Chemical formulae and chemical equations

1.21     write word equations and balanced chemical equations to represent the reactions studied in this specification

You will encounter these throughout this booklet.  Learn them!  If the exam just asks for “an equation”, assume they are talking about a balanced chemical equation.

You should be prepared to work out similar equations.  For example, if you know the equation for sodium reacting with water, you should easily be able to deduce the equation for anything else from Group I reacting with water.

Knowing the charges on ions (Section 1.30) is essential for you to be able to work out formulae and hence equations.

Here is an example of the sort of knowledge and logic you should be using to write equations.

e.g. Give a balanced chemical equation for the reaction between magnesium and hydrochloric acid

•  knowledge:                acid + metal → salt + hydrogen

•  knowledge:              hydrochloric acid makes chloride salts

•  deduction:                hydrochloric acid + magnesium → magnesium chloride + hydrogen

•  knowledge:               magnesium has a charge of +2 because it is in Group 2

•  knowledge:               chloride ions have a charge of -1 because chlorine is in Group 7

•  knowledge:               charges in compounds must balance out

•  deduction:                 there must be one Mg2+ to two Cl- ions, which gives MgCl2

•  knowledge:               hydrochloric acid is HCl and hydrogen gas is diatomic, H2

•  putting it together:     Mg + HCl → MgCl2 + H2

•  knowledge:             equations must be balanced so the same number of atoms appear on each side. This can only be done by putting big numbers in front of things and not by changing formulae.

•  deduction:                 Mg + 2HCl → MgCl2 + H2