Chemical reactions and equations

When a magnesium ribbon burns with a dazzling white flame and leaves a white powder, a new substance has been formed. Such a change is a chemical reaction. The substances we start with are the reactants; the new substances formed are the products.

We can tell that a reaction has taken place from one or more signs: a change in colour, a change in temperature, the release of a gas, or the formation of a solid (a precipitate) in a solution.

Writing a chemical equation

A reaction can be described in words:

Magnesium + Oxygen → Magnesium oxide

Chemists prefer symbols and formulae, which are shorter and say more:

Mg + O₂ → MgO

This is a "skeletal" equation. Count the atoms: there are two oxygen atoms on the left and only one on the right. But atoms cannot vanish. The law of conservation of mass says that matter is neither created nor destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides. We must balance the equation.

Balancing

We balance by putting numbers, called coefficients, in front of the formulae. We never change the formulae themselves.

2Mg + O₂ → 2MgO

Now there are 2 magnesium atoms and 2 oxygen atoms on each side. Another example, the formation of water:

2H₂ + O₂ → 2H₂O

The physical states may be shown in brackets: (s) solid, (l) liquid, (g) gas and (aq) for a solution in water.

Types of chemical reactions

1. Combination

Two or more substances combine to form a single product. Quicklime reacts with water to form slaked lime, giving out a lot of heat:

CaO + H₂O → Ca(OH)₂

2. Decomposition

A single substance breaks up into two or more simpler ones, usually on heating, passing electricity or exposure to light. Heating limestone gives quicklime and carbon dioxide:

CaCO₃ → CaO + CO₂

3. Displacement

A more reactive element pushes a less reactive one out of its compound. Dip an iron nail in blue copper sulphate solution: the nail gets a brown coat of copper and the solution turns pale green.

Fe + CuSO₄ → FeSO₄ + Cu

4. Double displacement

Two compounds exchange their parts. Mixing solutions of sodium sulphate and barium chloride gives a white precipitate of barium sulphate:

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

5. Oxidation and reduction

Gaining oxygen (or losing hydrogen) is oxidation. Losing oxygen (or gaining hydrogen) is reduction. The two always happen together, and such reactions are called redox reactions. When copper oxide is heated with hydrogen, the copper oxide is reduced to copper and the hydrogen is oxidised to water:

CuO + H₂ → Cu + H₂O

Heat in reactions

Reactions that give out heat are exothermic: burning of fuels, respiration, quicklime with water. Reactions that absorb heat are endothermic: the decomposition of limestone, and photosynthesis, which absorbs light energy.

Oxidation in daily life

• Corrosion: metals are slowly eaten away by air and moisture. Iron rusts; silver turns black; copper gets a green coat.
• Rancidity: oils and fats in food are oxidised and begin to smell and taste bad. Packets of chips are filled with nitrogen to prevent this, and food is stored in airtight containers.