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How to Balance Chemical Equations

What is a Chemical Equation?

A chemical equation is a way of showing the reactants and products of a chemical reaction with their ratios. A balanced equation shows the ratio in which the reactants react, and the ratio in which the products are produced.

It’s a shorthand way of clearly defining the reaction ratios (or ‘stoichiometry’) without having to write a paragraph of text.

And, because chemical symbols are international, an equation is written the same regardless of language. An equation that you write would be easily understood by a chemist who doesn’t speak English.

Chemical Equation vs Word Equation

You likely know that we can write an equation using words We do so by writing the names of the reactants on the left side of the arrow, and the names of the products to the right of the arrow.

A word equation describes the basics of what is happening, but doesn’t tell us any more.

It tells a story. But it leaves out important details.

A word equation misses out the ratio in which reactants react and that products are formed. It doesn’t usually show the state (solid, liquid, gas or aqueous). And, of course, it doesn’t include formulae.

The balanced chemical equation can show all of these simply. At a glance. That’s why we use chemical equations so widely, so frequently.

What is the Balanced Equation?

A balanced equation for a reaction shows the ratios, the states and the formulae of everything reacting and being produced.

In short, the balanced equation tells you the whole story in one simple line.

Well, almost all of the story.
You may need to add to it if you need to show the reaction conditions or the use of a catalyst. But usually, there is all the information you need.

In this episode:

  • How to write a chemical equation
  • The GOLDEN RULE of balancing equations
  • What do we mean by “balancing an equation”?
  • How to balance an equation
  • Definition of full, ionic and half equations

How to Balance a Chemical Equation

You may need to balance an equation for all sorts of reasons.

  • You’ve done a lab practical and need to include the equation in your report.
  • You need to do a calculation to find how many moles of a substance were made, or how many moles were used.
  • You need to do a calculation to find how many grams of substance are needed or how many grams were used.

Here’s the process you need to follow:

Step 1: Word Equation

Write the word equation.

The simplest way to do this is to just write the names of all the reactants on the left of the arrow, and the names of all the products on the right-hand side.
You can work out the products and reactants from a description of the reaction.

We’re going to use an example:
The reaction between sodium carbonate and nitric acid produces sodium nitrate, water and carbon dioxide. We could write the word equation like this…

sodium carbonate + nitric acid → sodium nitrate + water + carbon dioxide

Step 2: Replace Words with Formulae

The next step is to replace each substance’s name with its formula.

It is vital to use the correct formula for each substance. This is the golden rule of balancing equations – you can only balance equations if you use the correct formulae.

In our example…

Na2CO3 + HNO3 → NaNO3 + H2O + CO2

Step 3: Balance the Atoms

Count the number of atoms of each element on the left-hand side of the arrow, and do the same for the atoms on the right-hand side of the arrow.

If the number of atoms on each side match exactly then the equation is already balanced – meaning that 1 mole of each reactant produces exactly 1 mole of each product.

But if the number of atoms doesn’t match, you will need to do some more work to balance the atoms on each side. But how do you do that?

You need to change the number of moles of one or more of the substances in your equation so that the number of atoms on each side match.

Let’s work through our example to demonstrate this. So far we replaced the names with formulae:

Na2CO3 + HNO3 → NaNO3 + H2O + CO2

Next, we need to see if this is already balanced. We do this by counting the number of atoms of reactants and products to see whether they match.

Reactant side (to the left of the arrow):
Na – 2
C – 1
H – 1
N – 1
O – 6

Product side (to the left of the arrow):
Na – 1
C – 1
H – 2
N – 1
O – 6

They don’t match. The equation isn’t balanced. This isn’t the ratio in which the reactants react.

So we need to think about different numbers of moles for some of the substances.
Let’s start by noticing that there are two sodium atoms in the reactants and only one in the products. What happens if we consider getting two moles of the product sodium nitrate?
That would give two sodium atoms on the right side, as needed. But we need to use two moles of nitric acid. Why? So that there can be two moles of sodium nitrate in the products.

Let’s write out the equation with those adjustments, then count the atoms on each side of the equation:

Na2CO3 + 2HNO32NaNO3 + H2O + CO2

Reactant side:
Na – 2
C – 1
H – 2
N – 2
O – 9

Product side:
Na – 2
C – 1
H – 2
N – 2
O – 9

The number of atoms on each side now matches. This is the balanced equation.

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