Chemical reaction: A chemical reaction occurs when a substance or a mixture of substances undergo a change which produces new substances.

Reactants: The initial substances of a reaction.

Products: The new substances produced in a chemical reaction.

Chemical equation: A symbolic representation of a chemical reaction using chemical formulae.
Note a chemical equation is not a mathematical equation. The arrow does not mean equal.
Example: When gaseous dihydrogen, H₂, and dioxygen, O₂, are mixed at room temperature nothing happens, but when the mixture is sparked a large amount of heat is given out and water is formed.
Equation:

An arrow, normally from left to right shows the direction of the reaction. In a chemical reaction the atoms of the reactants rearrange to give new substances. Chemical bonds are broken and new chemical bonds made.

Chemical bonds: The forces holding atoms together in matter (see section 4)

Balanced chemical equation: A chemical equation in which the numbers of atoms of each element is the same on both sides
Example:

Stoichiometry: The relative proportions in which elements form compounds or in which substances react. (From the Greek word for element, stoikheion.)

Stoichiometric coefficient: The number immediately in front of the formula of the chemical substances in the balanced equation. The coefficients may be interpreted as the relative number of particles of reactants and products.
Example: For the above equation two molecules of dihydrogen reacts with one molecule of dioxygen to form two molecules of water.

Balancing chemical equations: Deriving the stoichiometric coefficients of reactants and products such that the equation is balanced. Although this can always be done by the mathematical method of linear equations, most equations can be balanced by inspection using the following hints.

1. If the number of atoms of one kind happen to be balanced when the formulae of reactants and products are written down keep this so by placing 1 in front of these formulae.
2. Leave formulae containing only one kind of atom for the last.
3. Start with a kind of atom appearing in only two formulae.

Two examples:

(In section 8 another interpretation of the stoichiometric coefficient is given.)

Summary:
A balanced chemical equation is a concise expression for conveying much information.
e.g. the equation:
implies:

• a molecule of sulfur contains 8 atoms of sulfur
• a molecule of oxygen contains 2 atoms of oxygen
• sulfur and oxygen react to form sulfur dioxide, each molecule of which contains one atom of S and 2 atoms of O (as the name suggests)
• one molecule of sulfur requires 8 molecules of dioxygen for a complete reaction.

Nett ionic equation:
A chemical equation for a reaction occurring in aqueous solution between ionic compounds, but showing only the reacting ions.
Example When solutions of silver nitrate, AgNO₃, and sodium chloride, NaCl, are mixed a precipitate of silver chloride forms:

The Na⁺ and NO₃¯ ions present in solution are not shown because they are not reacting. Such ions are sometimes called spectator ions. The (aq) and (s) in the equation stand for aqueous and solid respectively and make the equation more meaningful.

Nuclear equation:
An equation for a nuclear change or reaction.
Example The production and decay of cobalt-60:

Normal cobalt, ⁵⁹Co, absorbs a neutron (n) in a nuclear reactor to give ⁶⁰Co which decays with elimination of an electron from the nucleus to give ⁶⁰Ni with excess energy and this is lost as a gamma-photon. Note that the mass numbers (superscripts) balance and the atomic numbers or charge (subscripts) balance.

EXERCISES
Balance the following chemical equations:

balanced equations

Write nett ionic equations for the following reactions on mixing aqueous solutions of the following soluble ionic compounds (salts).

11. Solutions of thallium nitrate, TlNO₃ and potassium fluoride, KF,
    to give a precipitate of thallium fluoride.
12. Solutions of copper sulfate, CuSO₄, and sodium carbonate, Na₂CO₃,
    to give a precipitate of copper carbonate.
13. Solutions of calcium nitrate, Ca(NO₃)₂, and sodium phosphate, Na₃PO₄,
    to give a precipitate of calcium phosphate.
14. Solutions of magnesium sulfate, MgSO₄, and barium hydroxide, Ba(OH)₂,
    to give precipitates of barium sulfate and magnesium hydroxide.

balanced equations