Displacement reactions

A displacement reaction is a chemical change in which a free element — almost always a metal — takes the place of a less reactive element already combined in a compound, releasing that less reactive element in its free form. It is a single-replacement process: one element goes in, one element comes out.

The general pattern. For a metal $X$ reacting with the salt of a metal $Y$:

where: $X$ is the free (uncombined) metal added to the reaction, $YZ$ is the compound already containing metal $Y$ combined with a non-metal part $Z$ (a sulphate, chloride, or nitrate radical, for example), $XZ$ is the new compound formed once $X$ has taken $Y$'s place, and $Y$ is the displaced metal, now released as a free element. The reaction proceeds only if $X$ is more reactive than $Y$, as ranked by the reactivity series. If $X$ is less reactive than $Y$, nothing happens — the two substances can sit in contact indefinitely with no visible change.

Why the reactivity series is the deciding tool. Reactivity in this context means how strongly a metal "wants" to hold onto electrons once it has given them up to form a compound — a more reactive metal holds its combined state more loosely relative to a less reactive one only in the sense that the less reactive metal is more easily pushed out. In practice, you never need the underlying electron chemistry to answer an ICSE question: you only need to look up both metals on the reactivity series and check their order. This is why the reactivity series is described as the single tool that decides displacement questions before a single equation is written.

Reading the change. Two everyday clues confirm a displacement reaction has occurred: a colour change in the solution (as the old metal-salt colour, often blue for copper compounds, fades and a new salt colour appears or the solution becomes colourless) and the appearance of a solid deposit of the newly freed metal on the surface of the metal that was added.

Whether a displacement reaction actually happens is entirely a matter of position in the reactivity series, and reasoning through this needs no calculation — only a careful comparison, done in a fixed order.

Step 1: identify the two metals. In any displacement question there is a free metal being added and a metal already combined in a compound. Name both explicitly before doing anything else, since mixing them up is the single most common source of error.

Step 2: compare their positions on the reactivity series. Locate both metals on the standard series (from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold). If the free metal sits above the combined metal, it is more reactive and displacement will occur; if it sits below, no reaction takes place at all, however long the two are left in contact.

Step 3: write the products only if displacement is predicted. The free metal takes the place of the combined metal in the compound, and the displaced metal is released as the free element, matching the general form $X + YZ \rightarrow XZ + Y$. If Step 2 predicted no reaction, stop here — do not force an equation.

Step 4: balance for charge and formula ratio. Some displaced metals combine with the non-metal part in a different ratio than the metal doing the displacing (for example, one copper atom needs two nitrate-containing units to balance against two silver ions). Balancing must always be checked, not assumed to be one-to-one just because the first example you learned was. This four-step habit — name the metals, check the series, write the equation only if justified, then balance — is why displacement reactions are so predictable: the entire outcome is decided in Step 2, before any chemistry is even written down.

Question: A copper wire is dipped into a colourless solution of silver nitrate, $\text{AgNO}_3$. Predict whether a reaction occurs; if so, write the balanced equation with state symbols and describe what would be observed.

Step 1. Identify the metals involved: copper is the free metal being added, and silver is the metal already combined, in $\text{AgNO}_3$.

Step 2. Compare positions on the reactivity series. Copper sits above silver, so copper is more reactive than silver, and displacement is predicted to occur.

Step 3. Write the products: copper takes silver's place in the nitrate compound, and silver is released as the free metal, giving $\text{Cu(NO}_3)_2$ and Ag.

Step 4. Balance the equation. One copper atom forms $\text{Cu}^{2+}$ and needs two nitrate groups, so two $\text{AgNO}_3$ units are needed to supply both the nitrate and the two silver atoms released:

Checking atoms: 1 Cu, 2 Ag, 2 N, 6 O on each side — balanced. This is a displacement reaction: shiny silvery crystals of silver metal deposit on the copper wire, and the colourless solution slowly turns pale blue as copper nitrate forms.

Competitive stretch

If a strip of silver were instead dipped into zinc sulphate solution, Step 2 would show silver sits below zinc on the reactivity series, so no reaction occurs — the correct exam answer is a plain statement of "no reaction," not a forced equation.

• Skipping the reactivity series check. Writing an equation before comparing positions risks predicting a reaction (or "no reaction") wrongly — always do Step 2 first.
• Confusing displacement with double displacement. A single displacement reaction has exactly one free element among the reactants and exactly one among the products; if two compounds simply swap partners with no free element anywhere, that is double displacement instead.
• Copying a 1:1 ratio onto every equation. The $\text{Zn} + \text{CuSO}_4$ reaction is 1:1, but combining ratios change with the metal's usual charge, as the 1 Cu : 2 Ag ratio in the silver nitrate example shows.
• Omitting state symbols or the observation. ICSE answers expect $(s)$ and $(aq)$ symbols on every species, plus the colour change and metal deposit when asked to describe what is seen.

Ask the tutor: Want to test a few more metal-and-salt combinations against the reactivity series, or get help balancing a trickier displacement equation? Ask your AI tutor to work through them with you.