Single-replacement reactions also called single-displacement reactions are chemical reactions in which one element replaces another, creating two new chemical species.
The general form of the single displacement reaction (equation) is:
AX + B → A + BX
They are important in many natural and industrial processes, such as the corrosion of metals, the metabolism of living organisms, and the production of useful compounds. Understanding these reactions can help us predict the products of a reaction and design new chemical processes.
The starting materials are;
- Pure elements (B), such as a pure zinc metal
- Diatomic compounds AX)
When a replacement reaction occurs, a new aqueous compound (BX) and a different pure element (A) will be generated as products.
Zn + 2HCl → ZnCl2 + H2
Due to the formation of a new bond in the products (between B and X elements), single replacement reactions are exothermic reactions that release energy in form of heat or light.
Also Read; When is a reaction spontaneous?
Which elements can replace others?
In general, elements that form anions can replace the anions in a compound, and elements that form cations can replace the cations in a compound.
Some frequently encountered elements that undergo single replacement reactions are listed below:
- Metals (groups 1-2 and some elements of groups 13-14) form cations.
- Non-metals such as Halogen (group 17) form anions.
- Hydrogen usually forms the cation H+.
How to know if a single replacement reaction is going to occur or not?
It is important to mention that single-replacement reactions do not always occur, it depends on the elements involved in the reaction, as well as which element is replacing which one!
Single-replacement reactions only occur when the more reactive element replaces the less reactive element (spontaneity).
If the element is Halogen:
The replacement reaction will only occur if the halogen (pure element) is MORE reactive than the halogen in the compound.
The reactivity series of halogen is;
F2 > Cl2 > Br2 > I2
F2 + 2NaBr → 2NaF + Br2
Fluorine is more reactive than bromine, thus it can replace bromine in the reaction. There is no reaction if this case is reversed.
Br2 + 2NaF → no reaction
If the element is a Metal:
Same as halogens, replacement reaction will only occur if the metal (pure element) is MORE reactive than the metal already present in the compound.
The reactivity series of metals is shown below:
Ni + 2AgNO3 → Ni(NO3)2 + 2Ag
The reaction occurs because Ni is more reactive than Ag, therefore, replacement is possible.
What determines if a single displacement reaction will occur?
Single-replacement reactions only occur when the more reactive element replaces the less reactive element.
What is a single displacement reaction? Give an example.
The reaction in which one element replaces another in a compound. The starting materials are always pure elements, such as a pure zinc metal or diatomic gas, plus an aqueous compound (HCl). When a replacement reaction occurs, a new aqueous compound (ZnCl2) and a different pure element (H2) will be generated as products.
Zn + 2HCl → ZnCl2 + H2
How do you know if it’s a single replacement reaction?
A single replacement reaction is usually identified by the replacement of one element with another. This type of reaction has the general equation:
A + BC → B + AC
What are single displacement reactions vs double displacement reactions?
A single replacement reaction occurs when one element replaces another element in one compound, the reaction is represented by;
A + BC → B + AC
A double replacement reaction occurs when two ionic compounds exchange ions, producing two new ionic compounds, the reaction is represented by;
AB + CD → BD + AC
Why are single replacement reactions exothermic?
Single replacement reactions are exothermic reactions because they lead to the formation of a stable product (with low energy content).