Chemical Reaction — Definition & Overview

(Video) What Triggers a Chemical Reaction?

by Kareen Jarrah via TED-Ed

What do rotten bananas and a freshly-baked cake have in common? $$\color{#8925AE}{\text{The answer: Chemical reactions}}\\$$ In the video above, Kareen Jarrah explains how energy and matter come together in common chemical reactions in the form of entropy and enthalpy. Energy can be released (exothermic) or put in to (endothermic) reactions. Baking a cake is an example of an endothermic reaction, where energy is added in the form of heat to cause a chemical change.

Chemical Reactions in Biology

A chemical reaction is a process that changes or transforms one type of chemical into another. The key idea is that the bonds connecting atoms are changed. In the animation below, the blue atom (nitrogen) breaks a bond with the gray atom (hydrogen). The same blue atom is now able to form a bond with the black atom (carbon). Life is the result of countless reactions happening all at the same time, most often between carbon, hydrogen, nitrogen, and oxygen atoms.

It's also really, really important to remember that during chemical reactions the mass and energy must be conserved. You cannot try to create or destroy energy and mass; they are always constant. When living organisms eat food for energy, our bodies break it down into usable chemicals and energy. We describe this energy as calories. The energy from food will change forms, but it is never lost. Therefore, matter and energy is conserved.

If you look at the picture above, the left-hand side is called the reactants and the right-hand side is called the products. Reactants are the atoms or molecules that enter into a reaction. The compounds that are produced by the reaction are called products. The arrow between the reactants and products always points towards the products of the reaction.

So, the reactants react and the products are produced.

Sometimes we cannot see when reactions are happening, but indications for reactions occurring are color change, temperature change, and state change. The reactants above are both gases, but the water formed is a liquid. The formation of water is an indication that a reaction is occurring.

When making a cake, the flour, sugar, and other ingredients are reactants. After you add them together and bake them (add heat), you've made something pretty different from the original ingredients. The finished cake is the product and we cannot easily remove the original reactants without another reaction.

Reactions in Biology

We only need to worry about three types of chemical reactions in biology: decomposition, synthesis, and replacement reactions.

• A decomposition reaction happens when a molecule decomposes, or falls apart.
• Synthesis reactions happen when two atoms or molecules are joined together, or synthesized.
• The last type of chemical reaction is a replacement reaction, where one or more of the reactants are replaced.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Chemical Reaction}}$ : A process that changes or transforms one type of chemical to another

$\color{#8925AE}{\text{Reactant}}$ : The atoms or molecules entering into the reaction; the ingredients

$\color{#8925AE}{\text{Product}}$ : The atoms or molecules leaving the reaction; the finished product

What Is a Chemical Reaction?

It is a reaction which occurs when a molecule experiences a chemical change, rather than a physical change. This means that it has a different chemical identity or chemical composition than it started with. We can compare making soup to making cake to understand the difference in chemical and physical change. When you bake a cake, the individual ingredients cannot be removed from the cake once it is made. The cake formed from a chemical change. However, when we make chicken noodle soup, we can remove noodles from the rest of the soup. The soup, even though it was heated, is formed through a physical change.

There are many types of reactions that chemicals undergo: combustion reactions, acid-base reactions, oxidation-reduction reactions, and more. The reactions displayed below are synthesis, decomposition, single replacement, and double replacement (from top to bottom).

Parts of a Chemical Reaction

In each reaction, there are reactants and products. Reactants are written on the left side of the equation, before the arrow. Products are written on the right side of the equation, after the arrow.

Think of the arrow almost as an equals sign. It translates to "is converted into". So, the chemical equation reads "reactant is converted into product".

Major Types of Reactions

In biology, there are generally two main types of reactions: synthesis reactions and decomposition reactions. An example of a biochemical synthesis or chemical reaction in biology is cellular respiration, which creates ATP as the product. A decomposition reaction in the body is when large chemical compounds, like complex carbohydrates, are broken down into smaller components, simple sugars.

Here's a general description:

Key Vocabulary:

1. Reaction
2. Reactant
3. Product
4. Synthesis
5. Decomposition

Reactants and Products

$$A+B\rightarrow C+D$$ $$\textit{A generic chemical reaction}$$

A $\color{#3DB917}{\text{chemical reaction}}$ changes the configuration of atoms. It is key that there is the formation and breaking of chemical bonds. What this means is that within a reaction, some bonds will be broken and some bonds will be formed, but the amount and type of matter is constant. In a chemical reaction, the amount of matter and energy doesn't change, but the form each takes can change. For example, energy is released when bonds are formed. This energy can be released in the form of heat or an exothermic reaction.

There are two main parts of a chemical reaction: the $\color{3db917}{\text{reactants}}$ and the $\color{3db917}{\text{products}}$. The reactants are the chemicals on the left side of the arrow. They are the things that go in to the reaction to make it work. The products are on the right side of the arrow. They are what come out of a reaction. The arrow always points to the products.

There are four types of chemical reactions: synthesis, decomposition, single replacement, and double replacement. $\\$ In synthesis, two or more chemicals come together in order to make a larger chemical.

Decomposition is the reverse of synthesis. A large chemical breaks up into two or more smaller chemicals.

In single replacement, a part of one chemical dissociates from the rest of the chemical and bonds to a different species.

Double replacement is the same as single replacement, but it happens to two chemicals at the same time.

$$\color{#8925ae}{\text{Review}}$$

$\color{#8925ae}{\text{Chemical reaction}}$: A process than changes the configuration of bonds and atoms but does not change the total mass, energy, or charge

$\color{#8925ae}{\text{Reactants}}$: The chemicals that go in to a reaction. Typically on the left side of the arrow, and the arrow always points away from this

$\color{#8925ae}{\text{Products}}$: The chemicals that come out of a reaction. Typically on the right side of the arrow, and the arrow always points toward this

Definition of a Chemical Reaction

A chemical reaction is any process that changes the chemical composition of one or more substances. The change in bonds that join elements in a compound indicate a reaction.

Parts of a Reaction

• The materials that are present before the reaction are called the reactants.
• The materials that are created are called the products.