Chemical Reactions

Science background gives teachers more in-depth information on the phenomena students explore in this unit. Below is an excerpt from the science background section on chemical reactions.

Chemical Reactions

Every molecule is formed in a chemical reaction between atoms. A result of this process is that the substance formed has different properties than the original substances. For example, at normal room temperature, both oxygen and hydrogen are gasses. When hydrogen and oxygen join as a molecule of water, they change into a liquid instead of a gas at room temperature. You can also see evidence of these changes in the kitchen—when you toast bread or brown meats like steak or chicken, chemical reactions cause the changes you see.

In any chemical reaction, the atoms and molecules that interact together are called reactants. The atoms and molecules produced by the reaction are called products. When the reactants come together, energy breaks the bonds holding the reactants together and rearranges them to form new products. However, the total number of atoms does not change. The mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. This is because matter is never created or destroyed. Therefore, the more reactants you add to the chemical reaction, the more products will form.

An element’s mass at the beginning of a reaction will equal its mass at the end of the reaction. It is always true that mass is conserved in a chemical reaction. However, this can be difficult to measure in the real world because matter can interact with the environment. Remember that if a gas is produced, it will fill whatever space it is in. This is impossible to measure. For this reason, scientists sometimes conduct closed-system experiments, in which matter cannot be exchanged with the environment.

In every chemical reaction, the reactants form a system that interacts with the surrounding environment. A system is a set of connected, interacting parts that form a more complex whole. As the reactants combine and rearrange, energy is exchanged between the system and the environment.

Every chemical reaction needs energy to get started. This initial input of energy is called activation energy. For example, when someone strikes a match to light a candle, they provide the activation energy needed to start a fire, which is a chemical reaction.

Once the reaction begins, some reactions absorb more energy from the environment than they release. Others release more energy into the environment than they absorb. The difference between the different kinds of reactions can be understood by thinking about the energy required to break the bonds of the reactants compared to the energy needed to form the new bonds of the products. Whenever a process occurs in which the system absorbs heat, it is called endothermic. “Endo-” means to draw in.

Energy in Chemical Reactions

In an endothermic reaction, the reactants have less energy than the products. Because energy is never created or destroyed, the energy needed to form the products is absorbed from the environment into the system.

We can’t observe these changes at the molecular level, but we can measure the temperature change that results. We see evidence of this transfer of energy when the environment’s temperature decreases because it means that the reaction has absorbed energy from the environment.

A chemical ice pack uses an endothermic reaction to get cold. Many ice packs have ammonium nitrate and water kept in separate sections with a thin barrier between them. When you break the barrier, the water and ammonium nitrate combine. More energy is needed to break the bonds of the reactants than is released when new bonds form in the products. This is why the ice packs get cold. When the reactants have fully reacted, the chemical reaction stops.

Any process in which the system loses heat to the environment is called exothermic. “Exo-” means to give off. In an exothermic reaction, the reactants have more energy than the products. Because energy is never created or destroyed, energy is released by the reactants and transferred into the environment as heat.

Exothermic Reactions

There are many examples of exothermic reactions. For example, whenever you light a match, you are witnessing an exothermic reaction take place. The light and heat produced are evidence that energy is being released into the environment. Another common example of exothermic reactions occurs in certain animals that produce and release light. Called bioluminescence, this phenomenon occurs in animals that live in the ocean, as well as some land animals such as fireflies.

The strength of a chemical reaction can be measured by the amount of energy absorbed or released by the reaction. When more reactants are added, it increases the amount of energy that is absorbed or released.