Latent Heat & Thermodynamics

Latent Heat & Thermodynamics

The amount of heat required to change the state of a unit mass of a substance at constant temperature is called latent heat.

If Q heat is required to change the state of a substance of mass m at constant temperature and L is the latent heat, then:

Q = mL

• The S.I. unit of latent heat is joule/kilogram.

Any material has two types of latent heat:

1. Latent Heat of Fusion

It is the amount of heat energy required to convert a unit mass of a substance from a solid state to a liquid state at its melting point.

• It is also the amount of heat released by a unit mass of liquid when it changes into a solid at its freezing point.

Latent Heat of Water:

• Latent Heat in Cal: 80

• Latent Heat in Joule: 336 × 10³

2. Latent Heat of Vaporization

It is the amount of heat required to change a unit mass of a substance from a liquid state to a vapor state at its boiling point.

• It is also the amount of heat released when a unit mass of vapor is changed into liquid.

Latent Heat of Water:

• Latent Heat in Cal: 540

• Latent Heat in Joule: 2256 × 10³

Sublimation

• Sublimation is the process of conversion of a solid directly into a vapor.

• Sublimation takes place when the boiling point is less than the melting point.

• Examples: Camphor or ice in a vacuum.

Hoar Frost

• Hoar frost is the reverse process of sublimation, i.e., the direct conversion of vapor into solid.

Steam vs Water Burns

• Steam produces more severe burns than water at the same temperature because the internal energy of steam is more than that of water due to latent heat.

Relative Humidity

Relative humidity is defined as the ratio of the amount of water vapor present in a given volume of atmosphere to the amount of water vapor required to saturate the same volume at the same temperature, multiplied by 100 to express it as a percentage.

• Relative humidity is measured by a hygrometer.

• Relative humidity increases with the increase in temperature.

Air Conditioning

For a healthy and favorable atmosphere for humans, the conditions are:

1. Temperature: 23°C to 25°C

2. Relative humidity: 60% to 65%

3. Speed of air: 0.75 m/min to 2.5 m/min

Thermodynamics

First Law of Thermodynamics

Heat energy given to a system is used in two ways:

1. Increasing the temperature and internal energy of the system

2. Doing work by the system

If ΔQ = heat energy given, ΔU = increase in internal energy, and ΔW = work done, then:

ΔQ = ΔU + ΔW

• Internal energy may be potential or kinetic, with kinetic energy arising from molecular translational, rotational, and vibrational motion.

• The first law is equivalent to the principle of conservation of energy.

Isothermal Process

• If the temperature remains constant throughout the change, the process is called isothermal.

Adiabatic Process

If there is no exchange of heat energy between the system and surroundings, the process is adiabatic.

• Example: Sudden expansion of carbon dioxide into dry ice.

Second Law of Thermodynamics

While the first law guarantees energy conservation, it does not indicate whether a process will occur. The second law provides the answer.

• Kelvin’s statement: All of the heat can never be converted into work.

• Clausius' statement: Heat cannot flow by itself from a lower to a higher temperature.

Heat Engine

A heat engine converts heat energy into mechanical work continuously through a cyclic process.

• Components: 1. Source (hot body), 2. Sink (cold body), 3. Working substance

Types of Heat Engines

1. Internal Combustion Engine:

• In the engine, heat is produced in the engine itself.

Example:

• Otto engine or petrol engine (efficiency = 52%)

• Diesel engine (efficiency = 64%)

• Only a petrol engine uses a carburetor.

2. External Combustion Engine

• Heat is produced outside the engine.

• Example: Steam engine (efficiency 20%)

Reversible / Carnot Heat Engine

• An ideal heat engine free from imperfections is called a Carnot heat engine.

• The working substance undergoes a Carnot cycle.

• Efficiency: η = 1 – (T₂ / T₁), where T₁ = source, T₂ = sink

Carnot's Theorem

1. All reversible engines operating between the same source and sink have the same efficiency.

2. No engine can have more efficiency than a reversible engine operating between the same temperatures.

Refrigerator or Heat Pump

A refrigerator is an apparatus that transfers heat energy from a cold body to a hot body at the expense of energy supplied by an external agent. The working substance here is called refrigerant. In an actual refrigerator, vapors of freon (CCL₂F₂) act as a refrigerant. 

• A freezing chest in a refrigerator is kept near the top because it cools the remaining space by convection.

• Dry ice is the solid form of carbon dioxide and is used for temporary refrigeration.

Final Thoughts

Latent heat is the energy required to change the state of a substance without altering its temperature, such as melting ice or boiling water. It includes two types: latent heat of fusion, which converts a solid into a liquid, and latent heat of vaporization, which turns a liquid into a vapor. Everyday examples include water freezing or steam condensing.

Sublimation is the direct change of a solid into vapor, while hoar frost is the reverse. Steam burns are more severe than water burns because steam contains higher latent heat. The first law of thermodynamics explains that heat energy is either stored or used to do work, while the second law states that not all heat can be converted into work.

Heat engines, refrigerators, and Carnot cycles demonstrate these principles, showing how thermal energy powers machines and maintains comfort through systems like air conditioning.