__Higher recap__

What happens when a system at equilibrium is subjected to a change?

The system will adjust itself to counteract the applied change.

Le Chatelier's principle

The point when the rates of the forward and reverse reactions in a reversible reaction are equal is known as?

Equilibrium

What is meant by dynamic (or moving) equilibrium?

At equilibrium the forward and reverse reactions still continue, but at the same rate.

If [reactants]>[products], the position of equilibrium is said to lie to which side?

Left-hand side

If [products]>[reactants], the position equilibrium is said to lie to which side?

Right-hand side

The position of the equilibrium can be measured using K.

What is K?

K is the equilibrium constant

Consider the reaction represented by the equation

aA + bB ⇌ cC + dD

where A and B are reactants, C and D are products and a, b, c and d are the stoichiometric coefficients in a balanced reaction equation.

In terms of concentration, the equilibrium constant can be expressed as?

K = [C]^{c}[D]^{d}/[A]^{a}[B]^{b}

What are the units for the equilibrium constant?

No units

Consider the ionisation of ethanoic acid

CH_{3}COOH ⇌ H^{+} + CH_{3}COO^{-}

How would the equilibrium constant be expressed?

K = [H^{+}] [CH_{3}COO^{-}] / [CH_{3}COOH]

CH_{3}COOH ⇌ H^{+} + CH_{3}COO^{-}

If sodium ethanoate solid was added, what would happen to

1) the position of equilibrium?

2) the pH?

1) Equilibrium would shift to the left

2) pH would increase

This is because of a reaction between H^{+}(aq) and CH_{3}COO^{-}(aq) ions to make CH_{3}COOH(aq)

The dissociation of ethanoic acid has an equilibrium constant of 1.7x10^{-5}

Does equilibrium lie to the left or the right?

Calculating the equilibrium constant is done by the products divided by the reactants. A constant less than 1 must means more reactants than product, therefore equilibrium to the left.

The conversion of ozone to oxygen has an equilibrium constant of 1x10^{55}

Does equilibrium lie to the left or the right?

Calculating the equilibrium constant is done by the products divided by the reactants.

A number larger than 1 must mean more products than reactants, therefore, equilibrium to the right.

The decomposition of hydrofluoric acid has an equilibrium constant of 1x10^{-13}

Does equilibrium lie to the left or the right?

Calculating the equilibrium constant is done by the products divided by the reactants. A number smaller than 1 must mean more reactants than products, therefore, equilibrium to the left.

An equilibrium mixture of gaseous O_{2}, NO and NO_{2} at 500 K contains 1.0 x 10^{-3 }moll^{-1} O_{2}, 1.9 x 10^{-3} moll^{-1} NO and 5.0 x 10^{-2} moll^{-1} NO_{2}. Calculate the value of K.

O_{2} + 2NO ⇌ 2NO_{2}

K = [NO_{2}]^{2}/[NO]^{2}[O_{2}]

K = (5.0x10^{-2})^{2} / (1.9x10^{-3})^{2 }(1.0x10^{-3})

= 6.9x10^{5}