SAT Subject Test Chemistry




Introduction to Chemistry


Definition of Energy

The concept of energy plays an important role in all of the sciences. In chemistry, all physical and chemical changes have energy considerations associated with them. To understand how and why these changes happen, an understanding of energy is required.

Energy is defined as the capacity to do work. Work is done whenever a force is applied over a distance. Therefore, anything that can force matter to move, to change speed, or to change direction has energy. The following example will help you understand this definition of energy. When you charge a battery with electricity, you are storing energy in the form of chemical energy. The charged battery has a capacity to do work. If you use the battery to operate a toy car, the stored energy is transformed into mechanical energy that exerts a force on the mechanism that turns the wheels and makes the car move. This process continues until the charge or stored energy is completely used. In its uncharged condition, the battery no longer has the capacity to do work.

Work itself is measured in joules, and so is energy. In some problems, however, energy may be expressed in kilocalories. The relationship between these two units is that 4.18 × 108 joules ( J) equals 1 kilocalorie (kcal).

Forms of Energy

Energy may appear in a variety of forms. Most commonly, energy in reactions is evolved as heat. Some other forms of energy are lightsoundmechanical energyelectrical energy, and chemical energy. Energy can be converted from one form to another, as when the heat from burning fuel is used to vaporize water to steam. The energy of the steam is used to turn the wheels of a turbine to produce mechanical energy. The turbine turns the generator armature to produce electricity, which is then available in homes for use as light or heat, or in the operation of many modern appliances.

Two general classifications of energy are potential energy and kinetic energy. Potential energy is stored energy due to overcoming forces in nature. Kinetic energy is energy of motion. The difference can be illustrated by a boulder sitting on the side of a mountain. It has a high potential energy due to its position above the valley floor. If it falls, however, its potential energy is converted to kinetic energy. This illustration is very similar to the situation of electrons cascading to lower energy levels in the atomic model described in Chapter 2.

Types of Reactions (Exothermic Versus Endothermic)

When physical or chemical changes occur, energy changes are involved. Change of heat content can be designated as ΔH. The heat content (H ) is sometimes referred to as the enthalpy. Every system has a certain amount of heat. This changes during the course of a physical or chemical change. The change in heat content, ΔH, is the difference between the heat content of the products and that of the reactants. The equation is:

ΔH = Hproducts – Hreactants

If the heat content of the products is greater than the heat content of the reactants, ΔH is a positive quantity (ΔH > 0) and the reaction is endothermic. If, however, the heat content of the products is less than the heat content of the reactants, ΔH is a negative quantity (ΔH < 0) and the reaction isexothermic. This relationship is shown graphically in Figures 1 and 2. This topic is developed in detail in Chapter 8.

Conservation of Energy

Experiments have shown that energy is neither gained nor lost in physical or chemical changes. This principle is known as the Law of Conservation of Energy and is often stated as follows: Energy is neither created nor destroyed in ordinary physical and chemical changes. If the system under study loses energy, the reaction is exothermic and the ΔH is negative. Therefore, the system’s surroundings must gain the energy that the system loses so that energy is conserved.