Chemical EquilibriumPresentation Part 1

c!e-γ˚T 2010

Equilibrium – “balance of force”

What is chemical equilibrium?

Kinetics deals with the rate of the reaction while Equilibrium deals with the extent in which the reaction proceeds.

A chemical reaction is capable of attaining equilibrium when it is a reversible process.

When is equilibrium reached?

Rateforward = Ratereverse

amount of R and P are constant

A reaction in equilibrium state

No observable change is seen after a certain time (dynamic process)

The ratio of the amount of product raised to its stoichiometric coefficient to the amount of reactants also raised to its stoichiometric coefficient remains constant at constant temperature.

[CH3OH]

[CO][H2]2

14.2 M-2

14.2 M-2

14.2 M-2

[CH3OH]

[CO][H2]

[CH3OH]

[CO](2[H2])

0.596 M-1

1.09 M-1

1.28 M-1

1.19 M-1

2.17 M-1

2.55 M-1

CO(g) + 2 H2(g) CH3OH(g)k1

k-1

Reaction Quotient (Q) The ratio of the amount of product

raised to its stoichiometric coefficient to the amount of reactants also raised to its stoichiometric coefficient at any point in the reaction is known as Reaction Quotient (Q)- The amount can be in terms of molar

concentration or partial pressure

Equilibrium constant (KEQ) When the amount present at equilibrium

is the value used in Q, it becomes KEQ.

Law of Mass Action- when a system reaches equilibrium at a given temperature, the ratio of the quantities that make up the reaction quotient has a constant numerical value, KEQ

KEQ from Rate Law The following equations show how KEQ expression

can be derived from the Rateforward = Ratereverse.   Forward reaction: N2O4 (g) 2 NO2 (g)

• Rate law: Rate = kf [N2O4] Reverse reaction: 2 NO2 (g) N2O4 (g)

• Rate law: Rate = kr [NO2]2   Therefore, at equilibrium, Ratef = Rater

kf [N2O4] = kr [NO2]2

Rewriting, it becomes

Rules in writing KEQ and Q expression

amounts is be expressed in terms of molar concentration (for both aqueous and gas) or partial pressure (gas).

products are always in the numerator, and the reactants are always in the denominator

Coefficients in the chemical equations are the exponents used in the expression• The stoichiometric coefficient of each molecule or compound is

used in place of the experimentally determined exponents because the reaction is assumed to occur at a fast reversible single step.

solids and liquids are not included in the expression, why?

unit less

Practice writing KC and KP

expression for the following reactions:

1. formation of ozone from oxygen gas2. formation of ammonia from nitrogen and

hydrogen gas3. decomposition of phosphorus pentachloride

into phosphorus trichloride and chlorine gas4. formation of sulphur trioxide from oxygen and

sulphur dioxide5. reaction of the aqueous sodium carbonate and

calcium chloride to form solid calcium carbonate and sodium chloride solution

6. decomposition of solid calcium carbonate into lime and carbon dioxide