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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 3: Potential Energy Diagrams

To react, reactant molecules must collide with enough energy to break their old bonds. When OH- and CH3Br react to form CH3OH and Br-, the CH3–Br bond must break, and a new OH–CH3 bond must form.

Potential Energy Diagram

Figure 12-4. Potential Energy Diagram

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Potential Energy Diagram

Figure 12-4. Potential Energy Diagram

A potential energy diagram showing the potential energies of the chemical species as the reaction progresses. In this case, the carbon in the center has a very high energy because it is temporarily trying to bind to five things rather than its standard four bonds.

The reactant molecules (OH- and CH3Br) and the product molecules (CH3OH and Br-) are stable, and stability corresponds to low potential energy. But in the transition between reactants and products—in the instant that old bonds break and new bonds form—there is a high-energy, unstable state called an "activated complex." The potential energy of the chemical species as the reaction progresses is shown on a potential energy diagram.

Figure 12-4 shows that the reactants require a certain amount of energy to reach the activated complex—this amount of energy is called the "activation energy" (Ea) of the reaction. Often, this is referred to as "the hill," and in order for the reaction to happen, we have to climb the hill. If the two molecules do not collide with enough energy to overcome the activation energy and make it to the activated complex, then no products are formed; the collision is not productive. Also, the higher the hill is, the harder it is to have the energy to overcome it and the slower the reaction is.

Temperature and Activation Energy

Figure 12-5. Temperature and Activation Energy

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Temperature and Activation Energy

Figure 12-5. Temperature and Activation Energy

The lines on this graph represent the distribution of particles possessing a range of energy values. The orange line represents a low temperature, and the blue line represents a higher temperature. Note that at the higher temperature, a much larger fraction of the molecules will have more than enough kinetic energy to overcome the activation barrier.

At a low temperature, only a small fraction of the molecules will have enough energy to overcome the activation energy, and the reaction will proceed slowly. At a higher temperature, more molecules will have the energy needed, and the reaction will speed up. (Figure 12-5)

The dependence of the reaction rate on temperature has many far-reaching applications. To speed up the production of ammonia in the Haber-Bosch process, the reaction is conducted at a high temperature (see Unit 9); many other industrial chemical reactions are carried out at high temperatures to boost production. Bread rises faster in a warm environment because the chemical reactions inside yeast increase at higher temperatures. The unwanted microorganisms that cause food to spoil also grow faster in warm conditions; this is why perishable food is best refrigerated.

Glossary

Activated complex

A chemical species midway between reactant molecules and product molecules.

Activation energy

The amount of energy that the reactants need to reach the activated complex.

Potential energy diagram

A graph that shows the potential energies of reactants, the activated complex, and the products as the reaction progresses. It is also called a reaction coordinate diagram.

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