In a single-displacement reaction, one element displaces another element in a compound.
The general equation for a single-displacement reaction is:
A + B-C → A-C + B
A and B must be either different metals (including H) or different halogens.
Chemists have devised an Activity Series. It lists the metals and nonmetals in decreasing order of chemical activity. An element that is higher in the series will displace an element that is below it. This enables chemists to predict which combinations will undergo single displacement reactions.
For example, Mg is above H in the series. We predict that Mg should replace the H in HCl, forming MgCl₂ and gaseous H₂.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Also, Cu is above Ag. We predict that Cu should replace the Ag in AgNO₃, forming Cu(NO₃)₂ and solid Ag.
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Cl is above I. We predict that Cl₂ should replace the I in KI, forming KCl and I₂.
Cl₂(aq) + 2KI(aq) → 2KCl + I₂(aq)
Will there be a reaction between Br₂ and KCl? Br is below Cl in the series, so we predict no reaction.
Video comparing the activity of three metals (Cu, Mg and Zn)