Writing formulae from ionic charges and oxidation states, constructing balanced chemical and ionic equations with state symbols, and using the language of chemistry consistently.

What this topic is asking

Eduqas topic C1.1 wants you to write the formulae of elements, ions and compounds, deduce formulae from ionic charges and oxidation states, and construct balanced full and ionic equations with the correct state symbols. This is the grammar of chemistry: every later calculation and mechanism depends on getting it right.

Writing formulae of ions and compounds

The formula of a simple ion follows its position in the periodic table: Group 1 forms $1+$ ions, Group 2 forms $2+$, Group 6 forms $2-$ and Group 7 forms $1-$. Transition metals and some non-metals form more than one ion, so their charge is shown by a Roman numeral, as in iron(II), $\text{Fe}^{2+}$, and iron(III), $\text{Fe}^{3+}$.

Oxidation states as a formula tool

The oxidation state (oxidation number) is the charge an atom would carry if all bonds were ionic. The rules give it directly: elements are $0$, Group 1 is $+1$, Group 2 is $+2$, hydrogen is usually $+1$ (but $-1$ in metal hydrides), oxygen is usually $-2$ (but $-1$ in peroxides), and the sum equals the overall charge. Knowing that nitrogen is $+5$ in $\text{NO}_3^-$ or chlorine is $+5$ in $\text{ClO}_3^-$ lets you build and name oxoanion formulae with confidence.

Balancing chemical equations

A balanced equation obeys conservation of mass: atoms are neither created nor destroyed.

Ionic equations and spectator ions

When ionic compounds react in solution, only some ions take part. An ionic equation shows just the species that change; the spectator ions, which are identical on both sides, are cancelled.

For neutralisation of any strong acid by any strong alkali, cancelling the spectators leaves the single ionic equation $\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l})$, which is why all such reactions release a similar enthalpy of neutralisation.

State symbols

Every species in an Eduqas equation should carry a state symbol: $(\text{s})$ solid, $(\text{l})$ liquid, $(\text{g})$ gas and $(\text{aq})$ aqueous (dissolved in water). Marks are routinely lost for omitting them or for writing a precipitate as $(\text{aq})$.

Examples in context

Example 1. Half-equations combine into a balanced redox equation. The displacement of copper by zinc is the sum of $\text{Zn}(\text{s}) \rightarrow \text{Zn}^{2+}(\text{aq}) + 2\text{e}^-$ and $\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu}(\text{s})$, giving $\text{Zn}(\text{s}) + \text{Cu}^{2+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Cu}(\text{s})$. The electrons cancel because they appear equally on both sides, exactly like spectator ions.

Example 2. Naming guides the formula. "Iron(III) sulfate" tells you the cation is $\text{Fe}^{3+}$ and the anion is $\text{SO}_4^{2-}$; balancing the charges gives $\text{Fe}_2(\text{SO}_4)_3$. Reading the Roman numeral first prevents the most common formula error in inorganic questions.

Try this

Q1. Write the formula of ammonium carbonate. [1 mark]

• Cue. Ammonium is $\text{NH}_4^+$ and carbonate is $\text{CO}_3^{2-}$, so two ammonium ions balance one carbonate: $(\text{NH}_4)_2\text{CO}_3$.

Q2. Write the ionic equation, with state symbols, for adding dilute hydrochloric acid to solid calcium carbonate. [2 marks]

• Cue. $\text{CaCO}_3(\text{s}) + 2\text{H}^+(\text{aq}) \rightarrow \text{Ca}^{2+}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$. Chloride is the spectator ion.