Science Experiments | Chemical Energy - Introduction

Introduction

Chemical energyEnergy stored in chemical bonds. is the energy stored within the bonds of atomsThe smallest unit of an element, made up of protons and neutrons in a central nucleus surrounded by moving electrons. . A bondThe force that holds two atoms together. is the force that holds two atoms together. Different substances have bonds held together by different amounts of energy. When those bonds are released, a chemical reactionAny chemical change in which at least one new substance is formed. takes place, and a new substance is created. Chemical reactions that break these bonds and form new ones sometimes release the excess energy as heatA form of energy produced by the motion of molecules that make up a substance. and sometimes absorb heat energy from whatever is around them.

Thus, heat energy can be produced or absorbed during a chemical reaction. Reactions that release heat energy are called exothermicA chemical reaction that gives off heat energy.. Reactions that take in heat energy from the surrounding environment are called endothermicA chemical reaction that takes in heat energy.. Whether heat energy is given off or absorbed during a chemical reaction depends on the bonds that hold the atoms together.

In a chemical reaction, the original substances are called reactantsA compound present at the beginning of a chemical reaction.. The new substances that are formed are called productsA compound that is formed as a result of a chemical reaction. . When the bonding structure of the products requires less energy than the bonding structure of the reactants, the excess energy may be released as heat. When the bonding structure of the products requires more energy than the structure of the reactants, it gets that energy by removing heat from its surroundings.

For example, when iron rusts, the iron atoms are combining with oxygen moleculesThe smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms. in the air to form iron oxide. The chemical reaction of rusting breaks the bonds in the oxygen molecules, releasing heat energy. The bonds between the oxygen atoms and the iron atoms do not require as much heat energy as the bonds within the oxygen molecules, so a small amount of energy is released, making the reaction exothermic. The amount of heat released is quite small, and the reaction is normally quite slow, so rusting iron does not feel hot to us. Yet, the energy released can be measured with a thermometer. In the first experiment, you will observe the change in temperature resulting from rusting.

Some exothermic reactions are quite common. One is combustionAny chemical reaction in which heat, and usually light, is produced. It is commonly the burning of organic substances during which oxygen from the air is used to form carbon dioxide and water vapor., the burning of organic substances during which oxygen is used to form carbon dioxide and water vapor. The substances formed (ashes, for example) hold less heat energy than the substances burned held. The excess energy is released as heat. The reactions between some chemicals, such as aluminum oxide and iron oxide, can produce great amounts of heat. This reaction is used to produce very high temperatures for industrial purposes.

Endothermic reactions are more rare in nature, but scientists have found ways to create them. For example, an endothermic reaction occurs when you use a chemical cold pack. These packs contain a chemical in powder form that reacts with water. Squeezing the pack breaks down the wall separating the powder from the water. The reaction that occurs absorbs more energy than it releases, making the pack feel cold to you. In the second experiment, you will compare four chemical reactions and determine whether each one is exothermic or endothermic.