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
  • 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)
• TEMPERATURE

  A) free

  B) no intermolecular

  C) faster

  D) kinetic energy

  E) High

  F) Low

  
• Heating and Changes of State

  • When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and potential energy will increase instead
  • This breaks the bonds between the molecules, causing them to move further apart and change of state
  • 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

  The different sections of the graph show:

  • ORIGIN to A: heat energy increases kinetic energy of particles whilst a solid
  • A-B: heat energy is used to break the bonds, increasing the potential energy and melting the substance
  • B-C: heat energy increases the kinetic energy of particles whilst the substance is a liquid
  • C-D: heat energy is being used to break the bonds between the liquid molecules, increasing potential energy
  • D-E: 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: The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C
• SPECIFIC HEAT CAPACITY:

  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)
• 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)
• SPECIFIC LATENT HEAT:

  A) Vapourisation

  B) Fusion

  
• • 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