Energy And Chemical Reactions

In a chemical reaction, one or more substances change to produce one or more different substances. Energy is absorbed or released when chemical bonds are broken and new ones are formed. Living things undergo many thousands of chemical reactions every day. Reactions can vary from highly complex to very simple. The chemical reaction in Figure 2-6 takes place in your blood. Carbon dioxide is taken up from body cells and into the blood when it crosses the thin capillary walls. The carbon dioxide reacts with water in the blood to form carbonic acid. Carbon dioxide is then released into the lung's alveoli and exhaled when the carbonic acid breaks down to carbon dioxide and water.

If the reaction proceeds in only one direction, the reactants are shown on the left side of the equation. In the reaction in Figure 2-6, the reactants are carbon dioxide (CO2) and water (H2O). In a chemical reaction, bonds present in the reactants are broken, the elements are rearranged, and new compounds are formed as the products. The products of this reaction are shown on the right side. In this reaction, the product is carbonic acid (H2CO3). Notice that the number of each kind of atom must be the same on either side of the arrow. Some chemical reactions can proceed in either direction and a two-direction arrow (n) is used. For example, the equation in Figure 2-6 is reversible and can be written as CO2 + H2O n H2CO3.

The energy your body needs is provided by the sugars, proteins, and fats found in foods. Your body continuously undergoes a series of chemical reactions in which these energy-supplying substances are broken down into carbon dioxide, water, and other products. In this process, energy is released for use by your body to build and maintain body cells, tissues, and organs. Metabolism (MUH-TAB-uh-LIZ-uhm) is the term used to describe all of the chemical reactions that occur in an organism.

Activation Energy

For most chemical reactions to begin, energy must be added to the reactants. In many chemical reactions, the amount of energy needed to start the reaction, called activation energy, is large. Figure 2-7 shows the activation energy for a hypothetical chemical reaction.

Certain chemical substances, known as catalysts (KAT-uh-LISTS), reduce the amount of activation energy that is needed for a reaction to take place, as shown in Figure 2-7. A reaction in the presence of the correct catalyst will proceed spontaneously or with the addition of a small amount of energy. In living things enzymes act as catalysts. An enzyme is a protein or RNA molecule that speeds up metabolic reactions without being permanently changed or destroyed.

Activation Energy With and Without a Catalyst o -


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