Temperature changes and energy - EdexcelMultiple thermal changes
Changes in a material's temperature or state of matter are caused by changes to the internal energy. The energy required by different materials depends on their 'heat capacity' and 'latent heat'.
specific heat capacityThe amount of energy needed to raise the temperature of 1 kg of substance by 1°C. relates only to the energy required for a change in temperature. specific latent heatThe amount of energy needed to melt or vaporise 1 kg at its melting or boiling point. relates only to the energy required for a change in state. If a change in internal energyThe total kinetic energy and potential energy of the particles in an object. of a material will cause it to change temperature and change state, both equations can be used.
Example
What happens when 1 kilogram (kg) of water at 75 degrees Celsius (°C) is heated with 2.5 megajoules (MJ) (2,500,000 J)?
1. Some of the energy is used to raise the temperature of the water to 100°C, so the energy needed to raise 1 kg of water by 25°C is:
\(\Delta Q E= mc \Delta \theta\)
\(\Delta Q E= 1 \times 4,200 \times 25\)
\(\Delta Q E= 105,000~J\)
2. Some of the remaining 2,395,000 J is then used to turn the boiling water into steam, so the energy needed to change 1 kg of water at 100°C into steam at the same temperature is:
\(\Delta Q E = mL\)
\(\Delta Q E = 1 \times 2,260,000~J\)
\(\Delta Q E = 2,260,000~J\)
The final amount of energy 2,500,000 - 2,260,000 - 105,000 = 135,000 J, is used to raise the temperature of the steam, and as steam has a specific heat capacity of 1859 J/kg°C, the final temperature of the steam would be: