Writing a chemical equation

Why write equations?

When a chemist reports a reaction, she does not want to write a paragraph. She writes a short line that tells what went in, what came out, and in what state. This line is a chemical equation. Anyone in the world can read it, in any language.

From words to symbols

First, we can describe a reaction in words.

This is a word equation. The substances on the left are the reactants, the substances on the right are the products, and the arrow means gives or forms.

It is shorter to use the symbols and formulae of the substances. This is a chemical equation.

This is a skeletal equation: it has the formulae, but it is not balanced yet. We learn to balance it in the next lesson.

The steps to write one

1. Write the word equation, with the names.
2. Replace each name by the correct formula.
3. Check whether the equation is balanced. If it is not, balance it (next lesson).
4. Add the state symbols and the conditions, if you know them.

Example. Hydrogen burns in oxygen to give water.

• Word equation: hydrogen + oxygen $\rightarrow$ water.
• Skeletal equation: $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$.

Writing the formulae correctly

• Write the correct formula of every substance. From the last chapter, we know that oxygen is $\text{O}_2$, hydrogen is $\text{H}_2$ and magnesium oxide is $\text{MgO}$.
• Never change a formula to balance an equation. A formula is a fact about the substance. $\text{H}_2\text{O}$ is water, and $\text{H}_2\text{O}_2$ is a different substance (hydrogen peroxide). Change only the numbers in front of the formulae.
• Some elements are made of molecules, and we write them as molecules: $\text{H}_2$, $\text{O}_2$, $\text{N}_2$, $\text{Cl}_2$. Metals such as Mg, Fe and Cu are written with just the symbol.

State symbols

We can show the physical state of each substance with a small symbol after its formula.

Example:

Reading it: solid zinc reacts with sulphuric acid dissolved in water, and gives zinc sulphate dissolved in water, and hydrogen gas.

Conditions of a reaction

We can write a condition above or below the arrow: the temperature, the pressure, a catalyst, or heat, light or electricity.

In some books, a gas that is given out is shown with an arrow pointing up ($\uparrow$), and a precipitate with an arrow pointing down ($\downarrow$).

Counting atoms in a formula

To check an equation, we count the atoms of each element. For a formula with brackets, the number outside the bracket multiplies all the atoms inside.

• $\text{Ca(OH)}_2$ has $1$ Ca, $2$ O and $2$ H.
• $\text{Al}_2(\text{SO}_4)_3$ has $2$ Al, $3$ S and $12$ O.
• $2\,\text{H}_2\text{O}$ has $4$ H and $2$ O. A number in front multiplies the whole formula.

You should find that the oxygen count grows fastest, because the bracket and the number in front both multiply it.

Practise with new numbers

Try it in Python

This program counts the atoms of each element in a formula, even with brackets.

Common mistakes

• Changing a formula to balance. Never do this. Only the numbers in front can change.
• Writing oxygen as O and not $\text{O}_2$. Gases such as oxygen, hydrogen, nitrogen and chlorine are written as molecules.
• Forgetting the bracket. In $\text{Ca(OH)}_2$ the $2$ multiplies both O and H.
• Putting the products on the wrong side. Reactants on the left, products on the right.

Remember these

• A word equation uses names. A chemical equation uses formulae. The arrow means gives.
• State symbols: (s), (l), (g), (aq). Conditions go above or below the arrow.
• Never change a formula to balance an equation.
• The number outside a bracket multiplies all the atoms inside.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Write the word equation for carbon burning in oxygen to make carbon dioxide.

2. What does $(g)$ after a formula mean?

3. How many atoms of oxygen are there in $2\,\text{H}_2\text{SO}_4$?

4. Why do we write oxygen as $\text{O}_2$, not O, in an equation?