Chemical reactions rearrange the atoms of reactants into new combinations as products. These processes usually give off or absorb heat, allowing the reaction to be described as exothermic or endothermic, respectively. Classifying reactions allows chemists to understand what occurs during a reaction or even to predict the products of a reaction.

Endothermic Reactions

Endothermic reactions absorb heat, meaning the reaction uses more heat than is released in its products. Chemical reactions require energy to break bonds, but energy is released when new bonds form. In endothermic reactions, as bonds break, the reaction absorbs more energy than is released. You can observe this process by measuring the temperature of a substance before a reaction and comparing it to the temperature of the products after a chemical reaction takes place -- the product will have a lower temperature because heat was absorbed in the reaction.

Exothermic Reactions

During exothermic reactions, more energy is released as new bonds form than the energy used to break bonds in the reactants. The reaction releases heat, and the temperature of the product rises. In addition to measuring the temperature change in a reaction, you can calculate the enthalpy change of a reaction. Enthalpy change is the sum of the energy used in bond breaking in reactants with the energy released in bond making in products. If the enthalpy change is negative, the reaction is exothermic because more energy is released in the products than was used to break up the reactants. A positive enthalpy change represents an endothermic reaction.

Vinegar and Baking Soda

Because the acid-base reaction between vinegar, or acetic acid, and baking soda, or sodium bicarbonate, creates expanding foam with carbon dioxide bubbles, many people assume the reaction gives off heat and is exothermic. This reaction, however, is endothermic, which you can observe by measuring the temperature of vinegar and comparing it to the temperature of the mixture after you add sodium bicarbonate. Put 2 tablespoons of vinegar in a glass beaker or cup; place the end of a thermometer into the vinegar and record the temperature. Leave the thermometer in the cup and add 1/2 tablespoon of baking soda to the cup. Watch the thermometer for a temperature change; you will notice the temperature drop.

Vinegar and Steel Wool

The chemical reaction between vinegar and steel wool is an exothermic reaction. Place 2 tablespoons of vinegar in a glass beaker or cup, and put the end of a thermometer in the vinegar. Record the temperature of the vinegar, then remove the thermometer. Place a piece of steel wool into the cup, so the wool is completely covered by the vinegar. Wait two minutes, then pour out the vinegar. Put the thermometer into the steel wool and record the temperature. Vinegar breaks down the protective coating of steel wool, allowing the air to react with the metal in it. As the exposed iron in the steel wool reacts with oxygen in the air, it forms rust and gives off heat, making the reaction exothermic. You will notice the temperature shown by the thermometer rise.