Types of Chemical Reactions

Chemical reactions take place around us, and even inside us, every day. There are so many different chemical directions that scientists find it is helpful to classify them into groups. There are four main types of chemical reactions:

1. Synthesis
2. Decomposition
3. Replacement (also known as Displacement)
4. Combustion.

But before reading this, we strongly suggest you get familiar with the way we write chemical equations by reading the backgrounder on chemical equations. This will allow you to better understand the examples used below to explain the different types of reactions.

Synthesis Reactions

In a synthesis reaction, two or more atoms or molecules combine to form a compound. In other words, two or more reactants come together to form a product. We can use the symbol A to represent one of the reactants and B to represent the other reactant. In a synthesis reaction, A and B combine to form AB.

The general formula for a synthesis reaction is:

The physical and chemical properties of A, B and AB are different. An example of a synthesis reaction is the formation of water (H₂O). This reaction occurs when hydrogen gas (H₂) reacts with oxygen (O₂) gas. The resulting product is liquid water.

This is a pretty incredible reaction when you think about it. Hydrogen and oxygen gases are both highly combustible gases. But when combined, their product is a substance that can put out fires!

Not all synthesis reactions result in the combination of two substances into one. Some synthesis reactions can result in more than one product.

Photosynthesis is one of the most important chemical reactions on Earth. It allows plants and some microbes to convert water and carbon dioxide gas into storable sugar and oxygen. This oxygen is critical for life on Earth!

Here is a cool synthesis reaction that is sometimes called the Pharaoh's snake. In this reaction, calcium gluconate (C₁₂H₂₂CaO₁₄) reacts with oxygen (O₂) in the presence of a heat source to form water (H₂O), carbon dioxide (CO₂), calcium oxide (CaO) and carbon (C).

Decomposition Reactions

Decomposition reactions are essentially the reverse of synthesis reactions. This type of reaction involves a compound that breaks apart into two or more atoms or molecules. This often occurs when atoms or molecules are heated. AB is broken down to form A and B. Similar to synthesis, the physical and chemical properties of AB, A, and B are different.

The general equation for a decomposition reaction is:

A specific example of this reaction is the breakdown of water into hydrogen and oxygen gas. This happens when electricity passes through the water in a process called electrolysis.

An important decomposition reaction happens when we bake some of our favourite foods, like cookies. Many recipes include baking soda as an ingredient. Baking soda, or sodium bicarbonate NaHCO_3(s), is used to make foods moist and fluffy. But, how does it work?

When the cookie batter is put in the oven, heat is added to the mixture. When the temperature of the batter reaches temperatures above 80℃ (176°F), the baking soda breaks down into sodium carbonate (Na₂CO₃), water vapour (H₂0), and carbon dioxide gas (CO₂).

Did you know?

Sodium carbonate is also known as soda ash. Read more about soda ash in this backgrounder.

The bubbles of gas get trapped by the flour mixture as it hardens, giving us the fluffy cookies we love!

Replacement Reactions

As the name suggests, in a replacement reaction one element replaces another element. There are two types of replacement reactions. These are single replacement reactions and double replacement reactions.

Single Replacement Reactions

In a single replacement reaction, a single uncombined element C replaces another element in a compound AB. In this case, two reactants yield two products.

The general equation for a single replacement reaction is:

An example of a simple replacement reaction is the explosive reaction that happens when potassium reacts with water.

In this reaction, potassium (K) reacts with water (H₂O). This reaction produces hydrogen gas and solid potassium hydroxide (KOH) in solution.

The equation for the reaction between potassium and water is:

The reaction between potassium and water also releases energy in the form of heat. This makes it an exothermic reaction. Another reaction happens soon after the hydrogen gas is produced. Heat from the reaction of potassium and water causes the hydrogen gas to combust (see more about combustion reactions below). 

Double Replacement Reactions

Now that you know what happens in a single replacement reaction, what do you think happens in a double replacement reaction? If you thought that two parts switched then you would be right!

In a double replacement reaction, two reactant compounds form two new product compounds through the exchange of ions. Ions are charged molecules or atoms. Cations are positive ions (+) and anions are negative ions (-). Only cations can replace cations and only anions can replace anions.

The general formula for a double replacement reaction is:

In this case A and C are cations and B and D are anions. Therefore only A can replace C and only B can replace D.

The most well-known and widely used double replacement reactions are acid-base reactions.

Acid-Base Reactions

An acid–base reaction is a special type of double replacement reaction that occurs between an acid and a base. Acids are substances that are close to 0 on the pH scale. Acidic foods have a sour taste. This includes things such as vinegar and lemon juice. Bases are substances that are close to 14 on the pH scale. Bases are most often seen in our cleaning cupboards. This includes things such as bleach and detergent.

Did you know?

The pH scale gives a measurement of the amount of H⁺ in a solution. You can explore acids and bases in this hands-on activity.

Typically, an acid-base reaction results in the formation of a precipitate.

Acids and bases have a very unique and interesting relationship. When they react together, each is able to cancel out, or neutralize, the acidic or basic properties of the other. That is why acids and base reactions are also called neutralization reactions.

In many neutralization reactions, a positive hydrogen ion (H⁺) in an acid reacts with a negative hydroxide ion (OH^-) ion in a base, forming water.

It is important to note that the word “salt” is a general term which applies to the products of all such acid-base reactions. The table salt that we eat can be produced through this reaction. You can make sodium chloride, better known as table salt, by reacting hydrochloric acid and sodium hydroxide.

See this reaction in action by watching the video below:

Combustion Reactions

A combustion reaction occurs when oxygen gas (O₂) reacts with certain types of compounds. We often call the other compound fuel. A more familiar term for combustion reactions is burning. The most common products of combustion reactions are carbon dioxide and water. Water is rarely seen as it is its gaseous form.

When a fuel reacts in such a way that only carbon dioxide and water are produced, we call it a complete combustion reaction.

The burning of butane, which is used in lighters, is an example of complete combustion. The chemical equation for this is:

Sometimes fuel does not completely react. In this case we call it an incomplete combustion reaction. Other products that result from the incomplete combustion of carbon-based fuels. These include carbon monoxide, a dangerous odorless gas and soot, which are particles of carbon.

Let’s see how combustion works when a candle burns.

When you apply a flame to the wick of a candle, a combustion reaction starts to happen. Most candle wicks are made from cotton, which burns easily. The burning wick gets its oxygen from the air. Without oxygen, the wick will stop burning. The heat produced during this combustion reaction will melt the solid wax and turn it into a gaseous form of the wax. The tiny gaseous molecules of wax then undergo their own combustion reaction with the oxygen in the air. The major products are carbon dioxide and water vapour. There is also considerable soot, made from carbon atoms, as well as some carbon monoxide (CO).

A candle flame is a good example of incomplete combustion. The engines in vehicles also undergo incomplete combustion when they burn gasoline and diesel fuel.

Did you know?

In microgravity, a candle shape is round because there is no upward movement of hot gases. 

What is important to know about combustion reactions is that they are exothermic reactions.This means they produce heat. People often use combustion reactions for the heat and light that these reactions produce. Whether it is burning wood to create a toasty campfire, using natural gas to heat food and homes, or the combustion of acetylene with oxygen in a welder’s torch, combustion reactions play an important role in our lives.

Actually…

ALL types of chemical reactions play an important role in our lives!