Choosing a Route to a Pure Salt: Neutralisation, Precipitation or Displacement
A school needs a pure, dry sample of copper(II) sulfate crystals for a practical. Available starting materials: copper metal turnings, dilute sulfuric acid, copper(II) oxide powder, copper(II) carbonate powder, and sodium hydroxide solution. The technician wants an efficient route that avoids introducing sodium ions into the product and avoids using concentrated acid. She also wants to know how to remove any unreacted solid and how to obtain crystals. A colleague suggests simply mixing copper turnings with dilute sulfuric acid and warming.
1. Evaluate the colleague's suggestion of copper + dilute sulfuric acid, and choose the best route among the available materials. Justify with the reactivity series.
Read the worked answer
Copper sits below hydrogen in the reactivity series, so it does not displace hydrogen from dilute acids. Copper + dilute sulfuric acid gives no reaction under normal conditions. The colleague's route fails. (Only with hot concentrated sulfuric acid does copper react, via reduction of sulfate to sulfur dioxide - but the technician explicitly wants to avoid concentrated acid.)
Better routes use a copper compound that reacts with acid:
- CuO + H₂SO₄ → CuSO₄ + H₂O (neutralisation of a basic oxide).
- CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂ (acid + carbonate).
Both give only copper(II) sulfate and water/CO₂ - no sodium contamination. The CuO route is cleanest because it adds no gas and the endpoint is easy to see (excess black CuO stops dissolving). The NaOH route is rejected: it would precipitate Cu(OH)₂ and then require acid, reintroducing sodium ions into the solution as Na₂SO₄ impurity.
Best choice: warm CuO with dilute H₂SO₄, add CuO in slight excess.
2. Describe how to obtain pure dry crystals from the chosen route, explaining each separation step and why excess CuO is used.
Read the worked answer
React with excess CuO: warm the dilute sulfuric acid and add copper(II) oxide powder until no more dissolves (black solid remains). Excess ensures all acid is consumed, so the solution contains only CuSO₄ - no leftover acid impurity.
Filter while hot: remove the unreacted excess CuO (and any impurities) as residue. Filtering hot keeps the sulfate fully dissolved and prevents premature crystallisation in the filter.
Concentrate: gently evaporate the filtrate in an evaporating basin to the point of crystallisation (do not evaporate to dryness - that drives off water of crystallisation and gives anhydrous powder plus impurities).
Cool and crystallise: cool slowly so blue CuSO₄·5H₂O crystals form; larger, purer crystals grow on slow cooling.
Filter off crystals and dry: separate crystals, press between filter papers or dry in a warm (not hot) place to avoid losing water of crystallisation.
Each step removes a specific impurity: excess oxide by filtration, excess water by evaporation, and the rest by crystallisation.