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Discovering chemistry
Discovering chemistry

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4 Chemical equations involving ions

Many reactions involve charged species – ions.

The rules for balancing equations representing reactions involving ions are exactly the same as they are for the reactions of neutral species.

When the gas sulfur trioxide, SO3, dissolves in water, hydrogen ions, H+(hydrogen atoms that have lost an electron) and the sulfate anions, SO42, are produced. The sulfate anion comprises four oxygen atoms covalently bonded to a sulfur atom and the whole unit has two negative charges from two additional electrons represented by the superscript 2−.

The balanced chemical equation for the reaction is:

SO3 (aq) + H2O(l) = 2H+(aq) + SO42 (aq)
Equation label: (5.11)

There are equal numbers of atoms of each type on both sides of the equation: 1 sulfur, 4 oxygen and 2 hydrogen atoms.

For ionic equations such as this, there is one more test that must be applied to ensure the equation really is balanced: the total charge on each side of the equation must be the same.

In this case, there are only uncharged species on the left.

On the right, there are two hydrogen ions each with a single positive charge and a sulfate anion with a double negative charge.

So the total charge on the right is 2 − 2 = 0.

And the total charge on each side is zero.

To be clear, the total charge on each side of an ionic equation doesn’t have to be zero; it is just that it must be the same on each side.

So, at this point you are equipped with the tools required to communicate information about chemical reactions in the everyday language of the chemist. In the next section you will look further at reactions themselves, and the properties of molecules which impact on their reactions.