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How do we represent chemical species and reactions with formulae and balanced equations?

Names and formulae of common ions, binary and polyatomic compounds, the use of oxidation numbers in naming, and the construction of balanced full and ionic equations including state symbols.

An OCR H432 module 2 answer covering common ion formulae, naming with oxidation numbers, writing chemical formulae from charges, and constructing balanced full and ionic equations with state symbols.

Generated by Claude Opus 4.89 min answer

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  1. What this topic is asking
  2. Common ions and formulae
  3. Balancing equations
  4. Ionic equations
  5. Examples in context
  6. Try this

What this topic is asking

OCR specification point 2.1.2 wants you to recall the names and formulae of common ions, build formulae for binary and polyatomic compounds from ionic charges, use oxidation numbers in naming (such as iron(III)), and construct balanced full and ionic equations with correct state symbols.

Common ions and formulae

You are expected to recall the formulae and charges of common ions without being given them. Key polyatomic (molecular) ions are:

Ion Formula Ion Formula
nitrate NO3−\text{NO}_3^- sulfate SO42−\text{SO}_4^{2-}
carbonate CO32−\text{CO}_3^{2-} hydrogencarbonate HCO3−\text{HCO}_3^-
hydroxide OH−\text{OH}^- ammonium NH4+\text{NH}_4^+
phosphate PO43−\text{PO}_4^{3-} nitrite NO2−\text{NO}_2^-

To build a formula, balance the charges so the compound is neutral. For example, calcium (Ca2+\text{Ca}^{2+}) and nitrate (NO3−\text{NO}_3^-) combine as Ca(NO3)2\text{Ca(NO}_3)_2: one 2+2+ ion needs two 1−1- ions.

Balancing equations

A balanced equation conserves atoms and charge. Adjust only the stoichiometric coefficients in front of each formula, never the subscripts inside a formula. For example, the combustion of propane:

C3H8+5O2→3CO2+4H2O\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

Balance carbon first, then hydrogen, then oxygen last (because oxygen appears in two products). State symbols are (s)(s) solid, (l)(l) liquid, (g)(g) gas and (aq)(aq) aqueous.

Ionic equations

For a precipitation, neutralisation or displacement, writing the ionic equation focuses on the chemistry. For neutralisation of any strong acid by any strong alkali the ionic equation is always:

H+(aq)+OH−(aq)→H2O(l)\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)

Examples in context

Example 1. Hard-water scale. When hard water is heated, hydrogencarbonate ions decompose: Ca2+(aq)+2HCO3−(aq)→CaCO3(s)+H2O(l)+CO2(g)\text{Ca}^{2+}(aq) + 2\text{HCO}_3^-(aq) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g). Writing this balanced equation, including the gas and the solid limescale, explains the deposit in kettles and shows oxidation numbers staying constant in a non-redox change.

Example 2. Testing for carbonate. Adding dilute acid to a carbonate fizzes as carbon dioxide is released: CO32−(aq)+2H+(aq)→H2O(l)+CO2(g)\text{CO}_3^{2-}(aq) + 2\text{H}^+(aq) \rightarrow \text{H}_2\text{O}(l) + \text{CO}_2(g). The ionic equation captures the essential chemistry of this common qualitative test, ignoring whichever spectator cation and anion were present.

Try this

Q1. Write the formula of aluminium sulfate. [1 mark]

  • Cue. Al3+\text{Al}^{3+} and SO42−\text{SO}_4^{2-} balance as Al2(SO4)3\text{Al}_2(\text{SO}_4)_3.

Q2. Write the ionic equation for the reaction of magnesium with hydrochloric acid. [2 marks]

  • Cue. Mg(s)+2H+(aq)→Mg2+(aq)+H2(g)\text{Mg}(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{H}_2(g).

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20183 marksAqueous barium chloride reacts with aqueous sodium sulfate to form a precipitate of barium sulfate. (a) Write the full balanced equation including state symbols. (b) Write the ionic equation for the reaction.
Show worked answer →

(a) BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq)\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq) (1 for formulae and balancing, 1 for correct state symbols).

(b) Cancel the spectator Na+\text{Na}^+ and Cl−\text{Cl}^- ions to give Ba2+(aq)+SO42−(aq)→BaSO4(s)\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s) (1).

Markers reward the balanced molecular equation, correct states (the precipitate as (s)(s)), and the cancellation of spectator ions.

OCR 20203 marksIron(III) oxide reacts with carbon monoxide in a blast furnace to form iron and carbon dioxide. (a) Deduce the formula of iron(III) oxide. (b) Write the balanced symbol equation for the reaction.
Show worked answer →

(a) Iron(III) is Fe3+\text{Fe}^{3+} and oxide is O2−\text{O}^{2-}; balancing the charges gives Fe2O3\text{Fe}_2\text{O}_3 (1).

(b) Fe2O3+3CO→2Fe+3CO2\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 (2, one mark for reactants and products, one for correct balancing).

Markers reward using the oxidation number to fix the formula and a fully balanced equation.

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