Displacement Relations

Chemical Reaction And Equation of Class 10

DISPLACEMENT REACTIONS:

It involves displacement of one of the constituents of a compound by another substance and may be regarded as a displacement reaction.

The elements involved may be metals or non-metals. In displacement reactions more reactive metal may displace a less reactive metal or a more reactive non-metal may displace a less reactive non-metal from its compound.

Relative activities or reactivity’s of metals:

Metals have been arranged in decreasing order of their activities (or reactivities) in the activity series as follows:

Displacement Relations

It is clear from the series that the metals lying above the hydrogen are more reactive than the metals lying below the hydrogen. Thus, any metal can displace the metals lying below it from its solution.

Hydrogen is included in the series because, like metals, hydrogen can lose an electron to form positive (H+) ion. As the metals lying above hydrogen are more reactive, they can displace hydrogen from acids or water, i.e. they can react with an acid to give out hydrogen gas. On the other hand, metals lying below hydrogen being less reactive than hydrogen cannot displace hydrogen from acids to give out hydrogen gas.

Relative activities (or reactivities) of Non-metals:

Relative activities of non-metals like halogens is in the order:

F > Cl > Br > I

Thus, fluorine is most reactive and iodine is least reactive. So fluorine (F₂) can displace chlorine (Cl₂) from NaCl, Bromine (Br₂) from NaBr and so on. Similarly, chlorine can displace bromine (Br₂) from KBr and iodine (I₂) from KI and so on.

Ex. Reaction between iron and copper sulphate

Sol.

Experiment: Take about 10 ml of copper sulphate solution in a test tube. It is deep blue in colour. Take two iron nails and clean their surface by rubbing with a sand paper.

Now put one iron nail in test tube containing CuSO₄ solution. Keep the another iron aside for comparison.

Displacement Relations

Displacement reaction between iron (nail) and copper sulphate solution.

After 30 minutes we observe the following changes :

Blue colour of CuSO₄has faded and it changes into light green due to formation of iron sulphate (FeSO4).
The iron nail is covered with a red brown layer of copper metal.

These changes show that the following reaction has taken place :

Displacement Relations

Thus, more reactive metal iron, (Fe), displaces the less reactive metal copper from copper sulphate solution. So this is an example of displacement reaction.

Displacement reactions in which a more reactive metal displaces a less reactive metal from its compound:

Example:

(i) Displacement Relations

Thus, when zinc pieces are added to copper sulphate solution, then, zinc being more reactive metal than copper, displaces copper from its solution (CuSO₄) so that Cu is set free. The blue colour of CuSO₄ solution fades due to formation of ZnSO₄(colourless). A reddish brown deposit of copper metal is formed on the surface of zinc. Therefore it is an example of displacement reaction.

(ii) Displacement Relations

When a copper wire is dipped in silver nitrate solution, copper, being more reactive metal than silver, displaces silver from its solution (AgNO₃) so that silver is liberated. This silver is deposited on the copper wire giving it a white shining surface. The solution forms a blue colour due to formation of copper nitrate. Thus it is an example of displacement reaction.

(iii) Displacement Relations

In this reaction sodium, being more active than hydrogen, displaces hydrogen from water so that hydrogen gas is liberated along with the formation of sodium hydroxide. Thus, it is an example of displacement reaction.

(b) The displacement reactions in which more reactive non-metal displaces less reactive non-metal from its compound:

e.g. (i) Displacement Relations

When Cl2 gas is passed through an aqueous solution of potassium bromide, chlorine being more reactive than bromine, displaces bromine from KBr so that bromine gas (Br₂) is liberated. The solution forms light brown colour due to dissolution of Br₂gas in it. So it is an example of displacement reaction.

(ii) Displacement Relations

When chlorine gas is passed through potassium iodide solution, chlorine, being more reactive than iodine, displaces iodine from KI and liberates I2 gas.

The solution acquires violet colour due to dissolution of I₂ gas in it. So, it is also an example of displacement reaction.

Uses of Displacement Reactions:

Displacement reactions are used in the extraction of silver and gold. Silver or gold ore is dissolved in sodium cyanide solution. When zinc granules are added to the solution of the compound formed, zinc, being more active than silver and gold, displaces silver and gold from the solution of their compounds and thus silver and gold are extracted.

Displacement reactions are exothermic:

All displacement reactions are exothermic (heat producing) reactions, For example :

In the displacement reaction between zinc and dilute hydrochloric acid or dilute sulphuric acid, there is production of heat along with evolution of gas.
In displacement reaction between iron and copper sulphate solution, there is an increase in temperature due to production of heat.Thus, displacement reactions are heat producing or exothermic reactions.