5.3.5Chemical Energetics
The Pressure Law — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 12 Unit quiz
Section 1 of 5The Pressure Law
When the volume of a gas is held constant, its pressure depends on its kelvin temperature.
Pressure law
- For a fixed mass of gas at constant volume, pressure is directly proportional to the kelvin temperature.
Equation
- The ratio p ÷ T is constant, so p₁ ÷ T₁ = p₂ ÷ T₂, with T in kelvin.
Heating a gas
- Heating a gas in a sealed container increases the speed and collision rate of its particles, so the pressure rises.
Cooling a gas
- Cooling a sealed gas reduces particle speed and the force of collisions, so the pressure falls.
Section 2 of 5Particle Explanation
The pressure law follows directly from the kinetic theory of gases.
1
At constant volume the container walls stay the same distance apart, so the volume of the gas cannot change.
2
A higher temperature means the particles have more kinetic energy and move faster.
3
Faster particles strike the walls with greater force and more often, which raises the pressure.
4
Doubling the kelvin temperature doubles the pressure, which is the pressure law in action.
Section 3 of 5Applications
Pressure changes with temperature in many practical devices and situations.
Applications
Aerosol cans
- Warnings on aerosol cans advise against heating, because the pressure of the gas inside rises sharply.
Car tyres
- Tyre pressure increases as the tyres warm up on a hot day, so drivers check the pressure regularly.
Pressure cooker
- A sealed cooker raises the pressure inside, which raises the boiling point of the water.
Limitation
- The law only applies to a fixed mass of gas at constant volume, with temperatures in kelvin.
Section 4 of 5Worked Example
Question
- A sealed container holds a gas at a pressure of 2.0 × 10⁵ Pa and a temperature of 280 K.
- Calculate the pressure when the temperature rises to 350 K, with the volume kept constant.
- Write the equationp₁ ÷ T₁ = p₂ ÷ T₂.
- Rearrange to give p₂ = p₁ × T₂ ÷ T₁ = 2.0 × 10⁵ Pa × 350 K ÷ 280 K.
- p₂ = 2.5 × 10⁵ Pa.
Answer The pressure rises to 2.5 × 10⁵ Pa.
Section 5 of 5Exam Checklist & Tips
5.3.5 – Key Points for the Exam
- Pressure law For a fixed mass of gas at constant volume, pressure is directly proportional to kelvin temperature.
- Constant volume A volume that does not change during the process being studied.
- Kelvin The SI unit of temperature, in which the pressure law is applied.
- Directly proportional Two quantities that increase at the same rate, so doubling one doubles the other.
- Collision An impact between a gas particle and another particle or the walls of the container.
Common Mistakes
- Using degrees Celsius in the equation, which gives a wrong answer because the ratio of temperatures is not meaningful in Celsius.
- Saying pressure is inversely proportional to temperature;
- it is directly proportional when temperature is in kelvin.
- Forgetting to convert the temperatures into kelvin before substituting into the equation.
- Applying the pressure law when the volume of the gas also changes, which needs a different method.
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