Problem 57 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Each question contains STATEMENT-1 (Assertion) and STATEMENT-2 (Reason). Each question has 5 choices (a), (b), (c) and (d) out of which ONLY ONE is correct. Mark your answer as Statement \(-1\) For every chemical reaction at equilibrium, standard Gibbs energy of reaction is zero. Statement \(-2\) At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibbs energy.

Step-by-Step Solution

Answer

(d) Statement -1 is false, but Statement -2 is true.

Step 1: Understand the Meaning of Gibbs Energy

The Gibbs energy change (\(\Delta G\)) determines the spontaneity of a reaction. If \(\Delta G < 0\), a process is spontaneous.

Step 2: Analyze Statement -1

Statement -1 claims that the standard Gibbs energy of reaction is zero for every chemical reaction at equilibrium. This is incorrect as the standard Gibbs energy (\(\Delta G^0\)) is not zero at equilibrium; it's related to the equilibrium constant by \(\Delta G^0 = -RT \, \ln K\), and it can be zero, positive, or negative depending on the reaction.

Step 3: Analyze Statement -2

Statement -2 states that at constant temperature and pressure, reactions proceed spontaneously in the direction of decreasing Gibbs energy. This is true as a negative \(\Delta G\) indicates a spontaneous reaction.

Step 4: Determine the Relationship between the Statements

Since Statement -1 is incorrect and Statement -2 is correct, Statement -2 provides a truthful assertion about spontaneity but does not justify Statement -1 since it is false. Hence, the two are unrelated in terms of justification.

Step 5: Final Answer Selection

According to the assessment of both statements, the correct answer is (d): Statement -1 is false, but Statement -2 is true.

Key Concepts

Chemical Equilibrium

Chemical equilibrium is when a reaction's forward and reverse processes happen at the same rate. This might seem like the reaction has ceased, but in reality, the chemicals involved are constantly changing between the reactants and the products. At this point, the concentrations of reactants and products remain constant. This does not mean they are equal, just constant. At equilibrium, the system is in a state of balance. It's important to understand that the equilibrium position of a reaction depends on the conditions such as temperature, pressure, and concentrations. In terms of energy, even though the process might stop changing visibly, equilibrium doesn't necessarily mean the minimum energy state. It means the energy is distributed such that there's no net change in reactant and product concentrations.

Spontaneity of Chemical Reactions

The spontaneity of a chemical reaction is determined by the Gibbs energy change (\(\Delta G\)). When \(\Delta G < 0\), a reaction happens spontaneously. This means it can occur without external input of energy. When \(\Delta G > 0\), the reaction isn't spontaneous, meaning_energy is needed to drive the reaction forward. The term spontaneous does not imply fast or violent reactions. It just suggests that the reaction prefers to proceed in a specific direction under given conditions. Spontaneity covers both enthalpy and entropy changes - which balance energy within the system to drive these reactions.

Equilibrium Constant

The equilibrium constant (\(K\)) is a numerical value that indicates the ratio of the concentrations of products to reactants at equilibrium. This ratio is essential since it can tell us about the position of equilibrium. A large \(K\) value suggests that products are favored, while a small \(K\) value indicates reactants are preferred. The equilibrium constant is temperature dependent, reflecting how temperature changes can shift equilibrium - either favoring forward or reverse reactions. There's a direct relationship between equilibrium constant and Gibbs energy change (\(\Delta G^0 = -RT \, \ln K\)). Understanding this relationship reveals how energy terms help explain why certain equilibria are established.