5.1 - 5.3 Thermodynamics

Internal Energy

The total sum of randomly distributed kinetic energy and potential energy of atoms/molecules in a substance.

Thermal Equilibrium

No net heat flow between bodies in thermal contact

(The two bodies are at the same temperature).

Specific Heat Capacity

The energy required to raise the temperature of a unit mass of a substance by one kelvin.

(Specific) Latent Heat of Vaporisation

The thermal energy required per unit mass in order to change a liquid into a gas at constant temperature.

(Specific) Latent Heat of Fusion

The thermal energy required per unit mass in order to change a liquid into a solid at constant temperature.

Boyle’s Law

Pressure is inversely proportional to volume for a fixed mass of gas at constant temperature.

Describe the motion of atoms in a solid at a temperature well below its melting point.

The atoms vibrate about their fixed positions.

Describe the effect that a small increase in temperature would have on the motion of atoms in a solid at a temperature well below its melting point.

The atoms would vibrate with a greater amplitude/ greater frequency, still about their fixed positions.

Describe the effect on the internal energy and temperature of a solid when it melts.

• The internal energy increases (that is the potential energy of the solid's molecules increases whilst the kinetic energy remains constant);

• and the temperature remains constant.

State some conclusions that may be deduced about the motion of the molecules of a liquid/gas by observing the motion of pollen grains/smoke particles (that are suspended in the liquid/gas) under a microscope.

• The movement of the pollen grains/smoke particles is caused by the molecules of the liquid/gas moving in a random/haphazard manner;

• Pollen grains/smoke particles are visible but the liquid/gas molecules are not, hence the molecules of the liquid/gas are much smaller than the pollen grains/smoke particles;

• The pollen grains/smoke particles are continuously moving, which means the liquid/gas molecules are continuously moving

State the assumptions made in the development of the kinetic model of an ideal gas.

• The collisions between the gas molecules and the walls of the container are perfectly elastic;

• The force between molecules is negligible except during collisions;

• The volume of the molecules is negligible compared to the volume of the gas / container;

• The time during a collision is negligible compared to the time between collisions:

• There are a large number of particles in random, rapid motion.

Use the kinetic model of a gas and Newton's laws of motion to explain how a gas exerts a pressure on the walls of its container.

• When a molecule collides with a wall, its momentum is changed;

• in order to change its momentum, the wall must have provided a force which acted on the molecule as the rate of change of momentum is directly proportional to the force exerted (by Newton's 2nd Law):

• By Newton's 3rd Law, the force exerted on the wall by the molecule is equal in magnitude and opposite in direction to the force exerted on the molecule by the wall;

• The total pressure experienced on the wall is = (sum of all forces exerted by the molecules) / (area of the wall).

A constant mass of gas occupies a container of constant volume. Use the kinetic theory of gases to explain the increase in the force exerted on the wall of the container by the gas when the temperature is increased.

• As the temperature increases, the speed of the gas molecules increases;

• As a result, collisions between the molecules and the walls of the container are more frequent;

• and as the molecules are travelling (and rebounding) with larger speeds, the change in momentum of the molecules increases;

• and so the total force exerted on the walls of the container increases

A constant mass of gas occupies a container whose volume can change. Use the kinetic theory of gases to explain why the volume of the gas must increase if the pressure is to remain constant as the gas is heated.

• As the temperature increases, the speed of the gas molecules increases;

• As a result, collisions between the molecules and the walls of the container are more frequent;

• and as the molecules are travelling (and rebounding) with larger speeds, the change in momentum of the molecules increases;

• For a constant pressure fewer collisions per unit time are required;

• this can be achieved by increasing the volume as the molecules will have to travel a greater distance between collisions, decreasing the number of collisions per unit time they undergo.

State what is meant by the Avogadro constant.

The number of atoms of carbon-12 in 0.012 kg of carbon-12.

Use the kinetic theory of matter to explain why melting requires energy but there is no change in temperature.

• On melting, bonds between molecules are broken/weakened;

• Kinetic energy remains unchanged so there is no change in temperature;