Kinetic particle theory and internal energy

Particles that never stop moving

Matter exists as gas, liquid, solid and plasma. Kinetic particle theory explains these states and how they change into one another. The states differ only physically: water is the same compound as ice, liquid or steam.

Gas particles move randomly in all directions. Their collisions with the container walls cause pressure, and no energy is lost in these collisions. The particles are far apart, so their attractions are negligible.

The average kinetic energy of gas particles is directly proportional to the kelvin temperature. At the same temperature, it is the same for all gases.

Liquid particles are close but still move, so a liquid has no fixed shape. Inter-particle forces give it a fixed volume and a level surface. Solid particles are held in fixed positions and only vibrate, so a solid has a fixed shape and volume.

Gas and liquid particles show all three movements: translational, rotational and vibrational. The internal energy of a substance is its total energy: the kinetic energy of its particles plus the potential energy of the bonding between them. Heating increases it.

Internal energy = kinetic energy of the particles + potential energy of their bonding. Heating raises it; how freely particles move decides the state.

Key terms

Kinetic particle theory

Explaining the gas laws, the three states and changes between them.

Internal energy

The total energy a substance contains; heat increases it.

Translational motion

One of the three movements of gas and liquid particles.

Vibrational motion

The only movement of particles in a solid.

Short questions with model answers

1. Q1. Use kinetic particle theory to explain why a gas in a closed container keeps the same pressure at constant temperature.

   • Source of pressure.
   • No energy lost.
   • Same temperature.

   The particles keep hitting the walls just as often and as hard, so the pressure stays the same.

2. Q2. Why does a solid keep its shape while a liquid takes the shape of its container?

   • Solid.
   • Liquid.

   Fixed positions give a solid its shape; moving particles let a liquid flow, though its forces still keep a fixed volume.

Common mistakes

• ✗ Steam is a different chemical from ice.

  ✓ The states differ only in physical properties; water stays the same compound.

• ✗ Heavier gases have more average kinetic energy at the same temperature.

  ✓ At the same temperature, the average kinetic energy is the same for all gases.

• ✗ Internal energy is just the kinetic energy of the particles.

  ✓ It also includes the potential energy of the bonding between particles.

MCQs

1. 1. According to kinetic theory, the basic difference between solid, liquid and gas is due to:

   1. (a) The difference in the movements of the particles
   2. (b) The chemical properties of the particles
   3. (c) The size of the particles
   4. (d) The shape of the particles
   Show answer

   (a) Exercise MCQ 1 (p. 13): the states differ in how their particles move.

2. 2. The inter-particle attractions are the strongest in:

   1. (a) Liquids
   2. (b) Plasma
   3. (c) Gas
   4. (d) Solids
   Show answer

   (d) Exercise MCQ 3 (p. 13): solid forces are strong enough to fix the particles in place.

3. 3. Physical state in which the particles possess the maximum energy:

   1. (a) Solid
   2. (b) Liquid
   3. (c) Gaseous
   4. (d) Vapour
   Show answer

   (c) Exercise MCQ 5 (p. 14): the expected answer is the gaseous state, whose particles move freely with negligible attractions. A vapour is also a gas, so ask your teacher about option (d).

4. 4. Which movement do the particles of a solid have?

   1. (a) Translational, rotational and vibrational
   2. (b) Vibrational only
   3. (c) Translational only
   4. (d) Rotational only
   Show answer

   (b) p. 3: solid particles possess vibrational motion only.

5. 5. The average kinetic energy of gas particles is directly proportional to:

   1. (a) The mass of the gas
   2. (b) The volume of the container
   3. (c) The temperature on the kelvin scale
   4. (d) The temperature on the Celsius scale
   Show answer

   (c) p. 2: average kinetic energy is proportional to the kelvin temperature.

Quick revision

• Gas particles move randomly and far apart; pressure comes from their collisions with the walls.
• Liquid particles move but stay close; solid particles only vibrate in fixed positions.
• Internal energy is the particles' kinetic plus bonding potential energy; heating increases it.