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ICSE Class 8 Chemistry Question 2 of 15

Chemical Reactions — Question 4

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Question 4

Explain the following types of chemical reactions giving two examples for each of them:

(a) combination reaction

(b) decomposition reaction

(c) displacement reaction

(d) double displacement reaction

Answer

(a) Combination Reaction — A reaction in which two or more substances combine to form a single substance is called a combination reaction. It is also called a synthesis reaction.

Examples:

(i) Two elements combine to form a compound.

When iron and sulphur (both elements) are heated together they combine to form a compound iron sulphide.

Fe (s) + S (s) →Δ\xrightarrow{\enspace\Delta\enspace} FeS (s)

(ii) An element and a compound can also combine to form a product.

Carbon monoxide, a compound, burns in presence of oxygen, an element, to form a single product, carbon dioxide.

2CO (g) + O2 (g) →Δ\xrightarrow{\enspace\Delta\enspace} 2CO2 (g)

(b) Decomposition Reaction — A reaction in which a compound breaks up on heating into two or more simpler substances is called a decomposition reaction.

Examples:

(i) Calcium carbonate decomposes on strong heating to form two compounds, calcium oxide and carbon dioxide.

CaCO3 (s) →Δ\xrightarrow{\enspace\Delta\enspace} CaO (s) + CO2 (g)

(ii) Mercuric oxide when heated decomposes to form mercury and oxygen.

2HgO (s) →Δ\xrightarrow{\enspace\Delta\enspace} 2Hg (s) + O2 (g)

(c) Displacement Reaction — A reaction in which a more active element displaces a less active element from a compound is called a displacement reaction.

Examples:

(i) Zinc is more reactive than copper so it displaces copper from copper sulphate solution to form zinc sulphate.

Zn (s) + CuSO4 (aq) ⟶ ZnSO4 (aq) + Cu (s)

(ii) Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution.

2KBr + Cl2 ⟶ 2KCl + Br2

(d) Double Displacement Reaction — A chemical reaction in which two compounds in their aqueous state exchange their ions or radicals to form new compounds is called a double decomposition reaction.

AB + CD ⟶ CB + AD

Here AB and CD are reactants. They exchange their ions to form CB and AD which are the products.

Examples:

(i) When a solution of silver nitrate is added to a solution of sodium chloride a precipitate of silver chloride is formed.

AgNO3 (aq) + NaCl (aq) ⟶ NaNO3 (aq) + AgCl ↓

(ii) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed.

FeSO4 (aq) + 2NaOH (aq) ⟶ Fe(OH)2 ↓ + Na2SO4 (aq)