Chemical reactions can be grouped into different categories based on how the reactants and products change during the reaction. Scientists classify reactions by recognizing similar patterns in how substances combine, break apart, or exchange atoms and ions.

In a synthesis reaction, two or more substances combine to form a new substance. These reactions are sometimes called combination reactions because simpler substances combine to form a more complex compound.

In a decomposition reaction, the opposite occurs. A single compound breaks down into two or more simpler substances. These reactions often require an energy source such as heat, light, or electricity to occur.

Another type of reaction is a single-displacement reaction. In this reaction, one element replaces another element in a compound. This usually occurs when a more reactive element replaces a less reactive one.

Finally, a double-displacement reaction occurs when the positive ions of two compounds exchange places. These reactions often produce a precipitate, which is a solid that forms from two solutions, or they may produce water or a gas.

Chemical reactions involve changes in energy as bonds are broken and new bonds are formed. During some reactions, energy is released into the surroundings, while in others, energy is absorbed. Scientists classify these reactions based on how energy is transferred.

An exothermic reaction releases energy, usually in the form of heat or light. Because energy is given off, the surroundings often become warmer. Common examples of exothermic reactions include burning wood, exploding fireworks, and many forms of combustion.  In contrast, an endothermic reaction absorbs energy from its surroundings. These reactions often make the surrounding area feel cooler because heat is absorbed during the reaction. One important example is photosynthesis, in which plants absorb sunlight energy to produce glucose and oxygen from carbon dioxide and water.

The speed of a chemical reaction can also be affected by certain substances. A catalyst is a substance that speeds up a chemical reaction without being permanently changed or used up in the process. Catalysts help reactions proceed more quickly by lowering the activation energy required for the reaction to begin. Enzymes in the human body are examples of biological catalysts that speed up digestion and other life processes.

An inhibitor has the opposite effect of a catalyst. It slows down or prevents a chemical reaction. Inhibitors may interfere with reactants or block a catalyst’s action. For example, food preservatives act as inhibitors by slowing chemical reactions that cause food to spoil.

Review:

1. What happens during a synthesis reaction?
2. What happens in a single-displacement reaction?
3. Compare an exothermic reaction to an endothermic reaction.