## Types of Energy

The various types of energy include kinetic, potential, and chemical energy.

### Learning Objectives

Differentiate between types of energy

### Key Takeaways

#### Key Points

• All organisms use different forms of energy to power the biological processes that allow them to grow and survive.
• Kinetic energy is the energy associated with objects in motion.
• Potential energy is the type of energy associated with an object’s potential to do work.
• Chemical energy is the type of energy released from the breakdown of chemical bonds and can be harnessed for metabolic processes.

#### Key Terms

• chemical energy: The net potential energy liberated or absorbed during the course of a chemical reaction.
• potential energy: Energy possessed by an object because of its position (in a gravitational or electric field), or its condition (as a stretched or compressed spring, as a chemical reactant, or by having rest mass).
• kinetic energy: The energy possessed by an object because of its motion, equal to one half the mass of the body times the square of its velocity.

Energy is a property of objects which can be transferred to other objects or converted into different forms, but cannot be created or destroyed. Organisms use energy to survive, grow, respond to stimuli, reproduce, and for every type of biological process. The potential energy stored in molecules can be converted to chemical energy, which can ultimately be converted to kinetic energy, enabling an organism to move. Eventually, most of energy used by organisms is transformed into heat and dissipated.

### Kinetic Energy

Energy associated with objects in motion is called kinetic energy. For example, when an airplane is in flight, the airplane is moving through air very quickly—doing work to enact change on its surroundings. The jet engines are converting potential energy in fuel to the kinetic energy of movement. A wrecking ball can perform a large amount of damage, even when moving slowly. However, a still wrecking ball cannot perform any work and therefore has no kinetic energy. A speeding bullet, a walking person, the rapid movement of molecules in the air that produces heat, and electromagnetic radiation, such as sunlight, all have kinetic energy.

### Potential Energy

What if that same motionless wrecking ball is lifted two stories above a car with a crane? If the suspended wrecking ball is not moving, is there energy associated with it? Yes, the wrecking ball has energy because the wrecking ball has the potential to do work. This form of energy is called potential energy because it is possible for that object to do work in a given state.

Objects transfer their energy between potential and kinetic states. As the wrecking ball hangs motionlessly, it has $\text{0%}$ kinetic and $\text{100%}$ potential energy. Once the ball is released, its kinetic energy increases as the ball picks up speed. At the same time, the ball loses potential energy as it nears the ground. Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.

Potential energy vs. kinetic energy: Water behind a dam has potential energy. Moving water, such as in a waterfall or a rapidly flowing river, has kinetic energy.

### Chemical Energy

Potential energy is not only associated with the location of matter, but also with the structure of matter. A spring on the ground has potential energy if it is compressed, as does a rubber band that is pulled taut. The same principle applies to molecules. On a chemical level, the bonds that hold the atoms of molecules together have potential energy. This type of potential energy is called chemical energy, and like all potential energy, it can be used to do work.

For example, chemical energy is contained in the gasoline molecules that are used to power cars. When gas ignites in the engine, the bonds within its molecules are broken, and the energy released is used to drive the pistons. The potential energy stored within chemical bonds can be harnessed to perform work for biological processes. Different metabolic processes break down organic molecules to release the energy for an organism to grow and survive.

Chemical energy: The molecules in gasoline (octane, the chemical formula shown) contain chemical energy. This energy is transformed into kinetic energy that allows a car to race on a racetrack.

## Energy Changes in Chemical Reactions

Chemical reactions often produce changes in energy.

### Learning Objectives

Describe the types of energy changes that can occur in chemical reactions

### Key Takeaways

#### Key Points

• Chemical reactions often involve changes in energy due to the breaking and formation of bonds.
• Reactions in which energy is released are exothermic reactions, while those that take in heat energy are endothermic.

#### Key Terms

• endothermic: A description of a chemical reaction that absorbs heat energy from its surroundings.
• enthalpy: In thermodynamics, a measure of the heat content of a chemical or physical system. The change in enthalpy of a chemical reaction is symbolized as ΔH.
• exothermic: A description of a chemical reaction that releases heat energy to its surroundings.

Due to the absorption of energy when chemical bonds are broken, and the release of energy when chemical bonds are formed, chemical reactions almost always involve a change in energy between products and reactants. By the Law of Conservation of Energy, however, we know that the total energy of a system must remain unchanged, and that oftentimes a chemical reaction will absorb or release energy in the form of heat, light, or both. The energy change in a chemical reaction is due to the difference in the amounts of stored chemical energy between the products and the reactants. This stored chemical energy, or heat content, of the system is known as its enthalpy.

### Exothermic Reactions

Exothermic reactions release heat and light into their surroundings. For example, combustion reactions are usually exothermic. In exothermic reactions, the products have less enthalpy than the reactants, and as a result, an exothermic reaction is said to have a negative enthalpy of reaction. This means that the energy required to break the bonds in the reactants is less than the energy released when new bonds form in the products. Excess energy from the reaction is released as heat and light.

Chemical reaction: A thermite reaction, which produces molten iron.

### Endothermic Reactions

Endothermic reactions, on the other hand, absorb heat and/or light from their surroundings. For example, decomposition reactions are usually endothermic. In endothermic reactions, the products have more enthalpy than the reactants. Thus, an endothermic reaction is said to have a positive enthalpy of reaction. This means that the energy required to break the bonds in the reactants is more than the energy released when new bonds form in the products; in other words, the reaction requires energy to proceed.

The decomposition of water into hydrogen and oxygen: When water is heated to over 2000 degrees Celsius, a small fraction will decompose into hydrogen and oxygen. Significant heat energy is needed for this reaction to proceed, so the reaction is endothermic.