Single & Double Replacement Reactions
There are four different types of chemical reactions. In previous session, we discussed about:
1. Combination or Synthesis Reactions
2. Decomposition Reactions
In this session, We will focus on remaining two types of chemical reactions.
3. Single Replacement Reactions
4. Double Replacement Reactions
Previous learning sessions:
Single Replacement Reactions
As the word says, Single Replacement means one element can replace the another one element present in one compound.
So, any single replacement reaction to happen, it requires at least one compound and one element.
As shown in the diagram, compound AB which react with element C and here C is the single element which can replace an element present in AB compound.
Element C replaces an element A which was present in compound AB and after replacing A with C, a new compound CB is formed and A is left over as a single element.
Such kind of reactions are considered as Single Replacement reaction, where one element able to replace another element present in one compound.
There are many important factors on which it is decided that one single element will be able to replace another element in a compound or not.
- Element A and C both can be metallic or non-metallic in nature
- It means, a metallic element will be able to replace a metallic element and a non-metallic element will be able to replace non-metallic element
- Metallic element will not be able to replace non-metallic and non-metallic will not be able to replace metallic element
- C replaces A on the basis of reactivity series
- If single element C is more reactive in nature than element A in compound AB, then only element C will be able to replace an element A in a compound AB
- This reactive nature of elements are arranged in reactivity series
- Cations (Metals) can only replace Cations (Metals)
- Anions (Non-Metal) an only replace Anions (Non-Metal)
1. Cation Replacement
- 2Al(s) + Fe2O3(aq) → Al2O3(aq) + 2Fe(s)
- Aluminium (Al) and Iron(Fe) both are metallic in nature
- Aluminium is more reactive than Iron, that is why Aluminium is able to replace Iron in compound Ferric Oxide (Fe2O3) and form new compound Aluminium Oxide (Al2O3)
2. Anion Replacement
- Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)
- Chlorine(Cl) and Bromine(Br) both are non-metallic in nature
- Chlorine is more reactive than Bromine, that is why Chlorine is able to replace Bromine compound in Potassium Bromide (KBr) and form new compound Potassium Chloride (KCl)
3. Some metals are more reactive that Hydrogen, so they can replace it
- Hydrogen Replaced by Zinc
- Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
- Zinc(Zn) and Hydrogen(H) both are metallic in nature
- Zinc is more reactive than Hydrogen, that is why Zinc is able to replace Hydrogen in compound Hydrogen Chloride (HCl) and form new compound Zinc Chloride (ZnCl2)
- Hydrogen Replaced by Sodium
- 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
- Sodium(Na) and Hydrogen(H) both are metallic in nature
- Sodium is more reactive than Hydrogen, that is why Sodium is able to replace Hydrogen in compound Water (H2O) and form new compound Sodium Hydroxide (NaOH)
Double Replacement Reactions
As the word says, Double Replacement means the displacement of two different elements in two different compounds.
So, any double replacement reaction to happen, the elements in two different compounds are allow to replace each other to form two new different compounds.
As shown in the diagram, two compounds AB and CD react with each other. Element A in compound AB and element C in compound CD replaces each other forming two new different compounds CB and AD.
- Both A & C are Cations (+ve ions) and B & D are Anions (-ve ions)
- +ve ions (cation) and –ve ions (anion) of reactant compounds switch places to form new product compounds
Such kind of reactions are considered as Double Replacement reaction.
- Sodium Hydroxide + Calcium Bromide → Sodium Bromide + Calcium Hydroxide 2NaOH + CaBr2 → 2NaBr + Ca(OH)2
- Sodium Sulphide + Hydrochloric Acid → Sodium Chloride + Hydrogen Sulphide Na2S + 2HCl → 2NaCl + H2S
Na+OH– + Ca2+Br– → Na+Br– + Ca2+OH–
Na+S2- + H+Cl– → Na+Cl– + H+S2-