Heat Energy and Temperature

• Heat Energy & Temperature:

  • The molecules within a substance possess two forms of energy:
  • Kinetic energy (due to their random motion / vibration)
  • Potential energy (due to their position relative to each other)
• Together, these two form the total energy that makes up the internal energy of the system.

  • Internal energy is defined as:The total energy stored inside a system by the particles that make up the system due to their motion and positions
• Heating and Temperature Change:

  • Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles
  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules
• Heating and Temperature Change: (part 2)

  • The higher the temperature, the higher the kinetic energy of the molecules and vice versa
  • This means they move around faster
  • This increase in kinetic energy (and therefore internal energy) can:
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)
• As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy:

  
• Heating and Changes of State:

  • When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead
  • This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state
  • For example, liquid to gas
• When a substance changes its state:

  • The potential energy of the molecules increases, breaking the bonds between them and becoming further apart
  • The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated
• Heating curve of a substance showing the energy changes as temperature is increased:

  
• The different sections of the graph show:

  • ORIGIN to A: Added heat energy is being used to increase the kinetic energy of the particles while it is a solid
  • A to B: Added heat energy is being used to break the bonds between the solid molecules, increasing the potential energy and melting the substance
• The different sections of the graph show:

  • B to C: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a liquid
  • C to D: Added heat energy is being used to break the bonds between the liquid molecules, further increasing the potential energy and boiling the substance
  • D to E: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a gas
• Specific Heat Capacity:

  • If the temperature of the system increases, the increase in temperature of this system depends on:
  • The mass of the substance heated
  • The type of material
  • The energy input to the system
• • The specific heat capacity of a substance is defined as:

  The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C
• • Different substances have different specific heat capacities
  • If a substance has a low specific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature)
  • If a substance has a high specific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature)
• Low vs high specific heat capacity:

  
• Specific Latent Heat:

  • Energy is required to change the state of a substance
  • This energy is known as latent heat
  • The specific latent heat of a substance is defined as:

  The amount of thermal energy required to change the state of 1 kg of a substance with no change in temperature
• There are two types of specific latent heat:

  • Specific latent heat of fusion (solid to liquid and vice versa)
  • Specific latent heat of vaporisation (liquid to gas and vice versa)
  • Latent heat is represented by the symbol L with units joules per kilogram (J/kg)
• Heat supplied against temperature graph, showing the changes in state for a substance:

  
• The specific latent heat of fusion is defined as:

  The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

  • This is used when melting a solid or freezing a liquid
  • When a solid substance melts, its temperature stays constant until all of the substance has become a liquid
  • The latent heat of fusion is the energy needed to break the bonds between the molecules
• The specific latent heat of vaporisation is defined as:

  The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

  • This is used when vaporising a liquid or condensing a gas
  • When a liquid substance is heated up to its boiling point, the substance boils and turns into vapour
  • The latent heat of vaporisation is the energy needed by the particles to break away from their neighbouring particles in the liquid
• Specific heat capacity and specific latent heat are slightly different:

  • Specific heat capacity is used for a change in temperature in the same state
  • Specific latent heat is used for a change in state but no change in temperature
• The difference between specific heat capacity and specific latent heat: