Table Of Contents: Energy in Reactions
1. Collision Theory
All molecules possess a certain minimum amount of energy. The energy can be in the form of kinetic energy and/or potential energy. In order for a reaction to occur, particles must collide at exactly the right angle with the proper amount of energy. When two particles collide, the kinetic energy of their motion is converted to potential energy. The energy conversion can be represented by a spring between two particles. As the particles move closer together, the spring is compressed. The speed of the particles decreases and the kinetic energy of motion is changed to stored, or potential energy in the spring. If the potential energy is great enough, a reaction will occur. The minimum energy to cause a chemical reaction is called the activation energy.
2. Activation Energy
All reactions have a minimum energy, called the activation energy, which is necessary for a chemical reaction to occur. The activation energy is used to break the bonds between the atoms or molecules of the reactants. For a chemical reaction to take place, there must be an adequate number of molecules with energy equal to, or greater than, the activation energy. The amount of activation energy required to start a reaction is often called the energy barrier. The addition of heat or an appropriate enzyme, a catalyst, are two factors that can activate molecules to overcome the energy barrier. The activation energy barrier prevents most reactions from taking place spontaneously. This is particularly important for biological processes, which ensure a stable environment for all living things.
3. Exothermic Reactions
A chemical reaction that releases energy is called exothermic. In such reactions, some of the energy released from breaking the bonds of the reactants will be released into the surroundings. Because energy is released in an exothermic reaction, the products have a lower energy content than the reactants. The energy that is released can be in the form of heat, electricity and light. For example, in a chemical hot pack the reaction between calcium chloride and water releases heat. The chemical reactions in a battery release electrical energy. And the exothermic reaction in a glow stick releases light.
4. Endothermic Reactions
If energy must be added to reactants in order for the reaction to take place, the reaction is endothermic. In an endothermic reaction, energy is absorbed from the surroundings and therefore the products have higher energy content than the reactants. The energy that is absorbed can be in the form of light or heat. Photosynthesis is an example of an endothermic chemical reaction. During this process, plants use energy captured from the Sun to convert carbon dioxide and water into glucose and oxygen. A cold pack also uses an endothermic reaction. Ammonium nitrate and water absorb heat from the environment and cool off their surroundings.
5. Energy in Reactions
A graph of energy versus reaction progress is often used to show the energy relationships in a reaction. The energy of the reactants increases due to the collisions of the particles. This is shown by the rising curve where the maximum point is the activation energy. The curve then decreases until the energy of the products is reached. When the energy of the reactants is greater than the energy of the products, the reaction is exothermic. When the energy of the reactants is less than the energy of the products, the reaction is endothermic.
6. Reaction Rates
Reaction rates are affected by temperature, concentration, and surface area. As the temperature of a substance increases, the kinetic energy increases. At higher temperatures, more collisions have the necessary energy to occur, so the reaction rate increases. The concentration of a substance is the amount of that substance in a given amount of a solvent, such as water. A higher concentration means more collisions. If the probability of a favorable collision increases, so does the reaction rate. Increasing the surface area of solid reactants exposes more reactant particles to each other, which in turn increases the rate of reaction. Conversely, lowering the temperature, concentration, or surface area will slow down a chemical reaction.
Catalysts play an important part in many chemical processes. A catalyst is a substance that speeds up the rate of a chemical reaction by lowering its activation energy. A catalyst does not get permanently changed and does not get used up in the reaction. Catalysts are important in many industrial chemical applications, as well as in biochemical reactions. In biological systems, the most common catalysts are protein molecules, called enzymes. Enzymes act as catalysts in virtually all chemical reactions within cells.