5.3.2 Partial Pressure

What is Partial Pressure?

The partial pressure of a gas in a mixture is the pressure that each individual gas contributes to the total pressure of the gas mixture. It depends on the mole fraction of that gas in the mixture and the total pressure.

Deriving Partial Pressure from Mole Fraction and Total Pressure

Mole Fraction ( $x$ ):

The mole fraction of a gas $A$ in a mixture is the ratio of the moles of $A$ to the total moles of all gases in the mixture:

x_A = \frac{\text{moles of } A}{\text{total moles of all gases}}

Partial Pressure ( $p$ ):

The partial pressure of gas $A$ , denoted $p(A)$ , is calculated by multiplying its mole fraction by the total pressure $P$ of the gas mixture:

p(A) = x_A \times P

This relationship can be applied to each component gas in the mixture. The sum of the partial pressures of all gases in the mixture equals the total pressure.

infoNote

Example: Calculating Partial Pressures Suppose a mixture contains gases $A$ , $B$ , and $C$ with mole fractions $x_A = 0.4$ , $x_B = 0.3$ , and $x_C = 0.3$ , and the total pressure $P$ is 100 kPa.

To find the partial pressures:

p(A): 0.4 \times 100 \, \text{kPa} = :highlight[40 \, \text{kPa}]

p(B): 0.3 \times 100 \, \text{kPa} = :highlight[30 \, \text{kPa}]

p(C): 0.3 \times 100 \, \text{kPa} = :highlight[30 \, \text{kPa}]

Calculating Partial Pressures at Equilibrium Using I.C.E Tables

In equilibrium problems, an I.C.E (Initial, Change, Equilibrium) table is often useful to organize the moles of each species throughout the reaction stages.

infoNote

Example Calculation with I.C.E Table For the reaction:

\text{H}_2(g) + \text{Br}_2(g) \leftrightarrow 2\text{HBr(g)}

Suppose 1.00 mol of $\text{H}_2$ and 1.00 mol of $\text{Br}_2$ are initially present, and at equilibrium, there is 0.824 mol of $\text{HBr}$

Species	Initial Moles	Change in Moles	Equilibrium Moles
$\text{H}_2$	1.00	-0.412	1.00 • 0.412 = 0.588
$\text{Br}_2$	1.00	-0.412	1.00 • 0.412 = 0.588
$\text{HBr}$	0.00	+0.824	0.00 + 0.824 = 0.824

Explanation of Each Column

Initial Moles: Moles of each species before the reaction starts.
Change in Moles: The change in moles due to the reaction proceeding to equilibrium. Since 0.824 mol of $\text{HBr}$ is formed, half of this amount (0.412 mol) of $\text{H}_2$ and $\text{Br}_2$ reacts due to their 1:1 stoichiometry with $\text{HBr}$
Equilibrium Moles: Calculated by adding the change to the initial amounts.

Partial Pressure Simplified Revision Notes for A-Level AQA Chemistry

5.3.2 Partial Pressure

What is Partial Pressure?

Deriving Partial Pressure from Mole Fraction and Total Pressure

Mole Fraction ( $x$ ):

Partial Pressure ( $p$ ):

Calculating Partial Pressures at Equilibrium Using I.C.E Tables

500K+ Students Use These Powerful Tools to Master Partial Pressure For their A-Level Exams.

Other Revision Notes related to Partial Pressure you should explore

Partial Pressure Simplified Revision Notes for A-Level AQA Chemistry

5.3.2 Partial Pressure

What is Partial Pressure?

Deriving Partial Pressure from Mole Fraction and Total Pressure

Mole Fraction (xxx):

Partial Pressure (ppp):

Calculating Partial Pressures at Equilibrium Using I.C.E Tables

500K+ Students Use These Powerful Tools to Master Partial Pressure For their A-Level Exams.

Other Revision Notes related to Partial Pressure you should explore

Mole Fraction ( $x$ ):

Partial Pressure ( $p$ ):