Content in a Nutshell

The process by which one or more substances change into new substances is called a chemical reaction. This definition is a bit ambiguous and stuffy; chemical reactions are neither. When a reaction occurs it is usually obvious and often spectacular. Chemical reactions are the motion in our lives, and they are the heart of this science. Most of chemistry is devoted to learning more about these reactions-how much product they produce, how fast they proceed, and how much energy is involved. For students, chemical reactions supply the gee, the whiz-bang, the magic of chemistry. It is valuable for students to see many reactions via laboratory activities and teacher demonstrations, especially early in the course.

Chemical reactions involve the breaking and making of bonds, or stating it differently, a change in the location of electrons with respect to the nuclei of atoms. A driving force for reactions is that electrons find a lower energy environment, illustrated by the reaction of active metals in acid.

Mg(s) + 2 HCl(aq) ---> H 2 (g) + MgCl 2 (aq)

A major change here is that two of the electrons originally on the Mg atom have moved because the H nuclei can hold them more strongly. Chemical reactions are primarily movement of electrons so chemistry textbooks spend more time talking about electrons than they do about nuclei.

Breaking chemical bonds requires energy and making them produces energy so chemical reactions involve energy changes. Many times we do reactions to obtain the energy rather than the products of the reaction. The burning of fuels is an example. Several demonstrations in this module focus on the energy changes in chemical reactions.

Chemists use equations with symbols and formulas to represent the reactions that occur. It is critical that students understand that the equations we write on paper represent what happens in the laboratory. Balancing a chemical equation, in the early stages of the student's study, is best done by trial and error. Again, students should recognize that the numbers represent the quantities of substances that react in the laboratory.

The study of chemical reactions, for chemists and students, begins in the laboratory. Chemists mix substances together and look for evidence (such as formation of gas or precipitate, color change, or energy change) to indicate that a reaction has occurred. Additional experiments are conducted to identify the products. It is only through extensive experimentation that enough information is accumulated to enable us to predict the products of some reactions. Students should be encouraged not only to make the observations, but also to speculate about their meaning.

At first students may be bewildered by the wide variety of reactions that happen in the laboratory and the real world. They can gain some sense of control by developing and applying schemes for classifying reactions. Early in their exposure to chemistry they may find simple schemes such as synthesis, decomposition, replacement, and double replacement useful. As they learn more of the content of chemistry, their classification schemes will become more sophisticated and will include such concepts as acid-base and oxidation-reduction reactions.

Place in the Curriculum

The investigation of reactions as outlined in this module can come very early in the course. Laboratory activities and demonstrations create interest and provide an experiential basis for the study of later topics such as moles, atomic theory, electronic structure, and acids and bases. The topic of classification of reactions can be revisited later in the year when students can apply a greater knowledge base.

Central Concepts

  1. Chemical reactions are recognized by changes in the properties of the species involved.

  2. Many chemical reactions can be classified, at the simplest level, according to general kinds of change involved:

    A+ B ---> AB Synthesis

    AB ---> A +B Decomposition

    AB + C --->AC + B Replacement

    AB + CD ---> AD + CB Double replacement

  3. A chemical equation represents, in symbolic terms, the overall change in a chemical reaction at the atomic and molecular level (words, symbols, pictures in the mind).

  4. All chemical equations must be "balanced" to be consistent with the Law of Conservation of Matter.

Related Concepts

  1. Chemical Formulas and Symbols

  2. Law of Conservation of Mass

  3. Elements, Compounds, Substances, Mixtures

  4. States of Matter

Related Skills

  1. Observe changes that occur in the laboratory

  2. Use a balance

  3. Light and use a burner safely

  4. Recognize equation patterns

Performance Objectives

After completing their study of chemical reactions, students should be able to:

  1. write correct formulas given ion charges.

  2. interpret formulas.

  3. classify a given chemical equation as synthesis, decomposition, single replacement, or double replacement.

  4. recognize a chemical change in a laboratory setting.

  5. apply the Law of Conservation of Mass to an experiment involving a chemical change. 6. balance equations by inspection.

  6. complete simple chemical equations of the types listed earlier in this module.

  7. convert a word equation into a symbolic equation.

  8. draw "pictures in the mind" to represent the types of chemical reactions listed in this module.