Chapter 14: Representing chemical change

14.1 Introduction (ESADY)

As we have already mentioned, a number of changes can occur when elements are combined with one another. These changes may either be physical or chemical. In this chapter we will look at chemical changes. One way of representing chemical 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 \((\text{Fe})\) and sulfur \((\text{S})\) to form iron sulfide \((\text{FeS})\), we could represent these changes in a sentence, in a word equation or using chemical symbols:

Sentence: Iron reacts with sulfur to form iron sulfide.
Word equation: Iron \(+\) sulfur \(\rightarrow\) iron sulfide.
Chemical symbols: \(\text{Fe} + \text{S} \rightarrow \text{FeS}\)

Another example would be:

Sentence: Ammonia reacts with oxygen to form nitrogen monoxide and water.
Word equation: Ammonia \(+\) oxygen \(\rightarrow\) nitrogen monoxide \(+\) water.
Chemical symbols: \(4\text{NH}_{3} + 5\text{O}_{2} \rightarrow 4\text{NO} + 6\text{H}_{2}\text{O}\)

Compounds on the left of the arrow are called the reactants and these are needed for the reaction to take place. 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:

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

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

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Chemical symbols (ESADZ)

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.
Know the symbols for at least the first thirty six elements in the periodic table. You should also learn the symbols for other common elements that are not in the first thirty six.
Set a short test on naming elements and compounds for someone else in the class and then exchange tests with them so that you each have the chance to answer a test.

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Writing chemical formulae (ESAEA)

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.

The following exercise serves as revision. If you do not recall how to write chemical formulae refer back to Chapter 2.