See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The rate constant (k) of a reaction is related to the activation energy (Ea) by the Arrhenius equation: k = A·exp(-Ea/RT). The activation energy is the energy difference between the transition state and the reactants: Ea = E(transition state) - E(reactants). Step 1: Effect of reactant stability on rate constant. If the stability of the reactant is increased, the energy of the reactant ground state decreases. This increases the energy gap between reactants and the transition state, thereby increasing Ea and decreasing k. Conversely, if the stability of the reactant is DECREASED, the energy of the reactant ground state rises, making the energy gap smaller, lowering Ea, and increasing k. Therefore, a = decreasing. Step 2: Effect of transition state stability on rate constant. If the stability of the transition state is increased, the energy of the transition state decreases. This reduces the energy gap (Ea) between reactants and the transition state, thereby decreasing Ea and increasing k. Therefore, b = increasing. Step 3: Combining both conclusions. a = decreasing, b = increasing, which corresponds to option (c). Why other options fail: - (a) decreasing, decreasing: decreasing TS stability would raise Ea and lower k, so b cannot be decreasing. - (b) increasing, decreasing: increasing reactant stability raises Ea and lowers k, contradicting the premise; b = decreasing also lowers k. - (d) increasing, increasing: increasing reactant stability raises Ea and lowers k, not increases it. Therefore, the correct answer is C.