Investigation 4: Chemical Energy and Heat

In previous Investigations, students explored how mechanical potential energy can be converted into mechanical kinetic energy, how sound energy can be converted into mechanical energy, and how chemical potential energy can be converted into electrical energy. In turn, students discovered that electrical energy can be converted into light and heat energy as electrical current flows through a light bulb. In this Investigation, students will explore another facet of chemical energy transformation by studying how chemical energy can be converted to heat energy and heat energy to chemical energy.

All chemical reactions involve conversion between heat energy and chemical energy. A chemical reaction occurs when two or more chemicals, called reactants, interact to form a new product or products. The energy in a chemical is contained in its bonds. Atoms of the sugar molecule, sucrose (table sugar), are held together by chemical bonds, for example. The sucrose molecule with its many chemical bonds is shown in the animation here. When a chemical reaction occurs, chemical bonds are broken and new chemical bonds are formed. The atoms basically rearrange during a chemical reaction. 

In some reactions, the reactants contain a greater amount of chemical energy than the products. The excess energy is released from the reaction as heat and is detected as an increase in the temperature of the reaction. Thus, a conversion from chemical energy to heat energy has occurred. This is known as an exothermic reaction. An example of an exothermic reaction is the rusting of iron. Ice melting chemicals work by producing heat as a result of the exothermic reaction of the chemical granules with snow or ice. The heat generated by the reaction is great enough to melt the snow or ice and speed the drying of the water on the surface of the road or sidewalk.

In other reactions, the energy in the bonds of the products is greater than the energy in the bonds of the reactants. This type of chemical reaction requires the addition of energy in order for the reaction to proceed. In general, the addition of energy comes in the form of heat. The creation of a cake from a mixture of flour, salt, butter, milk, sugar, leavening agents, and flavorings is an example of a complex endothermic reaction involving many different chemicals. Because heat must be brought into the reaction from the outside, this type of chemical reaction is called an endothermic reaction. It should be noted that not all endothermic reactions require a heat source such as a hot plate or oven. Some endothermic reactions will occur at room temperature, which indicates that very little additional energy is needed to make the reaction proceed to its conclusion. Chemical ice packs are an example of an endothermic reaction that does not require a heat source other than the heat of its surroundings.

As with other examples of energy conversions observed by the students, endothermic and exothermic reactions are examples of the Law of Conservation of Energy. The heat released from an exothermic reaction is not created as a result of the reaction; instead, it is the result of the conversion of excess chemical energy to heat. The total amount of energy in the reaction remains constant. So, too, the heat absorbed into the endothermic reaction was simply transferred from outside the reaction and converted into chemical energy in the form of bonds.

In this Investigation, students will explore endothermic and exothermic reactions. Students will observe a demonstration of sugar being caramelized as an example of an endothermic reaction. Then students will combine non-chlorine powdered bleach with water to observe an exothermic reaction. In each case, students will identify that a chemical reaction has occurred by looking for signs of chemical change.