# Second law of Thermodynamics and Various Types?

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### Second law of Thermodynamics:

The second law of thermodynamics introduces another state function called entropy. Entropy is a measure of the molecular disorder (randomness) of a system.

It is defined as, dS = dqrev /T

The SI unit of entropy is JK-1. ### Kelvin Planck statement:

It is impossible to construct a machine that absorbs heat from a hot source and converts in to completely work by a cyclic process without transferring a part of heat to a cold sink. The second law of thermodynamics explains why even an ideal frictionless engine cannot give 100% efficiency.

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Efficiency = work performed / heat absorbed

### Clausius Statement:

It is impossible to transfer heat from a cold reservoir to a hot reservoir without doing some work.

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### Entropy change accompanying in the change of phase

When the substance undergoes change of state there is a change in entropy.

∆S = q rev / T = ∆H rev / T

### Entropy of fusion:

The heat absorbed when one mole of a solid melts at its melting point reversibly is called molar heat of fusion.

∆Sfusion = ∆Hfusion / Tf

Where is the Tf melting point.

### Entropy of vapourisation:

The heat absorbed when one mole of liquid boils at its boiling point reversibly is called molar heat of vapourisation.

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∆Sv= ∆Hv / Tb

Where is the Tb is the boiling point.

### Entropy of transition

The heat change when one mole of a solid changes reversibly from one allotropic form to another at its transition temperature is called enthalpy of transition.

∆St = ∆Ht/ Tt

Where is the Tt is the transition temperature.

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### Gibbs free energy:

Gibbs free energy is defined as

G = H – TS

It is an extensive property and it is a single valued state function.

### Criteria for spontaneity of a process:

• If the enthalpy change of a process is negative, then it is exothermic and may be spontaneous.
• If the enthalpy change of a process is positive, then it is endothermic and may occur spontaneously.
• The Gibbs free energy which is the combination of the above two should be negative for a reaction to occur spontaneously, i.e. the necessary condition for a reaction to be spontaneous is ∆H – T∆S<0.
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