5.3.1Chemical Energetics
Kinetic Theory — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 7 of 12 Unit quiz
Part 1 of 4Particles in Gases
The kinetic theory treats a gas as a large number of particles in constant, random motion.
Random motion
- Gas particles move continuously in random directions at high speeds.
Collisions
- Particles collide with each other and with the walls of the container, changing direction each time.
Gas pressure
- Pressure arises from the very many collisions of particles with a surface, each of which exerts a small force.
Temperature and energy
- The average kinetic energy of the particles increases as the temperature of the gas rises.
Part 2 of 4Explaining Pressure Changes
Changing temperature, volume or the number of particles changes the rate and force of collisions.
Explaining Pressure Changes
Higher temperature
- Particles move faster and hit the walls harder and more often, so the pressure increases.
Smaller volume
- Particles hit the walls more often in a smaller container, which increases the pressure.
More particles
- Adding particles increases the number of collisions per second, which raises the pressure.
Larger volume
- In a larger container collisions are less frequent, so the pressure is lower.
Part 3 of 4Building the Model
The model is simple, and its assumptions explain where it works well and where it does not.
1
The kinetic model treats gas particles as tiny hard spheres that move and collide with no loss of energy.
2
The model assumes the particles have negligible volume and exert no forces on one another except during collisions.
3
This ideal gas model explains the gas laws, such as Boyle's law and the pressure law, using particle motion.
4
Real gases deviate from the model at very high pressures or very low temperatures, when particles are close and attract each other.
Part 4 of 4Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Kinetic theory A model that explains the properties of matter in terms of the motion of its particles.
- Random motion Movement in no particular direction, as shown by gas particles.
- Collision An impact between two particles or between a particle and a surface.
- Gas pressure The force per unit area exerted by gas particles colliding with a surface.
- Ideal gas A model gas whose particles have negligible size and no forces between them.
Common Mistakes
- Saying gas particles stop moving when a gas is cooled; they slow down but do not stop above absolute zero.
- Stating that the particles themselves expand when a gas is heated; only their speed increases.
- Describing gas pressure as being caused by gravity pulling the particles towards the base of the container.
- Confusing particle speed with particle size when explaining why temperature changes pressure.
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