0.7 Representing chemical change  (Page 22/6)

Introduction

As we have already mentioned, a number of changes can occur when elements react with one another. These changes may either be physical or chemical . One way of representing these changes is through balanced chemical equations . A chemical equation describes a chemical reaction by using symbols for the elements involved. For example, if we look at the reaction between iron ( $\mathrm{Fe}$ ) and sulphur ( $\mathrm{S}$ ) to form iron sulphide ( $\mathrm{FeS}$ ), we could represent these changes either in words or using chemical symbols:

$\mathrm{iron}+\mathrm{sulphur}\to \mathrm{iron\; sulphide}$

or

$\mathrm{Fe}+\mathrm{S}\to \mathrm{FeS}$

Another example would be:

$\mathrm{ammonia}+\mathrm{oxygen}\to \mathrm{nitric\; oxide}+\mathrm{water}$

or

$4{\mathrm{NH}}_3+5{\mathrm{O}}_2\to 4\mathrm{NO}+6{\mathrm{H}}_2\mathrm{O}$

Compounds on the left of the arrow are called the reactants and these are needed for the reaction to take place. In this equation, the reactants are ammonia and oxygen. The compounds on the right are called the products and these are what is formed from the reaction.

In order to be able to write a balanced chemical equation, there are a number of important things that need to be done:

1. Know the chemical symbols for the elements involved in the reaction
2. Be able to write the chemical formulae for different reactants and products
3. Balance chemical equations by understanding the laws that govern chemical change
4. Know the state symbols for the equation

We will look at each of these steps separately in the next sections.

Chemical symbols

It is very important to know the chemical symbols for common elements in the Periodic Table, so that you are able to write chemical equations and to recognise different compounds.

Revising common chemical symbols

• Write down the chemical symbols and names of all the elements that you know.
• Compare your list with another learner and add any symbols and names that you don't have.
• Spend some time, either in class or at home, learning the symbols for at least the first twenty elements in the periodic table. You should also learn the symbols for other common elements that are not in the first twenty.
• Write a short test for someone else in the class and then exchange tests with them so that you each have the chance to answer one.

Writing chemical formulae

A chemical formula is a concise way of giving information about the atoms that make up a particular chemical compound. A chemical formula shows each element by its symbol and also shows how many atoms of each element are found in that compound. The number of atoms (if greater than one) is shown as a subscript.

Examples:

Number of atoms: $(1\times \mathrm{carbon})+(4\times \mathrm{hydrogen})=5$ atoms in one methane molecule

${\mathrm{H}}_2{\mathrm{SO}}_4$ (sulphuric acid)

Number of atoms: $(2\times \mathrm{hydrogen})+(1\times \mathrm{sulphur})+(4\times \mathrm{oxygen})=7$ atoms in one molecule of sulphuric acid

A chemical formula may also give information about how the atoms are arranged in a molecule if it is written in a particular way. A molecule of ethane, for example, has the chemical formula ${\mathrm{C}}_2{\mathrm{H}}_6$ . This formula tells us how many atoms of each element are in the molecule, but doesn't tell us anything about how these atoms are arranged. In fact, each carbon atom in the ethane molecule is bonded to three hydrogen atoms. Another way of writing the formula for ethane is ${\mathrm{CH}}_3{\mathrm{CH}}_3$ . The number of atoms of each element has not changed, but this formula gives us more information about how the atoms are arranged in relation to each other.