Displacement reactions — single displacement and the activity series

Watch this short introduction to displacement reactions — the pattern where one element "kicks out" another from a compound. Then make your prediction below before we go deeper into the activity series.

Displacement reactions

In some reactions, an element displaces another element in a compound. A single element might replace another element, or two elements of two different compounds may replace each other.

In single-displacement reactions, one element replaces another element

A displacement reaction occurs when an element replaces another element in a compound. When one element displaces another element in a compound, it is called a single-displacement reaction. This typically happens when a more reactive element replaces a less reactive one in a compound.

General form (Figure 4.7):

Element A is more reactive than element B, so A forms a chemical bond with C, causing B to be displaced from the compound.

Single-displacement reactions often occur between metals and ionic salts. (An ionic salt is a compound made up of a cation — a positive ion — and an anion — a negative ion.)

For example, when a piece of zinc metal is placed in copper(II) sulfate solution, zinc displaces copper to form zinc sulfate and solid copper.

In this case, zinc is more reactive than copper and, therefore, displaces it from the ionic solution.

What's actually doing the swapping — read it as ions

Zoom in on the same reaction at the particle level. CuSO₄ is an ionic salt (textbook key term: a compound made up of a cation and an anion), held together by two charged species:

• The cation (positive ion) — Cu²⁺, the copper(II) ion.
• The anion (negative ion) — SO₄²⁻, the sulfate ion.

When the zinc strip drops in, what really happens is a cation swap:

• The Cu²⁺ cation picks up two electrons from the zinc strip, becomes neutral Cu metal, and deposits as the reddish-brown coating you see.
• The Zn atom loses those two electrons, becomes a new Zn²⁺ cation, and enters solution.
• The SO₄²⁻ anion stays intact and stays in solution — it never broke up; it just changed which cation it sits with.

That's why we call this a displacement reaction: zinc displaces copper from the cation slot of the ionic salt. The anion is a passenger, the cation is what actually swaps. (Word "ion" is the umbrella term — any atom that has lost or gained electrons to become charged. Cu²⁺, Zn²⁺ and SO₄²⁻ are all ions; positive ones are cations, negative ones are anions.)

The activity series can be used to predict displacement reactions

To determine whether a displacement reaction will occur, we can use the activity series of metals. If pure, solid metal is more reactive than the metal cation in solution, then a reaction will occur. For example, zinc is above copper on the activity series, indicating that it will displace copper from its compound, forming solid copper.

This is a simplified activity series of metals (textbook Figure 4.9). It can be used to determine if a displacement reaction will occur when a metal is combined with an ionic compound in solution.

Note: you'll sometimes see this series ordered slightly differently in other resources — for example with K above Li instead of Li at the top. That's because there are two valid criteria for ranking metal reactivity: the thermodynamic ordering (based on standard electrode potentials, where Li sits at the top) and the kinetic / observational ordering (based on how vigorously the metals actually react in a beaker, where K can sit above Li because Li reacts more slowly than its thermodynamic position predicts). For VCE Year 10 we use the textbook Figure 4.9 ordering above.

Now try the rule interactively. Drop a metal into a metal-salt solution and see whether the sim shows a reaction:

Double-displacement reactions are Friday's L4 — today's focus stays on single displacement and the activity series.

Key terms

Work through all three questions.

Success criteria — where are you right now?

Next class (Fri 1 May, P2): double displacement and precipitation. You'll meet Table 4.2 — the solubility rules — and predict which precipitates fall out when two solutions are mixed.