Write formulae of common compounds from ion charges, write word equations, and balance symbol equations including state symbols

What this dot point is asking

The syllabus wants you to write the formula of a common compound from the charges on its ions, to write a word equation for a reaction, and to write and balance a symbol equation with state symbols. Balancing matters because atoms are never created or destroyed in a reaction, so each element must have the same number of atoms on both sides. These skills feed directly into the mole calculations later in this module.

The answer

Writing a formula from ion charges

To write the formula of an ionic compound, balance the charges so the compound is neutral:

• Find the charge on each ion (Group I is $1+$, Group II is $2+$, Group VII is $1-$, and so on, with common ions such as $\text{OH}^{-}$, $\text{NO}_3^{-}$, $\text{SO}_4^{2-}$, $\text{CO}_3^{2-}$ learnt by heart).
• Combine them so the total positive charge equals the total negative charge.

For example, $\text{Ca}^{2+}$ with $\text{Cl}^{-}$ needs two chlorides, giving $\text{CaCl}_2$.

Brackets for groups of atoms

When more than one of a group ion is needed, put it in brackets with a subscript. For example, calcium hydroxide is $\text{Ca(OH)}_2$, because one $\text{Ca}^{2+}$ needs two $\text{OH}^{-}$ ions.

Word equations

A word equation names the reactants and products with an arrow:

For example: zinc + hydrochloric acid $\rightarrow$ zinc chloride + hydrogen.

Balancing a symbol equation

A symbol equation uses formulae. To balance it, put big numbers (coefficients) in front of formulae until each element has equal atoms on both sides. Never change a formula; only change the numbers in front. Then add state symbols:

• $(s)$ solid, $(l)$ liquid, $(g)$ gas, $(aq)$ dissolved in water (aqueous).

Examples in context

Example 1. Reading a recipe for a reaction. A balanced equation is like a recipe: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ tells you two molecules of hydrogen react with one of oxygen. The numbers are the ratio you will use in mole calculations, so a correctly balanced equation is the starting point for working out masses.

Example 2. State symbols showing a precipitate. When two solutions react to form an insoluble solid, the state symbols make it clear: $\text{Ag}^{+}(aq) + \text{Cl}^{-}(aq) \rightarrow \text{AgCl}(s)$. The $(s)$ shows a precipitate forms, which links directly to the qualitative analysis tests for ions.

Try this

Q1. Write the formula of aluminium oxide, given the ions $\text{Al}^{3+}$ and $\text{O}^{2-}$. [2 marks]

• Cue. Balance the charges: two $\text{Al}^{3+}$ (total $6+$) with three $\text{O}^{2-}$ (total $6-$) gives $\text{Al}_2\text{O}_3$.

Q2. Balance the equation $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$. [2 marks]

• Cue. $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$, giving $4$ Fe and $6$ O atoms on each side.

Q3. Write the balanced symbol equation, with state symbols, for zinc reacting with hydrochloric acid to give zinc chloride solution and hydrogen gas. [2 marks]

• Cue. $\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$.