Gas phase equilibria and mole fractions
Gas phase equilibria are characterised by the equilibrium constant Kp where amounts of substances are expressed in terms of their partial pressures in the reaction mixture. Kp is constant at a particular temperature.
Amounts in moles and partial pressure are related to one another because they are both a measure of the number of particles of gas present.
pA = xA × ptotal
The two relationships given show that the partial pressure due to substance A in a mixture of gases can be calculated from a quantity known as the mole fraction xA and the total pressure due to the mixture of gases.
Useful checks in calculations:
For any mixture the sum of the mole fractions is 1
For any mixture of gases the sum of the partial pressures is the total pressure (Dalton's Law).
Thus Kp for a system at equilibrium can be calculated if the amounts in moles of the gases present are known as well as the total pressure due to the gases.
The form of the expression that relates the equilibrium constant Kp to the balanced equation for the reaction is shown for the general reaction below.
aA(g) + bB(g) 
cC(g) + dD(g)
A, B, C and D are gaseous substances
a, b, c and d are the stoichiometric coefficients in the balanced equation.
The unit of Kp is kPan where n can be deduced from cancelling powers of pressure.
cC(g) + dD(g) A, B, C and D are gaseous substances
a, b, c and d are the stoichiometric coefficients in the balanced equation.
| Kp = | p(C)cp(D)d |
| p(A)ap(B)b | |
| reaction quotient |
Amounts in moles and partial pressure are related to one another because they are both a measure of the number of particles of gas present.
| xA = | n(A) |
| ntotal |
pA = xA × ptotal
The two relationships given show that the partial pressure due to substance A in a mixture of gases can be calculated from a quantity known as the mole fraction xA and the total pressure due to the mixture of gases.
Useful checks in calculations:
For any mixture the sum of the mole fractions is 1
For any mixture of gases the sum of the partial pressures is the total pressure (Dalton's Law).
Thus Kp for a system at equilibrium can be calculated if the amounts in moles of the gases present are known as well as the total pressure due to the gases.