Question

Each of the statements given below is false. Explain why. a. The activation energy of a reaction depends on the overall energy change \((\Delta E)\) for the reaction. b. The rate law for a reaction can be deduced from examination of the overall balanced equation for the reaction. c. Most reactions occur by one-step mechanisms.

Short answer

The activation energy is the minimum energy needed for a reaction to occur and is not related to the overall energy change \((\Delta E)\). The rate law cannot be deduced from the balanced equation, as it depends on the reaction mechanism and is determined experimentally. Most reactions involve multi-step mechanisms, allowing for greater control over reaction rate and pathways, making one-step mechanisms relatively rare.

Step-by-step solution

Statement a: The activation energy of a reaction depends on the overall energy change \((\Delta E)\) for the reaction.

This statement is false because the activation energy of a reaction is the minimum amount of energy needed for the reactants to undergo a chemical reaction. It does not depend on the overall energy change \((\Delta E)\) of the reaction, which is the difference between the energy of the products and the energy of the reactants. The activation energy is a barrier that reactants must overcome to convert into products, whereas the energy change characterizes the overall energetics of the reaction.

Statement b: The rate law for a reaction can be deduced from examination of the overall balanced equation for the reaction.

This statement is false because the rate law for a reaction cannot be determined directly from the balanced chemical equation. The rate law is an expression that relates the rate of the reaction to the concentrations of reactants. It is determined experimentally, as it depends on the reaction mechanism and the molecularity of the elementary steps involved. The balanced equation only shows the stoichiometry of the reaction, that is, the ratio of reactants and products, but provides no information about the reaction mechanism and the order of the reaction with respect to each reactant.

Statement c: Most reactions occur by one-step mechanisms.

This statement is false because most reactions occur through multi-step mechanisms rather than one-step mechanisms. Multi-step mechanisms involve a series of elementary steps, where each step has its own transition state and activation energy. The overall reaction rate is determined by the slowest, or rate-determining step, in the mechanism. One-step mechanisms are relatively rare and are only observed in simple reactions. Most reactions, especially complex ones, tend to proceed through several intermediate steps before reaching the final products. Such multi-step mechanisms allow for greater control and regulation of the reaction rate and pathway.