6.2.7Chemical Change: Rate and Extent
The Effect of Pressure Changes on Equilibrium — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 16 of 16
Part 1 of 5When pressure matters (HT only)
Gases only
- Gases only Pressure changes affect equilibria in which at least one substance is a gas.
- Solids and liquids are barely affected by pressure.
Pressure and molecules
- Pressure and molecules At constant temperature, more gas molecules in a fixed volume produce a higher pressure.
Equal numbers
- Equal numbers If both sides of the equation have the same number of gas molecules, changing the pressure does not shift the position.
Part 2 of 5Predicting the shift (HT only)
1
Write the balanced equation with state symbols and mark which species are gases.
2
Count the number of gas molecules (using the balancing numbers) on the left and on the right.
3
If pressure is increased, the position moves towards the side with fewer gas molecules.
4
If pressure is decreased, the position moves towards the side with more gas molecules.
5
State the effect on yield of the product.
Part 3 of 5Pressure, shift and explanation (HT only)
| Pressure change | Shift towards | Reason |
|---|---|---|
| Increase | Side with fewer gas molecules | Reduces the number of molecules, which lowers the pressure again |
| Decrease | Side with more gas molecules | Raises the number of molecules, which raises the pressure again |
| Either, equal gas molecules each side | No shift | Nothing to oppose the change by moving |
Part 4 of 5Worked example: making ammonia (HT only)
1Equation
- Equation N2(g) + 3H2(g) ⇌ 2NH3(g)
2Count molecules
- Count molecules Left: 1 + 3 = 4 gas molecules; right: 2 gas molecules.
3High pressure
- High pressure The position moves right, towards fewer molecules.
4Result
- Result A higher pressure increases the yield of ammonia.
Part 5 of 5Industrial compromise
Choosing a pressure
Benefit
- Benefit High pressure raises the yield and also increases the rate by bringing particles closer.
Cost
- Cost Very high pressures need thick-walled reactors and powerful compressors.
Safety
- Safety Leaks and explosions are a greater hazard, so the Haber process uses about 200 atmospheres.
Key terms
| Term | Meaning |
|---|---|
| Gas molecules | The particles of a gas that determine its contribution to pressure. |
| Position of equilibrium | The balance of reactants and products when equilibrium is reached. |
| Pressure | Force per unit area exerted by particles colliding with a surface. |
| Balancing number | The number in front of a formula showing how many molecules or moles take part. |
| Compressor | A machine that increases the pressure of a gas. |
Avoid these mistakes
1
Counting solids or liquids when working out the number of gas molecules.
2
Saying the position moves towards the side with the larger volume when pressure is raised; it is towards fewer molecules.
3
Forgetting that when the number of gas molecules is equal, pressure changes have no effect on position.
4
Assuming high pressure is always best, ignoring cost and safety.
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