Match List I with List II
| List I | List II |
| (A) Activation energy | (I) Difference in the energy of activated complexes and reactants |
| (B) Threshold energy | (II) Minimum energy possessed by reactant molecule to form effective collisions |
| (C) Heat of Reaction | (III) Negative for exothermic reactions |
| (D) Role of a catalyst | (IV) To give new path of reaction |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
(A) Activation energy — This matches (I) Difference in the energy of activated complexes and reactants. Reactant molecules must first rise to the energy of the activated complex; this extra energy is the activation energy, Ea.
(B) Threshold energy — This matches (II) Minimum energy possessed by reactant molecule to form effective collisions. Only molecules with energy equal to or more than the threshold energy (and with proper orientation) collide effectively and form products.
Threshold energy = energy of reactants + Ea
(C) Heat of Reaction — This matches (III) Negative for exothermic reactions. The heat of reaction is the energy of the products minus the energy of the reactants. In an exothermic reaction the products lie lower, so heat is given out and ΔH is negative.
(D) Role of a catalyst — This matches (IV) To give new path of reaction. A catalyst provides an alternate pathway with a lower activation energy; it does not change the energies of the reactants and products, so ΔH stays the same.
NCERT (2026-27 edition):
Temperature Dependence of the Rate of a Reaction – “… H2(g) + I2(g) → 2HI(g)
According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate (Fig. 3.6). It exists for a very short time and then breaks up to form two molecules of hydrogen iodide. The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea).”
Effect of Catalyst – “… The action of the catalyst can be explained by intermediate complex theory. According to this theory, a catalyst participates in a chemical reaction by forming temporary bonds with the reactants resulting in an intermediate complex. This has a transitory existence and decomposes to yield products and the catalyst. It is believed that the catalyst provides an alternate pathway or reaction mechanism by reducing the activation energy between reactants and products and hence lowering the potential energy barrier as shown in Fig. 3.11.”
