Contents

- What is chemical thermodynamics?
- Spontaneous and non-spontaneous process
- Entropy
- Second law of thermodynamics
- Entropy and spontaneity
- Gibb’s free energy
- Gibb’s- Helmholtz equation
- Standard free energy
- Spontaneity (feasibility) of reaction on the basis of Enthalpy, Entropy and Gibb’s free energy
- Numerical Problems

**What is chemical thermodynamics?**

Thermodynamics is the branch of physical science that deals with the relationships between heat and other forms of energy (such as mechanical, electrical or chemical energy).

In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another.

**Spontaneous and non-spontaneous process**

**Spontaneous process:**

A process which may take place by itself or by initiation is called a spontaneous process.

In other words, a process which can take place by itself or has a tendency to take place is called spontaneous process.

Spontaneous process is simply a process which is feasible.

Examples of spontaneous process :

- Dissolution of common salt in water.
- Flow of water down a hill.
- Flow of heat from hot body to a cold body.
- Combination of H
_{2}and I_{2}to form HI.

- Combination of hydrogen and oxygen to form water (when initiated by passing an electric spark).
- Reaction between CH
_{4}and O_{2}to form CO_{2 }and H_{2}O (when initiated by ignition).

**Non-spontaneous process:**

A process which can neither take place by itself nor by initiation is called a non-spontaneous process.

Examples of non-spontaneous process:

- Dissolution of sand in water.
- Flow of water up a hill.
- Flow of heat from low pressure to a high pressure.

**Entropy**

Entropy is a thermodynamic state quantity that is a measure of the randomness or disorderness of the system. It is denoted by the letter ‘S’.

Greater the disorderness in molecules, greater will be the entropy. Generally, entropy is inversely proportional to the intermolecular force of attraction. Hence,

∆S_{gas} > ∆S_{liq} > ∆S_{solid}

Mathematically,

**Standard entropy: **Entropy at standard condition i.e. 25^{0}C, 1 atm pressure and 1 molar concentration is called standard entropy. It is denoted by ∆S^{0}.

**Types of entropy:**

**1. Entropy of vaporization: **The entropy change when one mole of a liquid changes into vapour (gas) is called entropy of vaporization.

**2. Entropy of fusion: **The entropy change when one mole of a solid changes into liquid is called entropy of fusion.

**3. Entropy of sublimation: **The entropy change when one mole of a solid changes into vapour (gas) is called entropy of sublimation.

**4. Entropy of combustion or oxidation: **The entropy change when one mole of any substance is completely burnt or oxidized is called entropy of combustion or oxidation.

Limitations of first law of thermodynamics:

>This law only explain about the total heat content in the system but does not say anything about the direction of flow of heat.

>This law does not say whether the process (reaction) is spontaneous or not.

**Second law of thermodynamics**

Second law of thermodynamics can be defined in a number of ways as follows:

**Kelvin Plank statement: **It is impossible to construct an engine operating in a complete cycle which will convert all the heat energy into work i.e. no any system (or engine) will have 100% efficiency.

**Clausius statement: **It is impossible for a self acting machine to transfer heat from a body at lower temperature to a higher temperature without any external actions (agencies).

**In terms of entropy: “**In any spontaneous process, there is always an increase in entropy of the universe”.

In other words- “It is not possible to have a process in which the entropy of an isolated system is decreased”.

**Entropy and spontaneity**

According to second law of thermodynamics, a process will occur spontaneously if entropy of the universe increases. Therefore;

**Gibb’s free energy**

Amount of energy available( required) to perform a certain work is called free energy. Generally, the amount of heat energy required to perform a certain work is called Gibb’s free energy. It is denoted by letter ‘G’.

Mathematically,

G = H – TS

Where,

H= Enthalpy(heat content)

S= Entropy of the system

T = The absolute temperature.

**Gibb’s- Helmholtz equation**

According to definition of Gibb’s free energy,

G = H – TS —(i)

Let us consider G_{1}, H_{1} and S_{1 }be the initial and G_{2}, H_{2} and S_{2} be the final free energy, enthalpy and entropy respectively. Then, for initial state:

G_{1} = H_{1} – TS_{1} —(ii)

Similarly, for final state:

G_{2} = H_{2} – TS_{2} —(iii)

From equation (ii) and (iii),

G_{2} – G_{1} = (H_{2}-TS_{2}) – (H_{1}-TS_{1})

Or, G_{2} – G_{1} = H_{2}-TS_{2} – H_{1} + TS_{1}

Or, G_{2} – G_{1} = (H_{2}– H_{1}) – T(S_{2} – S_{1})

Or, ∆G = ∆H – T∆S —-(iv)

This equation (iv) is called Gibb’s-Helmholtz equation.

**Standard free energy**

Free energy change in between reactant and product at standard conditions i.e. 25^{0}C, 1 atm pressure and 1M concentration is called standard free energy change. It is denoted by ∆G^{0}.

Mathematically,

∆G^{0} = ∆H^{0} – T∆S^{0}

For chemical reaction,

∆G^{0} = Ʃ∆G^{0}_{product} – Ʃ∆G^{0}_{reactant}

Free energyis also defined as thermodynamic function of a system which indicates the capacity of maximum useful work that can be done by the system.

**Spontaneity (feasibility) of reaction on the basis of Enthalpy, Entropy and Gibb’s free energy**

A reaction which occurs itself is called spontaneous or feasible reaction.