Simple chemical reactions

(1) Combination reactions

(2) Decomposition reactions

(3) Displacement reactions

(4) Double decomposition reactions

Let us study these reactions with suitable examples.

(a) Let us consider heating of magnesium ribbon in air. As seen earlier, Mg metal, when ignited, combines with oxygen and only one product is formed, i. e., magnesium oxide (MgO) which is the white powder. The chemical reaction is written as

2Mg + 02-* 2MgO

Single product

2HC11

(Hydrogen chloride) (Single product)

H2T + Cl2t (Hydrogen gas) (Chlorine gas)

Such reactions are called combination reactions. Thus we can define combination reaction as -

A chemical reaction in which two or more reactants combine to form a single product, is called combination reaction.

(b) Hydrogen gas combines with chlorine gas to form hydrogen chloride.

(c) Ammonia reacts with hydrogen chloride and forms ammonium chloride.

NH3T + HC1T -? NH4CI t

(Ammonia) (Hydrogen chloride) (Ammonium chloride)

(d) Reaction between quick lime or calcium oxide (CaO) and Water. Demonstration:

Take a beaker half filled with water; add few pieces of CaO to water. As the reaction proceeds, temperature of water increases and the pieces of CaO dissolve slowly. The reaction between CaO and water gives calcium hydroxide Ca(OH)2. The reaction is -

CaO + H20 _? Ca(OH)2 + Heat

(Single product)

As two r6actants'give a single product, it is a combination reaction. Ca(OH)2 so formed can be tested by passing carbon dioxide (C02) gas through this solution. It turns milky due to the following reaction.

Ca(OH)2 + C02T-? CaC03 + H20

Milky

Consider the following reactions:

(a) When mercuric oxide (HgO) is strongly heated, it decomposes or splits into mercury and oxygen gas. The decomposition is brought about by heat. So it is called thermal decomposition.

2HgO(s) Heat t 2Hg (/) +02t

(b) When electric current is passed through acidified water (water in which a few drops of sulphuric acid are added), then water splits or. decomposes into hydrogen and oxygen. As this decomposition is brought about by electrical energy, it is known as electrolytic decomposition or electrolysis.

2H20 (/) -? 2H2T + 02T

(Acidified water)

Now we are ready to define decomposition reaction.

A chemical reaction in which a compound is split into two or more simple substances is called as decomposition reaction.

As seen above, for decomposition reactions energy is required. When heat energy is used for decomposition, it is known as thermal decomposition. When electric energy is used, it is known as an electrolytic decomposition or electrolysis.

(c) Decomposition of calcium carbonate CaC03:

Demonstration :

Take a pinch of CaC03 in a hard glass test tube and close it with a cork fitted with a bent tube.Fix the test tube on a stand or hold it with the help of a test tube holder. Heat it using burner as shown in fig. 3.1.

On heating CaC03, a gas is evolved. The evolved gas is allowed to pass through the bent tube into lime water. It is observed that lime water turns milky. Thus the evolved gas is carbon dioxide (C02) and the residue left in the hard glass tube will be CaO. This reaction is represented by chemical equation as follows :

CaC03(s) Heat > CaO (s) + C02

As heat energy is required for above decomposition, it is a thermal decomposition reaction.

Consider the following reactions :

(a) Reaction between zinc (Zn) metal and hydrochloric acid (HC1).

In this reaction H2 gas is evolved and Zn metal dissolves slowly to form soluble zinc chloride (ZnCl2).

Zn (s) + 2HCl(aq.)--? ZnCl2 (aq.) + H2T

In this reaction hydrogen atom from HCl .is displaced by zinc.

(b) Chlorine gas (Cl2) when passed through potassium iodide (KI) solution, insoluble iodine I2 appears along with formation of water soluble potassium chloride.

2KI (aq.) + Cl2t-? 2KC1 (aq.) + I2 I (s)

In this reaction iodine from KI is displaced by chlorine. Now we can define displacement reaction. A chemical reaction in which an atom or group of atoms is displaced by another atom or group of atoms to form a new substance is called as displacement reaction.

(c) Reaction between iron metal and copper sulphate solution.

Demonstration :

Take CuS04 solution in beaker. Colour of solution is peacock blue. Dip a clean iron nail in CuS04 solution. Keep the beaker undisturbed for some time.

You will observe that peacock blue colour of CuS04 solution fades slowly and finally becomes green. The iron nail when taken out from the solution shows a pale brown shiny coat on the surface.

Colour change of solution and the shiny coat on nail is due to the reaction.

i. e., Copper from CuS04 solution is displaced by Fe and green coloured solution of FeS04 is formed while displaced copper metal is coated on nail. This is a displacement reaction.

Consider the following reactions:

(a) When hydrogen sulphide (H2S) gas is passed through copper sulphate solution [CuS04 (aq.) ] then black precipitate of copper sulphide (CuS) is formed along with sulphuric acid (H2S04).

CuS04(aq.) + H2ST -+ CuS 4 + H2S04(aq.)

In above reaction sulphate ion (S04)2" and sulphide ion S2~, mutually -exchange places to form two new compounds.

(b) If dilute sulphuric acid is added to barium chloride solution [BaCl2 (aq.)] then white precipitate of barium sulphate [BaS04] is formed along with hydrochloric acid.

BaCl2 (aq.) + H2S04 (aq.) --?BaS04«l+2HCl (aq.)

In this reaction also chloride ion (Cf) and sulphate radical (S04)2" mutually exchange places to form two new compounds.

Above reactions are double decomposition reactions and now we can define double decomposition reaction as;

A chemical reaction in which the constituents (ions or radicals) of two compounds mutually exchange places to form two new compounds is called as double decomposition reaction.

These reactions usually occur between ionic compounds.

(c) Reaction between silver nitrate (AgN03) solution and common salt (NaCl) solution:

+ ' Fe (s) Iron nail

> FeS04 (aq.) + Cu (s) 4 Green Sh'iny coat on nail

CuS04 (aq.) Peacock blue

Demonstration :

Take a small amount of NaCl solution in a test tube. Add small amount of AgN03 solution dropwise and stir.

After adding solution of AgN03 to NaCl solution, a white solid separates out, i. e., precipitate is formed. Reaction between NaCl and AgN03 is shown by the following chemical equation.

NaCl(aq.) + AgN03 (aq.)--? AgCl'4> +NaN03 (aq.)

In this reaction chloride ion, Cf and nitrate radical (white precipitate) (N03)" of reactants mutually exchange their places to form two new compounds, i. e., precipitate of silver chloride and soluble sodium nitrate (NaN03). Thus this is a double decomposition reaction.

Double decomposition reactions are also referred to as double displacement reactions.

All of the above reactions are classified on the basis of simple rearrangement of atoms or ions. Now let us see an example to study the rate of reaction or velocity of reaction.

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