Name: __________ KEY_________________________

CHEM 1212 Module 6 Reading Guide

Effect of Temperature on Rate

Reactant molecules must reach a higher energy state to begin the bond-breaking and bond-making processes that make up a chemical reaction. The minimum amount of energy needed for a reaction to occur is called the activation energy, Ea. As the activation energy increases, the rate constant decreases and the reaction proceeds more slowly. The rate constant varies non-linearly with ____________. For similar reactions under comparable conditions, the one with the smallest activation energy will occur most ____________.

A high-energy state in the rearrangement of bonds, called the transition state or activated complex, occurs during chemical reactions. These species are very short lived and usually undetectable by most analytical instruments. Because of the strain placed on the bonds that are breaking, the transition state is higher in energy than both the reactants and products regardless of the overall energy change, ?H, of the reaction.

The Arrhenius equation shows the relationship between the rate constant, activation energy, frequency of collisions, orientation, and temperature. The frequency factor found in the equation indicates the number of ____________ that occur with the correct orientation per unit time at a given temperature. ____________ concentrations usually result in more collisions, explaining why reaction rate often ____________ with increasing reactant concentration. These collisions must also occur with sufficient ____________ to penetrate the valence shell and rearrange the electrons to make new bonds. As the temperature increases, so does the average ____________ energy and the number of particles with energy sufficient to overcome the ____________ energy. If the activation energy is much larger than the average kinetic energy of the molecules, the reaction will occur ____________ since only a few fast-moving molecules will have enough energy to react. If the activation energy is much smaller than the average kinetic energy of the molecules, a large fraction of molecules will be adequately energetic and the reaction will proceed ____________.

It is easiest to determine the frequency factor through experiment in which the rate constant is measured at different temperatures. The form of the Arrhenius equation used for the determination of A is

And the two-point form is used to determine the rate constant at a different temperature or to determine activation energy depending on the data you already have.

Practice Problems

49. Rate constants for the reaction 2 NO2(g) ? 2 NO(g) + O2(g) are 0.77 M?1·s?1 at 603 K and 4.7 M?1·s?1 at 656 K.

a. Calculate the value of the activation energy in kJ/mol.

b. Calculate the rate constant at 626 K.

Reaction Mechanisms

Many reactions involve more than one molecular event and are best described as a series of elementary reactions, each of which has its own transition state, called a reaction mechanism. Unlike balanced equations representing an overall reaction, the equations for elementary reactions are explicit representations of the chemical change taking place. Unless you are explicitly told that a reaction is an ____________ step, you cannot assume that the rate law follows the reaction stoichiometry, but if it is an elementary step, the reaction orders can be determined from the ____________ of the balanced equation. Any proposed mechanism must account for the overall stoichiometry of the reaction and have a rate-determining step that is consistent with the ____________ rate law without containing any ____________.

The experimentally determined rate law for a reaction tells us something about the ____________ step of the mechanism, which is called the rate-determining step. The rate of the overall reaction can be no faster than the ____________ step. Matching the ____________ rate law and that predicted by the mechanism is easiest when there is one step that is clearly slower than the others. In general, when the rate-determining step is the ____________ step in a mechanism, the rate law for the overall reaction is the same as the rate law for this step. However, when the rate-determining step is preceded by a step involving a rapidly reversible reaction the rate law for the overall reaction may be more difficult to derive. When two or more slow steps have comparable rates, the experimentally determined rate laws can become complex.

A substance that appears as a product in one step of a mechanism and as a reactant in a subsequent step but is not in the overall balanced chemical reaction is called an ____________. Intermediates ____________ appear in the overall rate law.

The ____________ of a reaction identifies the number of reactant molecules that collide during an elementary step. A ____________ reaction involves the reaction of a single reactant species to produce one or more molecules of product.

Practice Problems

58. The formation of nitryl fluoride, NO2F, is thought to occur by the following two-step mechanism. Write the overall balanced reaction and rate law.

Step 1: NO2(g)+F2(g)?NO2F(g)+F(g) (slow)

Step 2: F(g)+NO2(g)?NO2F(g) (fast)

Catalysis

Catalysts speed up chemical reactions generally by ____________ the activation energy of the reaction by providing an alternative pathway with a ____________ activation energy. When a catalyst is present, a reaction may follow a different set of elementary reactions. Catalysts facilitate chemical reactions but are not ____________ by them. They are either consumed in one step and then later regenerated or do not participate in the reaction at all. Sometimes, the task of the catalyst is simply to bring the other reactants together in a way that makes the bonds ____________ to break.

____________ catalysts are in the same phase as the reactants, whereas heterogeneous catalysts are in a different phase than the reactants and are usually solids that provide a surface for the reactants to adsorb onto.

Practice Problems

66. Determine whether the following aspects of a reaction are increased, decreased, or unaffected by the addition of a catalyst.

a. rate of the forward reaction

b. rate of the reverse reaction

c. activation energy of the forward reaction

d. activation energy of the reverse reaction

e. the enthalpy of the reaction

Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/[email protected].; White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.; Chemistry LibreTexts, Libretexts, [online] Available from: https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/14%3A_Chemical_Kinetics/14.7%3A_Catalysis Reading guide style Adapted from “Ionic equations Study Guide” by Montgomery College is licensed under CC BY 4.0 Document not to be reposted on the internet without express permission.