A chemical reaction occurs when atoms combine or change their bonding partners. Consider the combustion reaction that takes place in the flame of a propane stove. When propane (C3H8) reacts with oxygen gas (O2), the carbon atoms become bonded to oxygen atoms instead of to hydrogen atoms and the hydrogen atoms become bonded to oxygen instead of carbon. As the covalently bonded atoms change partners, the composition of the matter changes and propane and oxygen gas become carbon dioxide and water. This chemical reaction can be represented by the equation
C3H8 + 5 O2 → 3 CO2 + 4 H2O + energy
In this equation, the propane and oxygen are the reactants, and the carbon dioxide and water are the products. In this case, the reaction is complete: all the propane and oxygen are used up in forming two products. The arrow symbolizes the direction of the chemical reaction. The numbers preceding the molecular formulas balance the equation and indicate how many molecules are used or are produced. In this and all other chemical reactions, matter is neither created nor destroyed. The total number of carbons on the left equals the total number of carbons on the right. However, there is another product of this reaction - energy. The heat and light of the stove’s flame reveal that the reaction of propane and oxygen releases a great deal of energy. Energy is defined as the capacity to do work but on a more intuitive level, it can be thought of as the capacity for change. Chemical reactions do not create or destroy energy but changes in energy usually accompany chemical reactions. In the reaction between propane and oxygen, the energy that was released as heat and light was already present in the reactants in another form, called potential chemical energy.
In some chemical reactions, energy must be supplied from the environment (for example, some substances will react only after being heated) and some of this supplied energy is stored as potential chemical energy in the bonds formed in the products. We can measure the energy associated with chemical reactions using a unit called a calorie (cal). A calorie* is the amount of heat energy needed to raise the temperature of 1 gram of pure water from 14.5°C to 15.5°C. Another unit of energy that is increasingly used is the joule (J). Compare data on energy: joules to joules and calories to calories. The two units can be interconverted: 1 J = 0.239 cal, and 1 cal = 4.184 J. Thus, for example, 486 cal = 2,033 J, or 2.033 kJ. Although defined in terms of heat, the calorie and the joule are measures of any form of energy — mechanical, electric, or chemical. Many biological reactions have much in common with the combustion of propane. The fuel is different — it is the sugar glucose, rather than propane — and the reactions proceed by many intermediate steps that permit the energy released from the glucose to be harvested and put to use by the cell. But the products are the same: carbon dioxide and water. These reactions were key to the origin of life from simpler molecules. Energy changes, oxidation– reduction reactions and several other types of chemical reactions that are prevalent in living systems will be presented in the sections that follow.