Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

# Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

## Section 1: Introduction

The speed of chemical reactions varies tremendously (Figure 12-1). Trinitrotoluene (TNT) detonates in a fraction of a second, while the iron in a car muffler takes years to rust through. The study of the rate of chemical reactions is called "chemical kinetics."

A rate always involves a change over time; the speed of a car is measured in distance over time: miles per hour. In a chemical reaction, we could measure how fast the reactants are used up or how fast the products are produced.

How can the rate of a chemical reaction be controlled? Sometimes, the goal is to speed up the reaction, for example, when synthesizing a new drug to make it available faster or when increasing the rate of fuel consumption to increase the thrust of a rocket. At other times, the goal is to slow down the reaction, for example, to prevent corrosion or the spoiling of food. To better understand and control chemical reactions, chemists have discovered that many factors can influence the reaction rate. Scientists studying chemical kinetics have developed equations that accurately describe reaction rates.

Figure 12-1. The Varying Speed of Chemical Reactions

© Left: Wikipedia, Creative Commons License 3.0. Author: BetacommandBot, 29 November 2008. Right: Wikimedia Commons, Public Domain.

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### Figure 12-1. The Varying Speed of Chemical Reactions

Chemical reactions happen at different rates. Low explosives, like the ones found in the fireworks at left, react relatively slowly compared with the high explosive on the right.

To fully understand why a reaction happens at a certain rate, we also must learn much more about how the chemical reaction occurs. A chemical equation shows reactants turning into products in a very straightforward manner; a simple arrow separates the two. As we will see in this unit, there is much more to that arrow than meets the eye. How exactly do reactants turn into products? Sometimes, the answer is simple: Two atoms bump into each other and form a bond. Most of the time, however, the process is much more complex.

Although nuclear reactions occur via a process completely different from chemical reactions, they both obey the same laws of kinetics. While humans have used chemical reactions for millennia, research into nuclear reactions began only slightly more than a century ago with the discovery of radiation. The latter half of this unit traces the early development of nuclear science. Luminaries such as Marie Curie made great strides in the study of radioactivity in the late 1800s. Through the next few decades, our understanding of radiation and nuclear chemistry grew until science achieved what was once considered impossible: splitting the atom. This discovery of nuclear fission by Lise Meitner and Otto Hahn in the 1930s set the stage for a frantic race to create the first atomic weapons and the subsequent arms race that defined cold war geopolitics for decades thereafter. While often associated with doomsday scenarios in the popular imagination, radioactivity has numerous nonviolent applications. The chain reactions that power nuclear weapons also power nuclear power plants, and radiation itself has many other peaceful uses, such as the radioactive dating in archeology and geology, medical imaging, and radiation therapy.

## Glossary

### Chemical kinetics

The study of the rates of chemical reactions.