Extracting Elements from Compounds
To extract a pure element from a compound, you must use a chemical process to break its chemical bonds; physical methods like filtration or evaporation are not sufficient.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Elements within a compound are chemically bonded together, not simply mixed.
- A chemical reaction is necessary to break these bonds and isolate a constituent element.
- Displacement reactions are a common method, where a more reactive element 'kicks out' a less reactive one from its compound.
- Electrolysis is another chemical process that uses electricity to decompose a compound, often used for very reactive elements.
Formulae
A + BC → AC + B To represent a single displacement reaction. This reaction will only proceed if element A is more reactive than element B.
Definitions
- Compound
- A pure substance consisting of two or more different elements that are chemically bonded together in a fixed ratio.
- Displacement Reaction
- A chemical reaction in which a more reactive element replaces a less reactive element in a compound. This is a type of redox reaction.
- Electrolysis
- The process of breaking down an ionic compound (either molten or in solution) into its constituent elements by passing a direct electric current through it.
Worked example
A student has a beaker of aqueous iron(II) sulfate (FeSO4). They want to produce a sample of pure, solid iron metal. Which of the following procedures would be successful? I. Gently heating the solution to evaporate the water. II. Adding a piece of copper wire to the solution. III. Adding magnesium powder to the solution.
- 1
Analyse the goal:
To extract the element iron (Fe) from the compound iron(II) sulfate (FeSO4).
This requires breaking the ionic bonds in the compound.
- 2
Evaluate option I:
Evaporation is a physical process.
It would remove the solvent (water), leaving behind solid iron(II) sulfate crystals.
It does not produce iron metal.
- 3
Evaluate option II:
This proposes a displacement reaction.
For copper to displace iron, it must be more reactive than iron.
Copper is less reactive than iron, so no reaction will occur.
- 4
Evaluate option III:
This proposes a displacement reaction.
Magnesium is more reactive than iron.
Therefore, magnesium will displace the iron from the solution:
Mg(s) + FeSO4(aq) → MgSO4(aq) + Fe(s).
This produces the desired solid iron metal.
- 5
Conclusion:
Only procedure III is a valid chemical method to displace the elemental iron from its compound.
Answer: III only
Common mistakes
- ×Confusing the decomposition of a compound with the separation of a mixture. You cannot filter an element out of a compound it is bonded into.
- ×Incorrectly applying the reactivity series. Remember, an element can only displace another element from a compound if it is more reactive.
- ×Assuming any reaction that forms a solid precipitate achieves the goal. For example, adding sodium hydroxide would precipitate iron(II) hydroxide, which is still a compound, not the pure element.
No-calculator tips
- ✓Focus on the core principle: are chemical bonds being broken? If not, it's a physical separation and won't work for this purpose.
- ✓Visualise the reactivity series as a 'league table'. An element higher up the table can displace any element below it.
- ✓Pay attention to state symbols in equations. The goal is often to turn an aqueous ion, like Fe2+(aq), into a solid metal, Fe(s).