Revision: Class 11 >> Chemical Thermodynamics NEET (UG) Chemical Thermodynamics

The enthalpy of neutralization is defined as the change in enthalpy of the system when one gram equivalent of an acid is neutralized by one gram equivalent of a base or vice versa in dilute solution.

\[\ce{H^+_{(aq)} + OH^-_{(aq)} -> H2O_{(l)}}\] = 57.32 kJ

The heat of combustion of a substance is defined as “The change in enthalpy of a system when one mole of the substance is completely burnt in excess of air or oxygen”. It is denoted by ∆H_C.

Enthalpy of a system is sum of internal energy of a system and the energy equivalent to PV work.

H = U + PV

The heat capacity for 1 mole of a substance, is called molar heat capacity (cm). It is defined as “The amount of heat absorbed by one mole of the substance to raise its temperature by 1 kelvin”.

By substituting equation W = −p_ex . ΔV in the equation ΔU = q + W, we get

ΔU = q − p_ex . ΔV  ...(1)

If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.

The equation (1) becomes ΔU = q_v

Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.

By substituting equation W = −p_ex . ΔV in the equation ΔU = q + W, we get

ΔU = q − p_ex . ΔV  ...(1)

If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.

The equation (1) becomes ΔU = q_v

Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.

Hess’s law of constant heat summation states that, “The change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps.”

The Hess’s law is a direct consequence of the fact that the enthalpy is a state function, and so the enthalpy change depends only on the initial and final states of the system and not on the path by which the reaction takes place. 
Example: The conversion of A to C can take place directly
in a single step.
A → C, ΔH° = ΔH₁

The reaction can also proceed in two steps, for which the ΔH° values are known.

Step (1): A → B, ΔH° = ΔH₂
Step (2): B → C, ΔH° = ΔH₃
______________________________
Overall: A → C, ΔH° = ΔH₂ + ΔH₃

According to Hess’s law, ΔH₁ = ΔH₂ + ΔH_3. The sequence of steps is represented in the figure.

1. Kelvin–Planck statement: Heat QH cannot be taken out of a hot reservoir and used in its whole for labour W. It is necessary for QC to exhaust (give away) some of its heat to a cold reservoir. This rules out the development of an ideal heat engine.
2. Clausius statement: Heat cannot transfer from a colder body to a warmer body unless some effort is made to do this. This rules out the creation of the ideal refrigerator.

The third law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero is zero. Otherwise, it can be stated that it is impossible to lower the temperature of an object to absolute zero in a finite number of steps. Mathematically,

`lim_(T->0)` S = 0 for a perfectly ordered crystalline state.