In a single-displacement reaction, one element replaces another in a compound. For metal displacement from a solution, a more reactive metal can replace a less reactive metal ion. For example, zinc can displace copper from copper sulfate: Zn+ CuSO4→ ZnSO4+ Cu. Copper does not displace zinc from zinc sulfate under the same simple rule.
Always use the reactivity order supplied in the question. The free metal must be above the metal in the compound for the predicted displacement. A lack of displacement is a valid result, not a missing product to invent. The displaced metal forms an element, while the incoming metal combines with the original counter-ion in an electrically neutral formula. This pattern is different from decomposition because an additional element reacts with the compound.
Example 1
Given Mg>Zn> Cu in reactivity, predict zinc added to copper sulfate.
Zinc is more reactive than copper, so Zn+ CuSO4→ ZnSO4+ Cu. Copper metal is displaced.
Example 2
Using the same order, predict copper added to magnesium sulfate.
Copper is less reactive than magnesium. The supplied rule predicts no metal-displacement reaction.
Watch out: Under the simple metal-displacement rule, the added metal must be more reactive than the metal ion it replaces.