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Discovering chemistry
Discovering chemistry

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2 Chemical equations

Chemical equations are representations of chemical reactions.

At this point you do have some experience of the use of numbers and symbols to represent elements and compounds (chemical formulas), chemical equations use these as a starting point.

Let’s begin by writing an equation for a simple reaction, one which is crucial to our everyday lives; the combustion of natural gas in air. Natural gas is almost pure methane whose molecules are made up of carbon and hydrogen atoms (i.e. it is hydrocarbon). And, methane burns in (reacts with) oxygen to yield the molecules carbon dioxide and water along with the production of heat.

In writing a chemical equation to represent this process, the first step is to identify the reactants and the products.

The reactants are written on the left and the products on the right to give, initially, a word ‘equation’:

methane and oxygen go to carbon dioxide and water

Even this simple word equation carries a lot of useful information in a relatively concise form, but you can go further. You know already that a formula reveals much more about a particular substance than does a name.

The molecular formula of methane is CH4.

  • What does the formula CH4 tell you about the methane molecule?

  • CH4 indicates that the methane molecule comprises one carbon atom and four hydrogen atoms.

So the next step is to replace the names of the compounds in the equation with appropriate formulas.

  • What other formulas do you need to write the equation for the combustion of methane?

  • You also need oxygen O2, and carbon dioxide CO2 and water H2O.

So now you could write:

cap c times cap h sub four postfix times and postfix times cap o sub two postfix times times go postfix times postfix times t times o cap c times cap o sub two postfix times and postfix times cap h sub two times cap o

The word and can be replaced by the plus sign, +, and the reaction representation becomes:

cap c times cap h sub four plus cap o sub two postfix times times go to cap c times cap o sub two plus cap h sub two times cap o

Even at this early stage of development of the equation, there is useful information about the chemistry going on.

  • What has happened to the hydrogen atoms in the methane molecule?

  • Hydrogen atoms of the methane molecules in the reaction end up as part of the product water molecules

  • What has happened to the carbon atom in methane?

  • The carbon atoms end up as part of the product carbon dioxide molecules.

It is important to note that the relationship provides information about the overall reaction. It does not say anything about how the reaction actually occurs, such as the way in which bonds are broken and formed to give the final products. Essentially it provides a summary of the reaction.