Iron reacts with sulfur to form iron(II) sulfide. The balanced symbol equation for this reaction is: Fe + S → FeS. State the type of substance that iron(II) sulfide is and explain why the formula FeS, rather than Fe₂S₂, is used as the chemical formula for iron(II) sulfide.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.5 Chemical formulae, equations and calculations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron reacts with sulfur when heated together. The balanced symbol equation for this reaction is: Fe + S → FeS.

Model answer (4 marks)

Iron(II) sulfide is a compound.

The empirical formula gives the simplest whole‑number ratio of atoms of each element in the compound. The ratio of Fe to S in iron(II) sulfide is 1:1. A formula such as Fe₂S₂ can be simplified by dividing the numbers of atoms by 2, giving FeS. Therefore FeS is the empirical formula of iron(II) sulfide.

Examiner tips

  • State the substance type first to secure the 1 mark. Explain that empirical formula is the simplest ratio. Show the simplification from Fe₂S₂ to FeS. Use correct chemical symbols and charge notation (Fe²⁺, S²⁻).

Common mistakes

  • Writing FeS as Fe₂S₂ and claiming it is the correct formula. Forgetting to explain that FeS is the empirical (simplest) ratio. Using the wrong charge notation or omitting the oxidation state.

Mark scheme (4 marks)

  1. Iron(II) sulfide is a compound
  2. The empirical formula gives the simplest whole number ratio of atoms of each element in the compound
  3. Fe₂S₂ can be simplified / divided by 2 to give FeS (ratio 1:1)
  4. FeS is the empirical formula (of iron(II) sulfide)

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