) ↔ 2 H2O(g) If the pressure is decreased, the reaction will shift to the _____. left 33

Changing Pressure 11) Consider the reaction CO2 (g) + H2 (g) + heat ↔ CO (g) + H2O (g) If the pressure is decreased, the reaction will shift to the _____. No shift 34

Adding a Catalyst If a catalyst is added, the reaction will NOT shift. Equilibrium will only be reached sooner. 35

Altering the Yield The yield of a product will be increased if the reaction shifts to the product side (the forward reaction). 36

Altering the Yield 12) N2 (g) + 3H2 (g) ↔ 2NH3 (g) ΔH = – 92.4 kJ How can the yield of NH3 be increased by manipulating the following? a) (add or remove) more N2 b) (increase or decrease) the pressure c) (increase or decrease) the temperature 37

Calculating Equilibrium If K > 1, products are favored. In other words, forward reaction is favored. If K < 1, reactants are favored. The reverse reaction is favored.

Calculating Equilibrium K, the equilibrium constant, it is only affected by temperature. K increases when a temperature change favors the products (the forward reaction). K decreases when a temperature change favors the reactants (the reverse reaction). 39

Calculating Equilibrium 13) H2(g) + I2(g) ↔2 HI ΔH = −10.4 kJ If temperature is increased, the reaction will shift _____. left Shifting this direction will cause the value of K to ______. decrease 40

Le Chatelier and Entropy Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. A transformation (shift) will not proceed in any direction that reduces the amount of total entropy. 41

Entropy Entropy of a solid Entropy of a liquid Entropy of a gas A solid has an orderly arrangement. A liquid has the molecules next to each other. A gas has molecules moving all over the place. 42

Problem 14a) Which side of the reaction has greater entropy? (left)

Problem b) Which side of the reaction has greater entropy? (right)

Problem 16a) Predict the change in entropy (increase or decrease). (increase)

Problem N2 (g) + 3 H2 (g) ↔ 2NH3 (g) (decrease) 16b) Predict the change in entropy (increase or decrease). N2 (g) + 3 H2 (g) ↔ 2NH3 (g) (decrease)