Kinetic Energy – IGCSE Chemistry Definition and Key Facts

Kinetic energy is a core idea that runs through multiple topics in the Cambridge 0620 syllabus: states of matter, diffusion, rates of reaction, and chemical energetics. Understanding it in the context of particle theory is essential.

The 0620 definition

Kinetic energy is the energy a particle possesses due to its motion. All particles (atoms, ions, molecules) are in constant motion, and the kinetic energy of the particles determines the state of matter and influences the rate of chemical reactions.

Kinetic energy and states of matter

State	Particle arrangement	Particle movement	Relative kinetic energy
Solid	Regular, closely packed	Vibrate about fixed positions	Low
Liquid	Close together, irregular	Move around each other, slide past	Medium
Gas	Far apart, random	Move rapidly in all directions	High

Changes of state and kinetic energy

• Melting: particles gain enough kinetic energy to overcome some of the forces holding them in fixed positions; they begin to move around each other
• Boiling/evaporation: particles gain enough kinetic energy to overcome the remaining attractive forces and escape as gas
• Condensing/freezing: particles lose kinetic energy; attractive forces pull them closer together

During a change of state, temperature remains constant even though energy is being supplied. The energy goes into overcoming attractive forces between particles, not into increasing kinetic energy. This is a commonly tested point.

Kinetic energy and temperature

Temperature is a measure of the average kinetic energy of the particles. When a substance is heated:

1. Particles gain kinetic energy
2. They move faster
3. This affects properties like pressure (in gases), diffusion rate, and rate of reaction

Important: at any given temperature, not all particles have the same kinetic energy. There is a distribution — some move slowly, some move quickly. The temperature gives the average.

Kinetic energy and diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The rate of diffusion depends on:

1. Kinetic energy of particles — higher temperature means faster diffusion
2. Mass of particles — lighter particles have higher speeds at the same temperature and diffuse faster

This explains why ammonia (Mr = 17) diffuses faster than hydrogen chloride (Mr = 36.5) in the classic cotton-wool-in-a-tube experiment.

Kinetic energy and collision theory

For a chemical reaction to occur, particles must:

1. Collide with each other
2. With sufficient energy — at least the activation energy

Increasing temperature increases the kinetic energy of particles. This means:

• Particles move faster, so collisions are more frequent
• More particles have energy equal to or greater than the activation energy
• Both effects increase the rate of reaction

The second point — more particles exceeding the activation energy — is usually the more important factor and earns the higher marks on Paper 4.

Kinetic energy and gas pressure

Gas particles collide with the walls of their container. Each collision exerts a tiny force on the wall. Gas pressure is the result of billions of these collisions per second.

• Increasing temperature → particles have more kinetic energy → they hit the walls harder and more often → pressure increases (in a fixed volume)
• Decreasing volume → particles hit the walls more often → pressure increases (at constant temperature)

Worked exam question

Explain, using kinetic particle theory, why increasing the temperature increases the rate of a chemical reaction. [3]

Increasing the temperature increases the kinetic energy of the particles [1]. The particles move faster and collide more frequently [1]. A greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful [1].

Common exam mistakes

1. Saying “particles gain more energy” without specifying kinetic energy — the question wants you to be specific.
2. Stopping at “particles collide more often” without mentioning that more particles exceed the activation energy — this second point typically carries a separate mark.
3. Confusing kinetic energy with heat energy — kinetic energy is the energy of movement; heat is the transfer of thermal energy between objects.