Chemical Equilibrium

Reversible Reactions A reaction that can occur in both the forward and reverse directions. Forward: N 2 (g) + 3H 2 (g)  2NH 3 (g) Reverse: 2NH 3 (g)  N 2 (g) + 3H 2 (g) These reactions are combined with the use of a double arrow: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) Reversible reactions

Reversible Reactions (cont’d.) Processes that can be easily reversed are phase changes that involve physical changes. Melting ice to form liquid water is easily reversed by refreezing the water Chemical changes such as burning wood, exploding firecrackers, or cooking food cannot be reversed.

Chemical Equilibrium A system is said to be in equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. A double arrow, ↔, indicates that the system is at equilibrium. At equilibrium, the concentrations of the reactants and products are constant, but not necessarily equal.

Equilibrium Constants The law of chemical equilibrium states that at a given temperature, a chemical system may reach a state in which the ratio of product concentrations to reactant concentrations has a constant value. The equilibrium constant, K c, is the mathematical way of expressing the ratio of product concentrations to reactant concentrations.

At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction: Rate forward = Rate reverse For the equation: aA + bB ↔ cC + dD The following equilibrium expression, K c, is obtained: K c = [C] c [D] d [A] a [B] b

The value of K c gives us some idea of the size of the product concentration relative to the reactant concentration. If K c is > 1: More products are present at equilibrium than reactants. If K c is < 1: More reactants are present at equilibrium than products.

Rules for Writing Equilibrium Constants The right side of the reaction is always considered the product side. The left side of the reaction is the reactant side. Aqueous and gas phases only appear in the equilibrium expression. Solids and liquids are never part of the K c. Coefficients become exponents in the K c.

Write the equilibrium expression for the following reactions: H 2 (g) + I 2 (g) ↔ 2HI(g) C 10 H 8 (s) ↔ C 10 H 8 (g) N 2 (g) + O 2 (g) ↔ 2NO(g) SO 3 (g) + H 2 O(l) ↔ H 2 SO 4 (l)

Calculate the equilibrium constant for the following: H 2 (g) + I 2 (g) ↔ 2HI(g) [H 2 ] = M [I 2 ] = M [HI] = M

At a certain temperature, K c = for the equilibrium: PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g) What is the [Cl 2 ] in an equilibrium mixture containing 0.865M PCl 5 and 0.135M PCl 3 ?

HW Assignment 3 due 5/14. Determine the value of K if [CO] = M, [H 2 ] = M, [CH 4 ] = M and [H 2 O] = M CO(g) + 3 H 2 (g)  CH 4 (g) + H 2 O(g)

ICE Charts Used to determine equilibrium concentrations when all information is not given. I = initial concentrations; products are always zero. C = change based on mole ratios; change for reactants is -, for products is + E = equilibrium concentrations, used to find K c.

H 2 (g) + I 2 (g)  2HI(g) Calculate K c

A chemist placed 4.95 moles of SO 3 in a L vessel and heated it to 527 o C. When equilibrium was established, there were moles of SO 2 in the container. What is the value of the equilibrium constant for the equation below? 2 SO 3 (g)  2 SO 2 (g) + O 2 (g)

A mixture of 2.5 moles of N 2 and 2.5 moles of H 2 are sealed in a 1.0-L container and heated to 400 o C. When equilibrium is established, the container holds 1.0 mole of NH 3. Calculate K for the equilibrium equation below. N 2 (g) + 3 H 2 (g)  2 NH 3 (g)

Calculate the value of K for the decomposition of hydrogen fluoride at 400K if 3.5 moles of HF are initially placed in a one liter vessel and allowed to reach equilibrium. The equilibrium concentration of hydrogen gas is 0.25 moles.

LeChatelier’s Principle If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. A stress may be a change in concentration, temperature or pressure. Equilibrium will always shift away from an increase and towards a decrease.

Heat + Co(H 2 O) Cl - ↔ CoCl H 2 O

2 NH 3 (g)  N 2 (g) + 3 H 2 (g) + heat Change: increase in [N 2 ] increase in temperature increase in pressure What is the effect on the concentration of: NH 3 H 2

2 NO(g)  N 2 (g) + O 2 (g) + heat Change: decrease [O 2 ] decrease in temperature increase in pressure What is the effect on the concentration of: O 2 NO

2 SO 3 (g) + heat  2 SO 2 (g) + O 2 (g) Change: increase in [SO 2 ] increase in temperature increase in volume What is the effect on the concentration of: SO 3 O 2