Partial Pressure (AQA A Level Chemistry)
Revision Note
Partial Pressure
The partial pressure of a gas is the pressure it exerts in a mixture of gases if it occupied the container on its own
Partial pressure is given the symbol p, so for a gas X, it is written as pX
The total pressure is the sum of the partial pressures (this is known as Daltons' Law)
The relationship between partial pressures in a mixture of gases and the total pressure
To find the partial pressure of a gas, you need two pieces of information
The total pressure in the container
The mole fraction Kp
The mathematical relationships are as follows
Partial pressure and mole fraction expressions
Worked Example
Working out mole fractionsA sample of 0.25 mole of nitrogen and 0.75 mole of hydrogen were reacted together to form ammonia. The equilibrium amount of nitrogen was 0.16 mole.
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Calculate the mole fractions of nitrogen, hydrogen and ammonia.
Answer
Write out the equation and record the initial, the change and the equilibrium amounts:
Examiner Tips and Tricks
You can check you have the mole fractions correct by adding them up and making sure they come to 1:0.195 + 0.585 + 0.220 = 1
Kp Calculations
Kp calculations are a step-by-step process in which you need to find
the mole fractions of the gases present
their partial pressures
the Kp expression
the value of Kp
The following worked example shows how this is achieved
Worked Example
Finding KpHydrogen and bromine were mixed in a flask in a 1: 1 ratio and allowed to reach equilibrium at 450 K. When equilibrium had been achieved the total pressure in the flask was 140 kPa and the mole fraction of bromine was 0.35.The equation for the reaction is
H2 (g) + Br2 (g) ⇌ 2HBr (g)
Determine the partial pressures for each gas at equilibrium and the value of Kp
Another style of Kp calculation involves being given the value of Kp and working backwards to deduce the partial pressure of one of the gases
Worked Example
Finding Partial PressureKp for the dissociation equilibrium reaction of nitrogen monoxide is 0.0021.
2NO (g) ⇌ N2 (g) + O2 (g)
If pure NO is introduced into a reaction flask at an initial pressure of 100 kPa, what is the equilibrium partial pressure of nitrogen?
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