4.1 Chemical Equations

Learning Objectives

  • Define chemical equation.
  • Identify the parts of a chemical equation.


A chemical reaction expresses a chemical change. For example, one chemical property of hydrogen is that it will react with oxygen to make water. This can be expressed as:

Hydrogen reacts with oxygen to make water

We can represent this chemical change more succinctly as:

hydrogen + oxygen → water

In this case, the + sign means that the two substances interact chemically with each other, and the → symbol implies that a chemical reaction takes place. But substances can also be represented by chemical formulas. Remembering that hydrogen and oxygen, both exist as diatomic molecules, we can rewrite our chemical change as:

[latex]\ce{H}_{2} + \ce{O}_{2} {\rightarrow} \ce{H_{2}O}[/latex]

This is an example of a chemical equation, which is a concise way of representing a chemical reaction. The initial substances are called reactants, and the final substances are called products.

Unfortunately, it is also an incomplete chemical equation. The law of conservation of matter says that matter cannot be created or destroyed. In chemical equations, the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the products. If we count the number of hydrogen atoms in the reactants and products, we find two hydrogen atoms. But if we count the number of oxygen atoms in the reactants and products, we find that there are two oxygen atoms in the reactants but only one oxygen atom in the products.

What can we do? Can we change the subscripts in the formula for water so that it has two oxygen atoms in it? No, you cannot change the formulas of individual substances because the chemical formula for a given substance is a representation of the chemical composition of that substance.

What you can do, however, is to change the number of molecules that react or are produced. We do this one element at a time, going from one side of the reaction to the other, changing the number of molecules of a substance until all elements have the same number of atoms on each side. In our example, the balanced chemical equation for the reaction between hydrogen and water is as follows:

[latex]\ce{2H}_{2} + \ce{O}_{2} {\rightarrow} \ce{2H}_{2}{O}[/latex]

Main Components of a Chemical Equation

  • Reactants: the starting materials of the reaction. Reactants are written on the left side of the equation.
  • Products: the materials obtained at the end of the reaction and written on the right side of the equation.
  • An arrow is drawn between the reactants and products, pointing towards the products to indicate a chemical change.
  • Conditions required for the reaction to occur: these include pressure and temperature and are written above the arrow.
  • Physical state of the substances involved: this is written next to the chemical formulas using brackets and abbreviations, as shown next. To illustrate Solid – (s), Liquid – (l), Gas – (g), Aqueous solution (a substance dissolved in water) – (aq).
  • Coefficient: the numbers written before the chemical formulas to balance the chemical equation.


Let’s take the photosynthesis reaction that occurs in plants as an example. Photosynthesis produces glucose and oxygen in the presence of a pigment known as chlorophyll using sunlight, carbon dioxide and water, as shown in the equation below.

[latex]\ce{6CO_{2}} + \ce{6H_{2}O} {\rightarrow} \ce{C_{6}H_{12}O_{6}} + \ce{6O_{2}}[/latex]

According to the photosynthesis equation shown above:

Reactants: [latex]\ce{CO}_{2}, \ce{H_{2}O}[/latex] as they are the starting materials and are written on the left side of the arrow.

Products: [latex]\ce{C_{6}H_{12}O_6}, \ce{O}_{2}[/latex] as these are the ending products and written on the right side of the arrow.

The conditions required for the reaction to occur are energy and chlorophylls.

The physical states of the reactants and products are written next to the chemical formula of reactants and products. For instance: [latex]\ce{CO}_{2}[/latex] is gaseous state, and the product, glucose in a solid state.

According to the balanced chemical equation, six [latex]\ce{CO}_{2}[/latex] molecules react with six [latex]\ce{H_{2}O}[/latex] molecules to produce one molecule of [latex]\ce{C_{6}H_{12}O_6}[/latex] and six molecules of [latex]\ce{O}_{2}[/latex].


Chemical Reactions in Everyday Life

Chemical reactions are an integral part of our everyday lives, and they occur in various forms and settings, from the functioning of our bodies to the products we use and consume. Here are some examples of chemical reactions along with their corresponding chemical equations:

  • Combustion in a car engine: when you start your car, gasoline (hydrocarbons) combine with oxygen to produce energy, carbon dioxide, and water vapour, as shown in the following equation.

[latex]\ce{2C_{8}H_{18}}{(l)} + \ce{25O}_{2}{(g)}{\rightarrow} \ce{16CO}_{2}{(g)} + \ce{18H}_{2}{O}{(g)}[/latex]

  • Baking bread: during the baking of bread, yeast consumes glucose and produces ethanol and carbon dioxide gas, causing the dough to rise, as displayed in the equation below.

[latex]\ce{C_{6}H_{12}O_{6}}{(aq)} {\rightarrow} \ce{2C_{2}H_{5}OH}{(aq)} + \ce{2CO}_{2}{(g)}[/latex]

  • Rusting of iron: when iron is exposed to moisture and oxygen, it forms iron oxide (rust):

[latex]\ce{4Fe}{(s)} + \ce{3O}_{2}{(g)} + \ce{6H_{2}O}{(l)} {\rightarrow} \ce{4Fe(OH)}_{3}{(s)}[/latex]


Image of a rusty bicycle with a cane basket.
Figure 4.1.1 Rusty bicycle. Image attribution: old rusty vintage bicycle © sakhorn38-stock.adobe.com.



Key Takeaways

  • A chemical equation is a concise description of a chemical reaction.
  • A chemical equation consists of several components, such as reactants, products, an arrow, chemical formulas, etc., each serving a specific purpose to describe the reaction.
  • Proper chemical equations are balanced.
  • Chemical reactions play a diverse and essential role in our daily lives.


Practice Questions





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