Question

For the reaction,
$SOCl_2 + H_2O \to SO_2 + 2HCl$
The enthalpy of reaction is 49.4 kilojoules $(kJ)$ and the entropy of reaction is 336 J/K. Calculate ΔG at 300K and predict the nature of the reaction.

Answer 1

Hint: In order to carry out a chemical reaction, the bonds between atoms need to break, reform or both and this can be made possible either by absorbing or releasing energy. Enthalpy is the term used for the energy ( in the form of heat) absorbed or released by the system, under constant pressure, and the change in enthalpy that results from that chemical reaction is termed as the enthalpy of reaction.
Complete solution:
Enthalpy is the term that measures the heat content, which is either absorbed or released by the system. It is denoted by $H$ and change in heat content, i.e., change in enthalpy is denoted by$\Delta H$.
$\Delta H$ for a system can be numerically calculated by using the Gibb’s free energy equation, $\Delta G = \Delta H - T\Delta S$ .
Here, $\Delta G$ refers to the change in Gibbs free energy of the system
$\Delta S$ represents the change in entropy of the system
T represents the temperature of the system
The enthalpy $H$ and entropy $S$ terms have different sign conventions, which is as follows-
For a reaction to be favorable, $\Delta H < 0$ and$\Delta S > 0$, and vice-versa for unfavorable reactions.
The reaction can be exothermic $\Delta H < 0$ or endothermic $(\Delta H > 0)$ on the basis of whether they release heat or absorb heat.
If after calculating, $\Delta G < 0$ then the reaction is said to be spontaneous otherwise non-spontaneous for positive value of $\Delta G$ .
Now, by using the formula, $\Delta G = \Delta H - T\Delta S$ , we will try to calculate the change in free energy.

Given: - $\Delta H = 49.4kJ$ $\Delta S = $ 336 J/K T= 300K
Therefore, ΔG= (49.4) – [(300 x 336)/ 1000]
                            = $ - 51.4kJ$
Since, the value of ΔG comes out to be negative, so the reaction is spontaneous.

Note: There is a term called state function, which describes the equilibrium state of a system or in simple words it is used to describe the type of system. For example, internal energy, enthalpy, and entropy are state functions because they can be used to describe quantitatively the state of a thermodynamic system, irrespective of how the system arrived in that state.