Assume all the reactions/process given below are spontaneous at temperature '$\mathrm{T}$' :-

Reaction I: ${\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to {\mathrm{PCl}}_5\left(\mathrm{g}\right)$

Reaction II : ${\mathrm{CO}}_2\left(\mathrm{g}\right)+\mathrm{CaO}\left(\mathrm{s}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)$

Reaction III : ${\mathrm{N}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NH}}_3\left(\mathrm{g}\right)$

The above reactions are Exothermic.

1. Thermodynamics:

The study of energy changes between system and surrounding is called thermodynamics. There are three types of systems, such as open, closed and isolated.

2. Zeroth Law of Thermodynamics:

If two systems are each in thermal equilibrium with a third one, they are also in thermal equilibrium with each other.

3. First Law of Thermodynamics:

Heat supplied $\left(Q\right)$ to a system is equal to algebraic sum of the change in internal energy $\left(\mathrm{\Delta }U\right)$ of the system and mechanical work $\left(W\right)$ done by the system.

(i) $Q=W+\mathrm{\Delta }U$ $\left[W=\int P\mathrm{d}V; \mathrm{\Delta }U=n{C}_{\mathrm{v}}\mathrm{\Delta }T\right]$

(ii) Sign convention:

(a) Heat absorbed by the system → positive

(b) Heat rejected by the system → negative

(c) Increase in internal energy (i.e., rise in temperature) → positive

(d) Decrease in internal energy (i.e., fall in temperature) → negative

(e) Work done by the system → positive

(f) Work done on the system → negative

(iii) For isochoric process:

(a) In this process, V= constant.

(b) P∝T

(c) W=0,

(d) $Q=\mathrm{\Delta }U=\mu C_v\mathrm{\Delta }T$

(iv) For isobaric process:

(a) In this process, P= constant.

(b) $V\propto T$

(c) $Q=\mu C_{\mathrm{p}}\mathrm{\Delta }T$

(d) $\mathrm{\Delta }U=\mu C_{\mathrm{v}}\mathrm{\Delta }T$

(e) $W=P\left(V_2-V_1\right)=\mu R\mathrm{\Delta }T$

(v) For adiabatic process:

(a) In this process, Q=0

(b) $P{V}^{\gamma }=$ constant.

(c) $T^{\gamma }{P}^{1-\gamma }=$ constant.

(d) $T{V}^{\gamma -1}=$ constant.

(e) $W=-\mathrm{\Delta }U=\mu C_V\left(T_1-T_2\right)=\frac{P_1{V}_1-P_2{V}_2}{\gamma -1}$

(vi) For isothermal process:

(a) In this process, T= constant or $\mathrm{\Delta }T=0$

(b) PV= constant.

(c) $\mathrm{\Delta }U=\mu C_{\mathrm{v}}\mathrm{\Delta }T=0$

(d) $Q=W=\mu RT\ln \left(\frac{V_2}{V_1}\right)=\mu RT\ln \left(\frac{P_1}{P_2}\right)$

(vii) For any general polytropic process:

(a) PVx= constant

(b) Molar heat capacity, $C=C_{\mathrm{v}}+\frac{R}{1-x}$

(c) Work done by gas, $W=\frac{nR\left(T_1-T_2\right)}{x-1}=\frac{\left(P_1{V}_1-P_2{V}_2\right)}{x-1}$

(d) Slope of $P-V$ diagram (also known as indicator diagram) at any point is $\frac{\mathrm{d}P}{\mathrm{d}V}=-x\frac{P}{V}$

(viii) For cyclic process,

(a) $\mathrm{\Delta }U=0$

(b) Q=W

4. Second Law of Thermodynamics:

(i) Kelvin–Planck statement: 

It is impossible to construct an engine operating in a cycle that will produce no effect other than extracting heat from a reservoir and performing an equivalent amount of work.

(ii) Rudolph Clausius statement:

It is impossible to make heat flow from a body at a lower temperature to a body at a higher temperature without doing external work on the working substance.

5. Heat Engines and Refrigerators:

(i) Carnot engine: It is a hypothetical engine with maximum possible efficiency.

Process $1\to 2 \text{and} 3\to 4$ are isothermal.

Process $2\to 3 \text{and} 4\to 1$ are adiabatic.

(ii) Efficiency of a cycle:

(a) For Carnot cycle:

$\eta =\frac{\text{Work done by working substance}}{\text{Heat supplied}}\mathrm{ }=\frac{W}{Q_1}=\frac{Q_1-Q_2}{Q_1}=1-\frac{Q_2}{Q_1}=1-\frac{T_2}{T_1}$

(b) For refrigerator:

Coefficient of performance, $\beta =\frac{Q_2}{W}=\frac{Q_2}{Q_1-Q_2}=\frac{T_2}{T_1-T_2}$

6. Bulk Modulus of Gases:

(i) $B=\frac{\mathrm{\Delta }P}{-\frac{\mathrm{\Delta }V}{V}}$

(ii) Isothermal bulk modulus of elasticity, $B_{\mathrm{Iso}}=-V{\left(\frac{\partial P}{\partial V}\right)}_{\mathrm{T}=\mathrm{constant}}$.

(iii) Adiabatic bulk modulus of elasticity, $B_{\mathrm{Adia}}=-\gamma V\left(\frac{\partial P}{\partial V}\right)\Rightarrow B_{\mathrm{Adia}}=\gamma B_{\mathrm{Iso}}$.

7. Thermodynamic Processes and Indicator Diagrams:

(i) Work done is least for monoatomic gas (adiabatic process).

(ii) Air quickly leaking out of a balloon becomes cooler as the leaking air undergoes adiabatic expansion.

(iii) First law of thermodynamics does not forbid the flow of heat from the lower temperature to the higher temperature.

(iv) First law of thermodynamics allows many processes which actually do not happen.