Chemistry Unit 1

Unit 1.7: Simple equilibria and acid-base reaction

Key Words

Reversible reaction – one that can go in either direction depending on the conditions

Dynamic equilibrium – is when the forward and reverse reactions occur at the same rate.

Position of equilibrium – is the proportion of products to reactants in an equilibrium mixture

Le Chatelier`s principle – if a system at equilibrium is subjected to a change, the equilibrium tends to shift to minimise the effect of that change.

Strong acid – fully dissociates in aqueous solution

Weak acid - is one that partially dissociates in aqueous solution

Acid – proton donor

Base – proton acceptor

Reversible reactions and dynamic equilibrium

The double arrow signifies a reversible reaction.

If in the above reaction the concentrations of A, B, C, D do not change, although the reaction is still in progress, then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.

At equilibrium, the concentrations of the reactants and products are constant, but are not necessarily equal.

Equilibrium constant

Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.

The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.

Position of equilibrium

Le Chateliers principle states: The position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.

This principle applies to any reaction that is in equilibrium.

When there are more productsthan reactants present, the position of equilibrium lies to the right.

When there are more reactantsthan products present, the position of equilibrium lies to the left.

The effect of concentration changes on equilibrium

Changing concentration of a reactant or product does not change the numerical value of the equilibrium constant, but it does change the position of the equilibrium.

In general, the position of the equilibrium is shifted towards the right if the concentration of a reactant is increased or to the left if the concentration of a product is increased.

At the start, when the change is made, the mixture is not at equilibrium, but equilibrium is eventually restored.

The effect of pressure changes on equilibrium

For a reaction involving gases, altering the pressure may cause a change in the position of the equilibrium.

For a reaction where there is an increase in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the left.

Where there is a decrease in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the right. The equilibrium constant remains the same.

The effect of temperature changes on equilibrium

The change that takes place when temperature is changed depends upon whether the forward reaction is exothermic or endothermic.

If the forward reaction is exothermic…