P1b Heating

Formulas:

  • Energy = Mass x Specific heat capacity x temperature change

  • Energy = Mass x Specific latent heat

Keywords:

Chemical change - A process in which a chemical reaction occurs meaning substances react to form different substances. It is irreversible

Conservation of mass - Mass before a reaction is equal to the mass after a reaction. It is true for both physical and chemical reactions

Heat - The transfer of thermal energy from one object to another because of a difference in temperature.

Immersion heater - An electric heater placed into a liquid (or solid). It transfers electrical energy to thermal energy

Intermolecular bonds - Forces of attraction between molecules that hold molecules together in solids and liquids

Joule - The unit of energy

Joulemeter - A device used to measure the amount of electrical energy transferred to another device

Physical change - Change, such as a change of state that does not result in a new substance being made. It is reversible

Specific heat capacity - The quantity of energy needed to raise the temperature of 1kg of that substance by 1•c

Specific latent heat - The energy needed to break intermolecular bonds and cause a change of state of 1kg of a material

Temperature - A measure of the average kinetic energy of the particles in a material

Latent heat of fusion - The energy needed to break intermolecular bonds and cause a change of state of 1kg of a material from solid to liquid

Latent heat of vaporisation - The energy needed to break intermolecular bonds and cause a change of state of 1kg of a material from liquid to gas

Heat flows as a result of temperature differences. The greater the temperature difference the greater the rate of thermal energy transfer. When objects reach the same temperature we say they are in thermal equilibrium.

Specific Heat Capacity Practical:

  1. Insert the immersion heater and thermometer into the block

  2. Connect the immersion heater with an ammeter and voltmeter using leads

  3. Record the initial temperature of the block

  4. Switch on the power supply (e.g. 12 V DC) and start the stopwatch

  5. Record current and voltage

  6. Heat the block until the temperature increases by a set amount (e.g. 10°C)

  7. Switch off the heater and stopwatch immediately and record the time taken

  8. Record the final (maximum) temperature reached.

  9. Calculate temperature change:
    Final temperature - initial temperature

  10. Calculate energy supplied:
    Energy = current x potential difference x time (seconds)

The accepted value for the specific heat capacity of steel is 420J/kg•c and water is 4200J/kg•c

What went wrong:

  • The immersion heater was not fully in the block

  • Heat was lost from the block to its surroundings

  • Heat was lost from the water

  • Some energy was used to heat the water

Improvement:

  • Deeper hole of shorter heater. Fully submerged in water experiment

  • Insulate it with bubble wrap

  • Put a lid on it

  • Pre-heat the heater before starting the experiment

! Rather than calculating energy from current, potential difference and time, we could have just connected a Joulemeter into our circuit. Every time the light flashes then 100J have been given to the immersion heater- we would just need to count the flashes !

For changes of state:

  • When you add energy to a substance, that energy goes into the molecules

  • Molecules move faster when they gain energy

  • The faster fhe molecules move, the higher the average kinetic energy

  • Temperatures measures the average kinetic energy of molecules, so the temperature rises

Specific Latent Heat of Fusion of ice Practical:

  1. Measure the mass of an empty beaker using a balance

  2. Place a funnel above the beaker and add ice into the funnel

  3. Insert the immersion heater into the ice

  4. Connect the ammeter and voltmeter with the heater

  5. Switch on the power supply (e.g. 12 V DC) and start the stopwatch at the same time

  6. Record the current and voltage

  7. Allow the ice to melt completely, collecting the water in the beaker

  8. Switch off the heater and stop the stopwatch. Record the time

  9. Measure the total mass of the beaker with the melted water

  10. Calculate mass of ice melted:

Mass of ice intself = Total mass - mass of empty beaker

  1. Calculate energy supplied:

Energy = current x potential difference x time (seconds)

  1. Calculate specific latent heat:

Specific latent heat = Energy / Mass

  1. Repeat and average results

The accepted specific latent heat of fusion for water is 334,000J/kg.