# 1.7.3 Equilibrium Constant

### Equilibrium Constant: Concentrations

#### Equilibrium expression & constant

• The equilibrium expression is an expression that links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
• So, for a given reaction:

aA + bB ⇌ cC + dD

the Kc is defined as follows:

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

• Solids are ignored in equilibrium expressions
• The Kc of a reaction is specific and only changes if the temperature of the reaction changes

### Mole Fraction & Partial Pressure

#### Partial pressure

• For reactions involving mixtures of gases, the equilibrium constant Kp is used as it is easier to measure the pressure than the concentration for gases
• The partial pressure of a gas is the pressure that the gas would have if it was in the container all by itself
• The total pressure is the sum of the partial pressure

Equation to calculate the total pressure in a mixture of gases

The diagram shows that each gas in the mixture has a partial pressure contributing towards the total pressure

#### Mole fraction

• The mole fraction of a gas is the ratio of moles of a particular gas to the total number of moles of gas present

Equation to calculate the mole fraction of a particular gas in a gas mixture

• To calculate the partial pressures of each gas the following relationship can be used:

Equation to calculate the partial pressure of a particular gas in a gas mixture

The sum of the mole fractions should add up to 1.00, while the sum of the partial pressures should add up to the total pressure.

### Equilibrium Constant: Partial Pressures

#### Equilibrium expressions involving partial pressures

• Equilibrium expressions in terms of partial pressures are written similarly to those involving concentrations with a few differences:

The diagram shows a comparison of writing equilibrium expressions in terms of partial pressures and concentrations