CH4(g) + 2 O2 (g) CO2 (g) + 2 H2O (g) + heat
A chemical reaction that produces energy is called an exothermic reaction, and a reaction that absorbs energy is called an endothermic reaction. Exothermic reactions produce heat and warm their surroundings; all combustion reactions are exothermic. There are many familiar examples of combustion reactions, including the burning of wood.
Endothermic reactions absorb heat, thus cooling their surroundings. “Cold Packs” that are used to treat some sports injuries are an example of an application of an endothermic reaction. Perhaps the most remarkable example of a combustion process is digestion.
Foods produce the same amount of energy, whether they are burned outside the body or broken down by the metabolic processes involved in digestion. Therefore, one method scientists use to determine the total energy content of food is to burn the food in a special container and measure the heat produced. This special container is called a calorimeter. Like a speedometer that measures speed, and a thermometer that measures temperature, a calorimeter measures heat (calories). The S.I. unit in which energy is measured is called the joule (J). Often another unit is used when measuring the energy content of food; this unit is called the calorie (cal). There are 4.184 joules in 1 calorie. More often the kilocalorie (kcal) is used; remember, there are 1000 calories in 1 kilocalorie. One calorie is the energy required to raise the temperature of 1 g of water by 1 oC. It is important to note that there is a difference between calories as defined above and the calories recorded on food packaging. It is common to list the…