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Chemistry Question 56 – NEET-UG 2023

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Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R : Assertion A : A reaction can have zero activation energy. Reasons R : The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to threshold value, is called activation energy. In the light of the above statements, choose the correct answer from the options given below :

First, evaluate the truthfulness of the Assertion about zero activation energy. Then, evaluate the truthfulness of the Reason, which is a definition.

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Ninja Strategy•Definition vs. Explanation

Recognize that Reason R is a general definition, not a specific explanation for the phenomenon described in Assertion A. A definition describes 'what' something is, not 'why' a specific instance occurs.

Step 1: Evaluate Assertion A✦ Active

Assertion A states that a reaction can have zero activation energy. According to the Arrhenius equation, k=Ae−Ea/RT, if the activation energy Ea=0, then the rate constant k=A. This means the reaction rate is independent of temperature. Such reactions, known as barrierless reactions, exist, particularly for the recombination of free radicals (e.g., 2CH3∙→C2H6). Therefore, Assertion A is a true statement.

Step 2: Evaluate Reason R○ Expand

Reason R provides the definition of activation energy. It is the minimum extra amount of energy that reactant molecules must absorb to reach the threshold energy, which is the minimum total energy required for a reaction to occur. This is the correct and standard definition of activation energy. Therefore, Reason R is also a true statement.

Step 3: Determine the Relationship between A and R○ Expand

While both statements are true, Reason R is merely the general definition of activation energy. It does not explain the specific conditions or types of reactions for which the activation energy can be zero. The definition describes 'what' activation energy is, but it does not explain 'why' it can be zero in certain cases. Therefore, Reason R is not the correct explanation for Assertion A.

💡 Teacher's Secret Hint

A correct explanation for Assertion A would involve discussing the potential energy surface of specific reactions, like radical recombinations, which lack an energy barrier.

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