M Prakash Institute Chemistry STD-VIII
L20 : Types of reactions : Displacement Reactions
Single displacement Reactions:
Expt.1: Dissolve a little copper chloride in water. Take the solution in a test tube. Put a clean iron
nail into the solution. In some time, the colour of the solution turns from blue to pale green. A
reddish coloured deposit is seen at the bottom of the test tube.
Fe(s) + CuCl2(aq) → FeCl2(aq) + Cu ↓
Expt. 2: Hydrogen is liberated by placing aluminium foil in HCl.
2 Al(s) + 6 HCl(aq) → 2 AlCl3 + 3H2 ↑
The above two experiments demonstrate single displacement.
Expt.3 : Put some iron filings in a solution of Copper sulphate in test tube -1 . In test tube-2, put
some Copper dust in a solution of Iron sulphate. In test tube -1, the colour of the solution changes
from blue to pale green. There is a reddish deposit at the base of the test tube.
F e + CuSO4 → Cu ↓ + FeSO4
In test tube -2, no reaction takes place. This shows that the reaction of displacement is a selective
reaction. In a displacement reaction, a more active element displaces or removes another element
from a compound. These reactions mostly occur in solution form. For example,
(a)Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(b) Pb(s) + CuCl2(aq) → P bCl2(aq) + Cu(s)
Zinc and lead being more active metals, displace the lesser active metal, copper from solution.
Generally, in these reactions, simple substances such as elements substitute the atoms in a molecule
of a compound substance. So these reactions are also called as substitution reactions. Following is
the reactivity series of metals.The metals are arranged in the order of their decreasing reactivities.
Reactivity series of metals:
Li > K > Ba > Sr > Ca > Na > Mg > Al > Mn > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > H > Cu > Hg >
Ag > Pt > Au
Highly reactive- Li > K > Ba > Sr > Ca > Na > Mg
Moderately Reactive- Al > Mn > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb
Least Reactive- Cu > Hg
Nobel Metals- Ag > Pt > Au
Reactivity series of metals:
The upper metal displaces the lower metal from its aqueous salt solution.
Meaning:
(a) Zn displaces all metals (for example Cu) underneath it in the reactivity series from their aqueous
salt solutions. BUT!! Cu can not displace Zn from its aqueous salt solutions.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu ↓
BUT ! Cu + ZnSO4(aq) → Reaction not possible!!
M Prakash Institute Chemistry STD-VIII
(b) Metals above hydrogen displace hydrogen from dilute acids.
Metal below hydrogen (Cu,Ag,Au,etc.) + HClaq) → No reaction.
Metal above hydrogen (Na,Mg,Al,etc) + HClaq) → Metal Chloride + Evolution of H2.
Metal below hydrogen (Cu,Ag,Au,etc.) + dilute H2SO4(aq) → No reaction.
Metal above hydrogen (Na,Mg,Al,etc) + dilute H2SO4(aq) → Metal Sulphate + Evolution of H2.
Reactivity series of Halogens:
The upper non-metal displaces the lower non-metal from its aqueous salt solution.
Order of reactivity : F2 > Cl2 > Br2 > I2
Meaning:
(a) F2 + 2NaCl(aq) → 2NaF + Cl2 ↑
F2 + 2KBr(aq) → 2KF + Br2 ↑
3F2 + 2AlI3(aq) ∆ 2AlF3 + 3I2 ↑
(b) Cl2 + KF(aq) → No Reaction!
Cl2 + NaCl(aq) → No Reaction!
Cl2 + 2KBr(aq) → 2KCl + Br2 ↑
3Cl2 + 3AlI3(aq) ∆ 2AlCl3 + 3I2 ↑
(c) Br2 + KF(aq) → No Reaction!
Br2 + NaCl(aq) → No Reaction!
Br2 + KBr(aq) → No Reaction!
3Br2 + 2AlI3(aq) ∆ 2AlBr3 + 3I2 ↑
(d) I2 + KF(aq) → No Reaction!
I2 + NaCl(aq) → No Reaction!
I2 + KBr(aq) → No Reaction!
I2 + AlI3(aq) ∆ No Reaction!
Double Displacement Reactions:
Expt.4: Mix a solution of Barium chloride with Sodium sulphate. A white precipitate of Barium
sulphate is immediately formed.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)↓ + 2NaCl(aq)
Here SO42- ions displace Cl− ions and Cl− ions displace SO42- ions.
Since such reactions involve displacement of two chemical species, these are known as double
displacement reactions. These reactions usually occur in ionic compounds. The reactions in which
the hydrated ions of two soluble salts move freely in aqueous phase are called as double
displacement reactions
(a) Hydrolysis of Metallic Salts
Metal Hydride + H2O → Hydroxide(aq) + H2 ↑
Metal Nitride + H2O → Hydroxide(aq) +NH3 ↑
Metal Sulphide + H2O → Hydroxide(aq) + H2S ↑
Metal Phosphide + H2O → Hydroxide(aq) + PH3 ↑
Metal Carbide + H2O → Hydroxide(aq) + Hydrocarbon ↑
M Prakash Institute Chemistry STD-VIII
For example!
NaH(s) + H2O → NaOH(aq) + H2 ↑
Mg3N2(s) + H2O → Mg(OH)2(aq) + NH3 ↑
Na2S(s) + H2O → NaOH(aq) + H2S ↑
Ca3P2(s) + 6H2O → 2PH3 + 3Ca(OH)2(aq) ↑
Be2C(s) + H2O → Be(OH)2(aq) + CH4 ↑
CaC2(s) + 2H2O → Ca(OH)2(aq) + C2H2 ↑
(b) Neutralisation reactions:
Neutralization reactions are a specific kind of double displacement reaction. An acid-base reaction
occurs, when an acid reacts with equal quantity of base. The acid base reaction results in the
formation of salt (neutral in nature) and water.
On mixing an aqueous solution of hydrochloric acid with an aqueous solution of sodium hydroxide,
sodium chloride and water are formed.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Oxides of metals (Basic oxides) also react with acids to form salt
and water.
MgO(aq) + 2HNO3(aq) → Mg(NO3)2(aq) + H2O(l)
Oxides of non metals (Acidic oxides)in aqueous solutions react with bases to form salt and water.
CO2(aq) + H2O → H2CO3(aq)
Ca(OH)2(aq) + H(aq)CO3 → CaCO3 + 2H2O
Do you know you can dissolve gold in a mixture of acids?
What? How’s that even possible?
Here's how
Click to watch it! That’s your homework!
