Consider the following reaction at temperature \(T\) : $$ \begin{aligned} &\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}(g) \\ &\Delta_{r} H^{\circ}=-217.5 \mathrm{~kJ} / \mathrm{mol}, \quad \Delta_{r} S^{\circ}=-233.9 \mathrm{~J} / \mathrm{K}-\mathrm{mol} \end{aligned} $$ Reaction is supported by : (a) entropy (b) enthalpy (c) both (a) \& (b) (d) neither

Understanding Gibbs free energy (abla G) is crucial for students tackling thermodynamics in chemistry, particularly when preparing for competitive exams like JEE. Gibbs free energy combines two important thermodynamic quantities: enthalpy (abla H) and entropy (abla S). It is defined by the equation: abla G = abla H - Tabla S, where T is the absolute temperature in Kelvin.

Gibbs free energy indicates the spontaneity of a reaction at constant pressure and temperature. If abla G is negative, the reaction is said to be thermodynamically favorable and will occur spontaneously. A positive value suggests the reaction is nonspontaneous, and zero indicates the reaction is at equilibrium. This concept not only helps in understanding chemical reactions but is also a pivotal part of many JEE Physical Chemistry questions. By analyzing abla G, we can deduce which factors—enthalpy or entropy—dominate to drive or impede a reaction.

Enthalpy (abla H) is a concept in thermodynamics that represents the total heat content of a system. It is a state function, meaning it depends only on the current state of the system, not on how it got there. In the context of a chemical reaction, the change in enthalpy (abla_r H) is the heat absorbed or released under constant pressure.

A negative abla_r H (exothermic reaction) means the system releases heat, often making it a driving force for a reaction, as seen in the provided textbook solution. Many JEE aspirants find questions on enthalpy challenging, so a clear understanding of this concept is indeed beneficial. The enthalpy change is what we commonly refer to as 'heat of reaction,' and it is a critical factor in assessing the energy profile of chemical reactions.

Entropy (abla S) is a measure of the disorder or randomness in a system and is a key concept in not only chemistry but in thermodynamics as a whole. A positive change in entropy (abla_r S) means that the disorder of the system increases, which is often associated with favorable natural processes, like the mixing of gases or melting of ice.

In the JEE Physical Chemistry section, understanding entropy can be a real game-changer. The entropy part of the Gibbs free energy equation can actually determine whether a reaction that is non-favorable in terms of enthalpy could still proceed spontaneously due to an increase in disorder. As with our textbook reaction, however, a negative entropy change indicates a decrease in disorder, which doesn't support the reaction's spontaneity on its own.

The Joint Entrance Examination (JEE) in India is a challenging test that requires a deep understanding of Physical Chemistry concepts like Gibbs free energy, enthalpy, and entropy. These topics are not isolated; they interconnect to explain how chemical reactions occur and how energy is exchanged.

Students preparing for the JEE should focus on building a strong conceptual understanding of thermodynamics, as it is integral to many problems they will encounter. Regular practice with textbook exercises, like our example problem, helps students familiarize themselves with the application of these concepts in various scenarios. The ability to analyze a reaction's thermodynamic favorability quickly and accurately is a skill that will be highly beneficial for scoring well in the Physical Chemistry portion of the JEE.