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Chapter 12: Problem 14

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

Expert verified
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

01

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.
02

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.
03

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.

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Most popular questions from this chapter

Rate Laws from Experimental Data: Initial Rates Method. The reaction $$2 \mathrm{NO}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NOCl}(g)$$ was studied at \(-10^{\circ} \mathrm{C}\). The following results were obtained where $$\text { Rate }=-\frac{\Delta\left[\mathrm{Cl}_{2}\right]}{\Delta t}$$ $$ \begin{array}{ccc} {[\mathrm{NO}]_{0}} & {\left[\mathrm{Cl}_{2}\right]_{0}} & \text { Initial Rate } \\ (\mathrm{mol} / \mathrm{L}) & (\mathrm{mol} / \mathrm{L}) & (\mathrm{mol} / \mathrm{L} \cdot \mathrm{min}) \\ 0.10 & 0.10 & 0.18 \\ 0.10 & 0.20 & 0.36 \\ 0.20 & 0.20 & 1.45 \end{array} $$ a. What is the rate law? b. What is the value of the rate constant?

The combustion of carbohydrates and the combustion of fats are both exothermic processes, yet the combustion of carbohydrates is a faster process. How can this be?

The decomposition of iodoethane in the gas phase proceeds according to the following equation: $$ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{H}(g) $$ At \(660 . \mathrm{K}, k=7.2 \times 10^{-4} \mathrm{s}^{-1} ;\) at $720 . \mathrm{K}, k=1.7 \times 10^{-2} \mathrm{s}^{-1}$ What is the value of the rate constant for this first-order decomposition at \(325^{\circ} \mathrm{C} ?\) If the initial pressure of iodoethane is 894 torr at \(245^{\circ} \mathrm{C},\) what is the pressure of iodoethane after three half-lives?

The decomposition of \(\mathrm{NO}_{2}(g)\) occurs by the following bimolecular elementary reaction: $$ 2 \mathrm{NO}_{2}(g) \longrightarrow 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) $$ The rate constant at 273 \(\mathrm{K}\) is $2.3 \times 10^{-12} \mathrm{L} / \mathrm{mol} \cdot \mathrm{s}\( , and the activation energy is 111 \)\mathrm{kJ} / \mathrm{mol}$ . How long will it take for the concentration of \(\mathrm{NO}_{2}(g)\) to decrease from an initial partial pressure of 2.5 \(\mathrm{atm}\) to 1.5 \(\mathrm{atm}\) at \(500 . \mathrm{K}\) ? Assume ideal gas behavior.

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