Main Idea
A single replacement reaction is a chemical reaction where one element replaces another element inside a compound.
The reactants must be one element plus one compound. The products are a different element plus a different compound.
| Part | Meaning |
|---|---|
| A | A single element |
| BC | A compound |
| AB | A new compound |
| C | The element that got replaced |
Basic Examples
Example 1
Na + KCl K + NaCl
Sodium is the single element, and potassium chloride is the compound. Sodium replaces potassium.
Example 2
F2 + 2LiCl 2LiF + Cl2
Fluorine is the single element. It replaces chlorine in lithium chloride, and chlorine leaves as Cl2.
Metals in Single Replacement
Metals can replace other metals in compounds. They can also replace hydrogen in acids or water.
Potassium and aluminum nitride
K + AlN ->
Potassium is a metal by itself. Aluminum is the metal inside aluminum nitride, so potassium can replace aluminum if it is more active.
Products
K3N + Al
Balanced
3K + AlN K3N + Al
Zinc and hydrochloric acid
Zn + HCl ->
Zinc can replace hydrogen from an acid because zinc is active enough. The hydrogen leaves as hydrogen gas.
Products
ZnCl2 + H2
Balanced
Zn + 2HCl ZnCl2 + H2
Water in Single Replacement
For these reactions, water can be thought of as HOH. The metal replaces one hydrogen and combines with hydroxide.
2Na + 2HOH 2NaOH + H2
2Na + 2H2O 2NaOH + H2
How to Know if a Reaction Happens
Not every single replacement reaction happens. To decide, use activity.
More active replaces less active.
In the activity series of metals, a metal higher on the list can replace a metal lower on the list. Hydrogen is included too: metals above hydrogen can replace hydrogen from acids.
Practice Examples with Metals
Fe + CuSO4 FeSO4 + Cu
Reaction happensIron is more active than copper, so iron replaces copper. This reaction happens.
Pb + KCl No reaction
No reactionLead is less active than potassium, so lead cannot replace potassium.
2Al + 6HCl 2AlCl3 + 3H2
Reaction happensAluminum is above hydrogen in the activity series, so it replaces hydrogen from acid.
Why Hg and Ag Being Near the Bottom Matters
Mercury, Hg, and silver, Ag, are near the bottom of the activity series. They are not very active metals, so they cannot replace many other metals. Metals near the bottom can only replace elements lower than themselves, and there are not many below them.
Nonmetals in Single Replacement
Nonmetal replacement is mostly limited to the halogens. These elements are diatomic, so they are written with a subscript 2.
| Halogen | Formula |
|---|---|
| Fluorine | F2 |
| Chlorine | Cl2 |
| Bromine | Br2 |
| Iodine | I2 |
The activity order for halogens is:
F2 > Cl2 > Br2> I2
Higher replaces lower, so fluorine can replace chlorine, bromine, or iodine. Iodine cannot replace fluorine, chlorine, or bromine.
Practice Examples with Nonmetals
F2 + 2HCl 2HF + Cl2
Fluorine is more active than chlorine, so fluorine replaces chlorine. This reaction happens.
Br2 + KCl No reaction
Bromine is less active than chlorine, so bromine cannot replace chlorine.
Summary Rules
| Situation | Rule |
|---|---|
| Metal replacing metal | A more active metal replaces a less active metal |
| Metal replacing hydrogen in acid | The metal must be above hydrogen |
| Metal replacing hydrogen in water | Very active metals form hydroxides and H2 |
| Halogen replacing halogen | A more active halogen replaces a less active halogen |
| Less active replaces more active | No reaction |