Latent Heat & Specific Latent Heat

Latent Heat

Latent heat of a substance is the amount of energy absorbed or released by the substance during a change in its physical state that occurs without changing its temperature.

• SI unit of latent heat is the joule ($\text{J}$).
• The latent heat associated with melting a solid or freezing a liquid is called the latent heat of fusion ($L_{f}$); that associated with vapourizing a liquid or a solid or condensing a vapour is called the latent heat of vaporization ($L_{v}$).

Specific Latent Heat Of Fusion

Specific latent heat of fusion, l_f, of a substance is defined as the amount of heat required to change a unit mass of the substance from solid to liquid state, without any change in the temperature.

$$Q = ml_{f}$$

, where
$Q$ = amount of thermal energy absorbed or released
$m$ = mass of substance
$l_{f}$ = specific latent heat of fusion.

• SI unit of specific latent heat of fusion, $l_f$, is joule per kilogram ($\text{J kg}^{-1}$)

Specific Latent Heat Of Vapourization

Specific latent heat of vapourization, $l_{v}$, of a substance is defined as the amount of heat required to change unit mass of the substance from liquid state to gas state without a temperature change.

$$Q = ml_{v}$$

, where
$Q$ = amount of thermal energy absorbed or released
$m$ = mass of substace
$l_f$ = specific latent heat of vapourization.

• SI unit of specific latent heat of vapourization, $l_{v}$, of a substance is joule per kilogram ($\text{J kg}^{-1}$)

Latent Heat In Terms Of Molecular Behaviour

Latent heat energy is absorbed or given out while a substance undergoes state change. The average kinetic energy of the molecules does not change so that the temperature remains constant.

• During melting, heat absorbed by the solid is used to break the inter-molecular bonds between the molecules of solid substance.
• During vapourization, heat absorbed by the liquid is used to break the inter-molecular bonds completely between the molecules of liquid substance.

Determining The Specific Latent Heat Of Fusion For Ice

In this section, we will guide you through the steps to determine the specific latent heat of fusion for ice.

Materials Required:

1. Ice cubes or crushed ice
2. Thermometer
3. Insulated container
4. Heater or heat source
5. Stopwatch or timer
6. Calorimeter (optional)

Experimental Procedure:

1. Preparation of Ice:
   Begin by preparing a sufficient amount of ice. You can use ice cubes or crushed ice, but ensure that it is free from impurities that may affect the results.
2. Insulated Container:
   Place the ice in an insulated container. An insulated container helps to minimize heat exchange with the surroundings, allowing for a more accurate determination of the specific latent heat of fusion.
3. Initial Temperature Measurement:
   Measure the initial temperature of the ice using a thermometer. This serves as the starting point for the experiment.
4. Heating the Ice:
   Apply heat to the ice using a heater or a heat source. As the ice absorbs heat, it will start melting and transitioning from a solid to a liquid state.
5. Continuous Temperature Monitoring:
   Continuously monitor the temperature of the ice as it melts. Record the temperature at regular intervals using the thermometer. It’s essential to maintain a constant temperature during this phase of the experiment.
6. Time Measurement:
   Simultaneously, use a stopwatch or timer to measure the time taken for the ice to completely melt. This time measurement is crucial for calculating the rate at which heat is absorbed.
7. Calculation of Specific Latent Heat of Fusion:
   Once the ice has completely melted, calculate the specific latent heat of fusion using the formula:
   $$Q = ml_{f}$$
   ,Where:
   • $Q$ is the heat energy absorbed (in joules),
   • $m$ is the mass of the melted ice (in kilograms),
   • $l_f$ is the specific latent heat of fusion for ice (in joules per kilogram). The specific latent heat of fusion can be determined by rearranging the formula:
     $l_f = \frac{Q}{m}$ Ensure that all units are consistent to obtain accurate results.
8. Calorimeter Option:
   For a more precise determination, you can use a calorimeter to measure the heat absorbed by the melted ice. The calorimeter helps trap the heat and allows for a more controlled environment.

Determining The Specific Latent Heat Of Vaporization For Water

In this section, we will walk you through the steps to determine the specific latent heat of vaporization for water.

Materials Required:

1. Water
2. Heater or heat source
3. Thermometer
4. Insulated container
5. Stopwatch or timer
6. Calorimeter (optional)

Experimental Procedure:

1. Initial Temperature Measurement:
   Begin by measuring the initial temperature of the water using a thermometer. This provides a baseline for the experiment.
2. Insulated Container:
   Pour a specific amount of water into an insulated container. Insulation helps minimize heat exchange with the surroundings, ensuring that most of the energy supplied goes into the phase change.
3. Heating the Water:
   Apply heat to the water using a heater or a heat source. As the water absorbs heat, it will gradually reach its boiling point and transition into vapor.
4. Continuous Temperature Monitoring:
   Continuously monitor the temperature of the water as it heats up. Record the temperature at regular intervals using the thermometer. It’s essential to maintain a constant temperature during this phase of the experiment.
5. Boiling Point Observation:
   Observe and record the temperature at which the water starts boiling. The boiling point is a critical reference point for calculating the specific latent heat of vaporization.
6. Time Measurement:
   Simultaneously, use a stopwatch or timer to measure the time taken for the water to completely vaporize. This time measurement is crucial for calculating the rate at which heat is absorbed.
7. Calculation of Specific Latent Heat of Vaporization:
   Once the water has completely vaporized, calculate the specific latent heat of vaporization using the formula:
   
   $$Q = ml_v$$
   ,Where:
   • $Q$ is the heat energy absorbed (in joules),
   • $m$ is the mass of the vaporized water (in kilograms),
   • $l_v$ is the specific latent heat of vaporization for water (in joules per kilogram). The specific latent heat of vaporization can be determined by rearranging the formula:
     $l_v = \frac{Q}{m}$ Ensure that all units are consistent to obtain accurate results.
8. Calorimeter Option:
   For a more precise determination, you can use a calorimeter to measure the heat absorbed during the vaporization process. The calorimeter helps trap the heat and provides a controlled environment.