Using chemical equations

The chemical principles discussed in this book are often illustrated using equations. It is useful to know a few of the ground rules chemists have adopted to construct these. Let us begin by looking at an equation depicting the process of rusting metallic iron:

Firstly, the arrow shows that the reaction is favoured in one direction (we will demonstrate this later when discussing energy needed to drive reactions). Next we can see that the reaction balances, i.e. we have four atoms of iron and six atoms of oxygen on both sides of the equation. When chemical reactions take place, we neither gain nor lose atoms. Finally, the subscripted characters in brackets represent the status of the chemical species. In this book l = liquid, g = gas, s = solid and aq = an aqueous species, i.e. a component dissolved in water.

It is important to realize that these reactions are usually simplifications of the actual chemical transformations that occur in nature. In equation 2.6 we are representing rusted or oxidized iron as Fe2O3, the mineral haematite. In nature, rusted metal is a complex mixture of iron hydroxides and water molecules. So equation 2.6 summarizes a series of complicated reaction stages. It illustrates a product we might reasonably expect to form without necessarily depicting the stages of reaction or the complexity encountered in nature.

Many of the equations in this book are written with the reversible reaction sign (two-way half-arrows; e.g. eqn. 2.5). This shows that the reaction can proceed in either direction and this is fundamental to equilibrium-based chemistry (see Box 3.2). Reactions depicting dissolution of substances in water may or may not show the water molecule involved, but dissolution is implied by the (aq) status symbol. Equation 2.7, read from left to right, shows dissolution of rock salt (halite).

The reverse reaction (right to left) shows crystallization of salt from solution.

When writing chemical equations, the sum of charges on one side of the equation must balance the sum of charges on the other side. On the left side of equa tion 2.7, NaCl is an electrically neutral compound, whilst on the right side sodium and chloride each carry a single but opposite charge so that the charges cancel (neutralize) each other.

In this book the symbol O, placed underneath the reaction arrow (e.g. see Box 4.16), is used to denote the presence of microorganisms involved in the reaction. We use the symbol where the presence of microorganisms is proven, although we should remember that in many other reactions microorganisms are often suspected to play a part.

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