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Chapter 14: Problem 99

Briefly describe each of the following ideas, phenomena, or methods: (a) the method of initial rates; (b) activated complex; (c) reaction mechanism; (d) heterogeneous catalysis; (e) rate-determining step.

Short Answer

Expert verified
(a) The method of initial rates involves studying the speed of reaction at the starting point to determine the reaction order. (b) Activated complex, or transition state, is the temporary molecule formed with the highest energy during a reaction. (c) Reaction mechanism is the sequence of elementary steps leading from reactants to products. (d) Heterogeneous catalysis is a process in which the reactants and catalyst are in different phases, the reaction occurs on the catalyst's surface. (e) The rate-determining step is the slowest step in a reaction and it determines the speed of the entire reaction.

Step by step solution

01

Explanation of the Method of Initial Rates

The method of initial rates is a procedure used to determine the reaction order by studying the rate at which the reaction proceeds, and specifically focusing on the initial stages of the reaction. The initial rates of reactions are determined by measuring the rate immediately after the reaction starts, and various tests are conducted changing the concentration of one reactant at a time while keeping others constant.
02

Explanation of Activated Complex

An activated complex, also known as the transition state, is a temporary, unstable arrangement of atoms formed when reactant molecules collide with sufficient energy. It is the highest energy configuration of a system during a reaction. The activated complex cannot be isolated as it quickly either dissociates back into reactants or proceeds to form products.
03

Explanation of Reaction Mechanism

A reaction mechanism is the description of the sequence of elementary steps leading from reactants to products for a chemical reaction. Each step details which bonds are broken, the intermediates formed, and which bonds are ultimately formed to produce the final product. The sequence of elementary reactions must sum to the overall balanced equation of the reaction.
04

Explanation of Heterogeneous Catalysis

Heterogeneous catalysis refers to the process where the catalyst and the reactants are in different phases, usually solid and gas or liquid. The reaction occurs on the surface of the catalyst. It is typically faster than the same reactions without a catalyst, due to reduced activation energy. One common example is the catalytic converter in a car, which speeds up reactions to reduce the emissions of harmful gases.
05

Explanation of Rate-Determining Step

The rate-determining step is the slowest step of a chemical reaction that determines the speed (rate) at which the overall reaction proceeds. This step has the highest activation energy. In multitask reactions, the speed of the slowest task (rate-determining step) effectively determines the overall speed of the reaction.

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Most popular questions from this chapter

In the reaction \(A \longrightarrow\) products, 4.40 min after the reac- tion is started, \([\mathrm{A}]=0.588 \mathrm{M}\). The rate of reaction at this point is rate \(=-\Delta[\mathrm{A}] / \Delta t=2.2 \times 10^{-2} \mathrm{M} \mathrm{min}^{-1}.\) Assume that this rate remains constant for a short period of time. (a) What is \([\mathrm{A}] 5.00\) min after the reaction is started? (b) At what time after the reaction is started will \([\mathrm{A}]=0.565 \mathrm{M} ?\)

The first-order reaction \(A \longrightarrow\) products has \(t_{1 / 2}=180 \mathrm{s}\) (a) What percent of a sample of A remains unreacted \(900 \mathrm{s}\) after a reaction has been started? (b) What is the rate of reaction when \([\mathrm{A}]=0.50 \mathrm{M} ?\)

Explain why (a) A reaction rate cannot be calculated from the collision frequency alone. (b) The rate of a chemical reaction may increase dramatically with temperature, whereas the collision frequency increases much more slowly. (c) The addition of a catalyst to a reaction mixture can have such a pronounced effect on the rate of a reaction, even if the temperature is held constant.

The mechanism proposed for the reaction of \(\mathrm{H}_{2}(\mathrm{g})\) and \(\mathrm{I}_{2}(\mathrm{g})\) to form \(\mathrm{HI}(\mathrm{g})\) consists of a fast reversible first step involving \(\mathrm{I}_{2}(\mathrm{g})\) and \(\mathrm{I}(\mathrm{g}),\) followed by a slow step. Propose a two-step mechanism for the reaction \(\mathrm{H}_{2}(\mathrm{g})+\mathrm{I}_{2}(\mathrm{g}) \longrightarrow 2 \mathrm{HI}(\mathrm{g}),\) which is known to be first order in \(\mathrm{H}_{2}\) and first order in \(\mathrm{I}_{2}.\)

For the reaction \(A \longrightarrow\) products, derive the integrated rate law and an expression for the half-life if the reaction is third order.
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