3.2 Internal Energy & Energy Transfers

A lidded beaker contains water vapour.

Describe the movement of the particles that make up the lidded beaker and the movement of the particles of water vapour.

The water vapour is allowed to cool and the temperature is taken at regular intervals.

Sketch a graph to show the change in temperature with time.

Explain what is meant by the specific heat capacity of a substance.

A mass of 170 g of water is needed to make a cup of instant hot chocolate for which the optimum temperature is 80 ºC. The water is heated in a kettle from a temperature of 55 ºC.

The specific heat capacity of water = 4200 J kg^–1 K^–1.

Calculate the energy required to heat the water to the optimum temperature.

Energy required = ....................................... J

Define and state the differences between latent heat of fusion and latent heat of vaporisation.

The energy needed to boil 530 g of a liquid is 0.6 MJ.

Calculate the specific latent heat of the liquid and state whether it is the latent heat of vaporisation or fusion.

The group of students doing this experiment used the equipment shown in Figure 1 to determine the energy transferred to the liquid.

Figure 1

The students were provided with the liquid, an electric water bath, and were allowed to use water from the tap.

Suggest why the students were given an electric water bath rather than a Bunsen burner.

The calculation of specific latent heat using this method may give a value that is different to the true answer.

Explain why.

A thermos flask, as shown in Figure 3, is designed to minimise the transfer of thermal energy between the contents and the surroundings. 

The claim made by the manufacturer is that the thermos can keep hot water warm for up to 6 hours.

Figure 3

14 800 J of energy are transferred from the thermos to the surroundings in 6 hours.

The mass of water in the flask is 360 g

The specific heat capacity of water is 4200 J / kg°C

The initial temperature of the water is 95°C

Calculate the temperature of water in the flask after 6 hours, and state whether or not you agree with the manufacturer’s claim.

By how much has the internal energy of the water in the flask changed?

What is meant by ‘specific latent heat of vaporisation’?

When the water in a saucepan boils, 0.074 kg of water changes to steam.

Calculate the amount of energy required for this change.

Specific latent heat of vaporisation of water = 2.3 10⁶ J / kg.

The graph in Figure 4 shows how the temperature of a substance changes as it is heated.

Figure 4

Explain what is happening to the substance in sections XY and YZ of the graph.

State what change of state is taking place from point Z onwards.

Explain whether the specific heat capacity of the substance in Figure 1 is greater for its solid or liquid form.

The heater used to heat the substance in Figure 1 outputs a power of 1.2 kW  mass of substance being heated = 0.060 kg

Use information from Figure 1 to calculate the specific latent heat of fusion of the substance.

Figure 5 shows how an old-fashioned storage heater is constructed.

There are ceramic bricks inside, which are heated by the heating element during the night. 

During the daytime, the heating element is switched off and the blocks transfer their stored internal energy to the room.

Figure 5

The reason for heating the room in this way is that electricity can be bought at a lower price between midnight and 7am.

The heating elements in the storage heater have a power output of 2.1 kW.

Calculate the amount of energy that will be transferred to the blocks between the hours of midnight and 7am.

By 7am the temperature of the blocks has risen from 20 °C to 750 °C.

Calculate the total mass of the blocks inside the heater.

Specific heat capacity of the blocks = 800 J / kg °C.

A student wants to carry out an investigation to calculate the specific latent heat of fusion of water. 

She uses a 250 ml beaker to hold the water.

Suggest what other apparatus she would need to carry out the investigation.

Describe the measurements the student would have to take to determine the specific latent heat of fusion of water, and explain how she would determine its value.