What is released when a chemical reaction happens?

Chris
Chemical reactions: They’re fundamental to chemistry; they make new things by rearranging other things. They can blow stuff up ... or freeze things quickly. In short, they are awesome.

Table of Contents Show

What Is a Chemical Reaction?
Same Atoms, New Bonds
Types of Chemical Reactions
Energy In and Energy Out
What happens when chemicals react?
What energy is released when the chemical?
What happens when a chemical reaction releases energy?

Brittny
But why do some chemical reactions release massive amounts of energy, while others absorb energy? In a chemical reaction, the main change that occurs relates to the way atoms are connected (or bonded) to each other. In order to change those connections, bonds must be broken and new bonds must be formed. Let’s break down how energy is transferred in these reactions.

Chris
To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:

It takes energy to break bonds.
Energy is released when bonds are formed.

To understand this, consider the chemical reaction between vinegar and baking soda. That’s right —the classic baking soda volcano experiment. The chemical reaction behind this science fair favorite involves baking soda—also known as sodium bicarbonate to chemists—and vinegar, otherwise known as acetic acid.

These compounds react to form the molecules sodium acetate, water, and carbon dioxide. The baking soda and vinegar are called the reactants. The sodium acetate, water, and carbon dioxide that are formed are called the products.

Before the atoms in acetic acid and sodium bicarbonate can be rearranged to form the products, some of the bonds between the atoms in those molecules must be broken, and because the atoms are attracted to one another, it takes energy to pull them apart.

Then, when the products are formed (sodium acetate, water, and carbon dioxide) energy is released because atoms that have an attraction for one another are brought back together.

By comparing the energy absorbed when bonds in the reactants are broken with the energy released when bonds in the products are formed, you can determine whether a chemical reaction releases energy or absorbs energy overall.

Brittny
Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture.

Chemical reactions that absorb (or use) energy overall are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products. Endothermic reactions are accompanied by a decrease in temperature of the reaction mixture.

Chris
You can use energy level diagrams to visualize the energy change during a chemical reaction. To understand these diagrams, compare the energy level of the reactants on one side with that of the products on the other side.

Consider, for example, a diagram that charts the energy change when a candle burns. Wax (C34H70) combusts in the presence of oxygen (O2) to yield carbon dioxide (CO2) and water (H2O). Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic.

Brittny
All of this stuff relates to thermodynamics—the study of heat and its relationship to energy and work. Using thermodynamics, you’ll learn how to calculate the precise amount of energy used or released by chemical reactions. Classifying a chemical reaction as exothermic or endothermic is simple. It comes down to weighing the energy needed to break bonds in the reactants with the energy released when the products are formed.

It’s a simple idea, but one with a lot of power.

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Figure \(\PageIndex{1}\) (Credit: Wisewire - "reaction of calcium carbonate with HCl"; Source: https://www.flickr.com/photos/158504583@N02/36897234506(opens in new window); License: CC-BY 2.0)

Does the term chemical reaction bring to mind an image like this one? In the picture, a chemist is mixing chemicals in a lab. Many chemical reactions take place in labs. However, most chemical reactions do not. Where do they occur? They happen in the world all around you. They even happen inside your own body. In fact, you are alive only because of the many chemical reactions that constantly take place inside your cells.

What Is a Chemical Reaction?

A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right).

Reactants → Products

Chemical reactions may occur quickly or slowly. Look at the two pictures in the figure below. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust.

Figure \(\PageIndex{2}\) (Credit: Fire extinguisher: Official U.S. Navy Page; Screw: Paulnasca; Source: Fire extinguisher: http://www.flickr.com/photos/usnavy/7949633444/(opens in new window); Screw: http://commons.wikimedia.org/wiki/File:Rust_screw.jpg(opens in new window); License: CC by 2.0(opens in new window))

Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from?

A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form.

Same Atoms, New Bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products.

Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The figure below represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water.

Figure \(\PageIndex{3}\): (Credit: CK-12 Foundation; Source: CK-12 Foundation; License: CC BY-NC 3.0(opens in new window))

Types of Chemical Reactions

The chemical reaction in the figure above, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The table below compares these four types of chemical reactions.

Four Types of Chemical Reactions

Type of Reaction	General Equation	Example
Synthesis	A+B → C	2Na + Cl2 → 2NaCl
Decomposition	AB → A + B	2H2O →2H2 + O2
Single Replacement	A+BC → B+ AC	2K + 2H2O → 2KOH + H2
Double Replacement	AB+ CD → AD + CB	NaCl+ AgF → NaF + AgCl
Combustion	fuel + oxygen → carbon dioxide + water	CH4 + 2O2 → CO2 + 2H2O

Q: The burning of wood is a chemical reaction. Which type of reaction is it?

A: The burning of wood—or of anything else—is a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH4).

Energy In and Energy Out

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions.

In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light.
In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment.

Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell?

A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire.

Summary

A chemical reaction is a process in which some substances, called reactants, change into different substances, called products. During the reaction, chemical bonds break in the reactants and new chemical bonds form in the products.
Types of chemical reactions include synthesis, decomposition, replacement, and combustion reactions.
All chemical reactions involve energy. Exothermic reactions release more energy than they use. Endothermic reactions use more energy than they release.

Review

What is a chemical reaction?
Write a general chemical equation that shows the relationship of products to reactants in a chemical reaction.
Contrast exothermic and endothermic chemical reactions.

What happens when chemicals react?

Chemical reactions involve breaking chemical bonds between reactant molecules (particles) and forming new bonds between atoms in product particles (molecules). The number of atoms before and after the chemical change is the same but the number of molecules will change.

What energy is released when the chemical?

chemical energy, Energy stored in the bonds of chemical compounds. Chemical energy may be released during a chemical reaction, often in the form of heat; such reactions are called exothermic. Reactions that require an input of heat to proceed may store some of that energy as chemical energy in newly formed bonds.

What happens when a chemical reaction releases energy?

Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture.