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Overview of Chemical Thermodynamics

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Estimated time: 36 minutes
Maharashtra State Board: Class 12

Definition: Chemical Transformations

Thermodynamics is concerned with the energy changes in physical and chemical transformations.

Maharashtra State Board: Class 12

Definition: State Function

The property which depends only on the state of the system and not on the path followed is called state function.

Maharashtra State Board: Class 12

Definition: Path functions

The properties which depend on the path followed are called path functions.

Maharashtra State Board: Class 12

Definition: Thermodynamic equilibrium

A system is said to be in thermodynamic equilibrium when its state functions do not change with time is called thermodynamic equilibrium.

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Definition: Enthalpy

The sum of internal energy and pressure–volume energy of a system is called enthalpy.

Maharashtra State Board: Class 12

Definition: Enthalpy of fusion

Enthalpy change accompanying conversion of one mole of solid into liquid at constant temperature and pressure is called enthalpy of fusion.

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Definition: Enthalpy of vaporization

Enthalpy change accompanying vaporization of one mole of liquid at constant temperature and pressure is called enthalpy of vaporization.

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Definition: Enthalpy of sublimation

Enthalpy change for conversion of one mole of solid directly into vapour at constant temperature and pressure is called enthalpy of sublimation.

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Key Points: Types of system

Type of System Exchange of Energy Exchange of Matter Example
Open System Energy is exchanged with surroundings Matter is exchanged with surroundings Example: Open cup of hot coffee. It releases heat to surroundings and water vapour escapes into air.
Closed System Energy is exchanged with surroundings Matter is not exchanged with surroundings Example: Hot coffee covered with saucer. It loses heat but water vapour does not escape.
Isolated System No exchange of energy No exchange of matter Example: Insulated cup of coffee. Neither heat nor water vapour escapes to surroundings.
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Definition: Intensive property

A property which is independent of the amount of matter in a system is called intensive property.

Examples: Pressure, temperature, surface tension, viscosity, melting point, boiling point, specific heat. 

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Definition: Extensive property

A property which depends on the amount of matter present in a system is called an extensive property.

Examples: Mass, volume, internal energy, heat capacity, number of moles.

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Key Points: Nature of heat and work

  1. In mechanics, work is defined as force multiplied by displacement, given by the formula W = f × d.
  2. In thermodynamics, the work involved is pressure–volume work (PV work), which is done when a gas expands or contracts against an external opposing pressure.
  3. PV work is given by the expression W = −Pₑₓₜ ΔV, where ΔV is the change in volume.
  4. When a gas expands, it does work on the surroundings; when it is compressed, work is done on the system by the surroundings.
  5. Heat (Q) is a form of energy that is exchanged between the system and surroundings due to temperature difference.
  6. According to sign convention, +Q means heat absorbed by the system, −Q means heat released, +W means work done on the system, and −W means work done by the system; both heat and work are path functions.
Maharashtra State Board: Class 12

Key Points: Expression for pressure-volume (PV) work

  1. Pressure–volume (PV) work is done when a gas expands or compresses against an external pressure in a cylinder fitted with a movable piston.
  2. The force exerted by the gas on the piston is equal to external pressure multiplied by area, given by
    f = −Pₑₓₜ A.
  3. Work done is equal to force multiplied by displacement, therefore
    W = f × d.
  4. Since change in volume is equal to area × displacement,
    ΔV = A × d,
    the expression for PV work becomes
    W = −Pₑₓₜ ΔV = −Pₑₓₜ (V₂ − V₁).
  5. During expansion (V₂ > V₁), work is done by the system on the surroundings and W is negative; during compression (V₂ < V₁), work is done on the system and W is positive.
  6. In free expansion (expansion in vacuum), external pressure is zero (Pₑₓₜ = 0), therefore
    W = 0, and no work is done.
Maharashtra State Board: Class 12

Law: First law of thermodynamics

  1. The First Law of Thermodynamics states that energy can neither be created nor destroyed but can only be converted from one form to another.
  2. According to this law, the total energy of the system and surroundings remains constant during any physical or chemical change.
  3. When a system exchanges heat or work with surroundings, its internal energy changes.
  4. If heat (Q) is supplied to the system and work (W) is done on the system, the internal energy increases.
  5. The mathematical expression of the first law is
    ΔU = Q + W.
  6. For infinitesimal changes, the first law is written as
    dU = dQ + dW.
  7. For special processes:
    • Isothermal process: ΔU = 0
    • Adiabatic process: Q = 0 and −ΔU = W
    • Isochoric process: ΔV = 0 and ΔU = Qᵥ
    • Isobaric process: Qₚ = ΔU + PΔV.
 
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Definition: Bond enthalpy

The enthalpy change required ·to break particular covalent bond in one mole of gaseous molecule to produce gaseous atoms and/or radicals, is called bond enthalpy.

Maharashtra State Board: Class 12

Definition: Entropy

A measure of molecular disorder or randomness of a system is called entropy.

Maharashtra State Board: Class 12

Definition: Spontaneous process

A process which occurs on its own without any external influence is called spontaneous process.

Maharashtra State Board: Class 12

Law: Second low of thermodynamics

  1. The Second Law of Thermodynamics states that the total entropy of a system and its surroundings increases in a spontaneous process.
  2. For a process to be spontaneous, the total entropy change must be positive, given by
    ΔSₜₒₜₐₗ = ΔSₛᵧₛ + ΔSₛᵤᵣᵣ > 0.
  3. The entropy change of surroundings is calculated using
    ΔSₛᵤᵣᵣ = −ΔH / T (at constant temperature).
  4. If ΔSₜₒₜₐₗ > 0, the process is spontaneous; if ΔSₜₒₜₐₗ < 0, the process is non-spontaneous.
  5. At equilibrium, the total entropy change is zero, that is
    ΔSₜₒₜₐₗ = 0.
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