Specific Latent Heat (OCR A Level Physics)

Specific Latent Heat of Fusion & Vaporisation

• Energy is required to change the state of substance

• Examples of changes of state are:

  • Melting = solid to liquid

  • Evaporation/vaporisation/boiling = liquid to gas

  • Sublimation = solid to gas

  • Freezing = liquid to solid

  • Condensation = gas to liquid

The example of changes of state between solids, liquids and gases

• When a substance changes state, there is no temperature change

• The energy supplied to change the state is called the latent heat and is defined as:

  The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

• There are two types of latent heat:

  • Specific latent heat of fusion (melting)

  • Specific latent heat of vaporisation (boiling)

 The changes of state with heat supplied against temperature. There is no change in temperature during changes of state

• The specific latent heat of fusion is used when a solid is melting or a liquid is freezing

  • It is defined as:

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

• The specific latent heat of vaporisation is used when a liquid is vapourising or a gas is condensing

  • It is defined as:

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

Determining Specific Latent Heat

• The amount of energy E required to melt or vaporise a mass of m with latent heat L is:

 E = mL

• Where:

  • E = amount of thermal energy to change the state (J)

  • L = latent heat of fusion or vaporisation (J kg⁻¹)

  • m = mass of the substance changing state (kg)

• Specific latent heat of fusion is represented by L_f 

• Specific latent heat of vaporisation is represented by L_v 

• The values of latent heat for water are:

  • Specific latent heat of fusion = 330 kJ kg⁻¹

  • Specific latent heat of vaporisation = 2.26 M J kg⁻¹

• Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water

• The reason for this is to do with intermolecular forces:

  • When ice melts: energy is required to increase the molecular separation until they can flow freely over each other

  • When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between the molecules,

    • This requires much more energy

Procedures to Determine Specific Latent Heat

Determining the Specific Latent Heat of Fusion, L_f

Equipment List

• Crushed ice

• Two funnels with filter paper

• Three retort stands

• Two thermometers

• Two electric balances

• An appropriate heater (e.g., an immersion heater)

• A power source

• A voltmeter, ammeter and stop-clock

Method

• Place a beaker on each balance

  • Leaving the beaker on the balance, zero the scale

• Arrange a funnel, clamped above each beaker

• Set up an immersion heater

  • Connect to the power source

  • Add an ammeter in series and a voltmeter in parallel

• Place the immersion heater in one of the funnels

• Measure out 50-100g of ice

  • Add the same mass of ice to each beaker

  • Record this value

• Turn on the immersion heater and start the stop watch

• Record the potential difference and current

• After a suitable period of time (around 5-10 minutes) remove the funnels, stop the stop watch and turn off the heater

• Record the mass of water in the beaker

Analysis

• The energy supplied to the ice can be calculated using the equation:

energy = current x potential difference x time

• Using the values for current, potential difference and time, calculate the energy supplied

• The specific latent heat of fusion can be calculated using the equation: 

energy = mass x specific latent heat

• The change mass is equal to the mass of water collected

  • To take into account melting due to heat transfer from the surroundings find the difference in mass between the two beakers of water

  • This gives the change in mass due to the energy supplied by the heater

• Calculate the mass of the melted ice and convert it into kg

  • Δm = m_A - m_B

  • Mass in g ÷ 1000 = Mass in kg

• Calculate the specific latent heat of fusion of ice to water using the equation for specific latent heat

Evaluation

• Errors may be introduced due to precision of the instruments

• Water may be absorbed by the filter paper

  • This will reduce the mass and therefore give a higher value for specific latent heat

Determining the Specific Latent Heat of Vaporisation, L_v

Equipment List

• A double-walled glass vessel with an inner flask containing water

• An appropriate electric heater (e.g., an immersion heater)

• A condenser with a collecting flask

• A power source

• A voltmeter, ammeter and stop-clock

• An electric balance

Method

• Connect the double-walled glass vessel to the condenser

  • Place the collecting flask at the end of the condenser

• Set up an immersion heater

  • Connect to the power source

  • Add an ammeter in series and a voltmeter in parallel

• Place the immersion heater in the fluid

• Turn on the immersion heater and start the stop watch

• Record the potential difference and current

• After a suitable period of time (around 5-10 minutes), stop the stop watch and turn off the heater

• Record the mass of water collected in the conical flask

Analysis

• The energy supplied to the water can be calculated using the equation:

energy = current x potential difference x time

• Using the values for current, potential difference and time, calculate the energy supplied

• The specific latent heat of vaporisation can be calculated using

  • The mass of water collected

  • The energy supplied calculated

  • The equation: 

energy = mass x specific latent heat

Evaluation

• Errors may be introduced due to precision of the instruments

• Not all of the vapour which enters the condenser may make it to the beaker