CHEMICAL REACTION AND EDUATIONS

CHEMICAL REACTION AND EDUATIONS

1.      Introduction of chemical Reaction

Chemical reaction are the processes in which new substance with new properties are formed.

During a chemical reaction, atom of one element do not change into those of another element. Only a rearrangement of atom takes place in a chemical reaction.

1.      The substance which take part in a chemical reaction are called reactants.

2.      The new substance produced as a result of chemical reaction are called products.

Magnesium              +          Oxygen           Magnesium oxide.

The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.

2.      Characteristics of Chemical Reaction:

The important characteristics of chemical reaction are:

i.                    Evolution of a gas,

ii.                 Formation of a precipitate,

iii.               Change in colour,

iv.               Change in temperature, and

v.                 Change in state.

3.      Chemical Equations:

The method of representing a chemical reaction with the help of symbols and formulae of the substance involved in it is known as chemical equation.

Zinc    +          sulphuric acid        à       zinc sulphate          +         hydrogen.

Now, symbol of zinc is Zn

            Formula of sulphuric acid is H₂SO₄

                Formula of zinc sulphate is ZnSO₄

And, formula of hydrogen is H₂

So, putting the symbols and formula of all the substances in the above word-equation, we get the following chemical equation.

                       à                   

 The substance which combine or react are known as reactants.

The new substance produced in a reaction are known as products.

4.      Balance and Unbalance chemical Equation:

1.      A balanced chemical equation has an equal number or atoms of different elements in the reactants and products.

Zn       +          H₂SO₄                           à        ZnSO₄            +            H₂

Let us count the number of atoms of all the element in the reactants and products separately.

                                                In reactants              in products

No. of Zn atoms:                             1                                  1

No of H Atoms:                               2                                  2

No. of S atoms:                                1                                  1

No. of O atoms:                               4                                  4

A Balance chemical equation has equal masses of various elements in reactants and products.

2.      An unbalance chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

H₂       +          O₂                  à                    H₂O

                                                In reactants                          in products

No of H atoms:                                2                                              2

No of O atoms:                                2                                              1

An unbalance equation masses of various elements in reactants and products.

Problem: 1. Write a balanced equation for the following reaction:

Methane burns in oxygen to form carbon dioxide and water.

Solution: this reaction can be written in the form of a word equation as:

Methane       +          oxygen                       à        carbon dioxide        +         water

Now,  formula of methane is CH₄

            Formula of oxygen is O₂

            Formula of Carbon dioxide is CO₂

And,   formula of water is H₂O

Writing the formulae of all substances in the above word equation, we get

CH₄    +          O₂      à        CO₂               +          H₂O

Atoms Name	No of Reactants	No of Products
C	1	1
H	4	2 X 2 = 4
O	2	3

Let us count the number of various atoms in reactants and products   :

We multiply H₂O by 2 and writing 2H₂O. Thus,

CH₄                +           O₂                  à                    CO₂    +          2H₂O

Counting the number of various atoms and both the sides again, we get: 

Atoms Name	No of Reactants	No of Products
C	1	1
H	4	4
O	2 X 2 = 4	4

We multiply O₂  by 2 and writing 2O₂ :

CH₄                +            2O₂                           à                    CO₂                +          2H₂O

Let us count the number of various atoms on the two side once again:

Atoms Name	No of Reactants	No of Products
C	1	1
H	4	4
O	4	4

This chemical equation contains an equal number of various types of atoms in the reactants and products, so this is a balanced equation.

5.      Types of Chemical Equations:

Some of the important types of chemical reactions are:

1.      Combination Reactions,

2.      Decomposition Reactions,

3.      Displacement Reactions,

4.      Double Displacement Reactions, and

5.      Oxidation and Reduction reactions.

1.      Combination Reactions

Those reactions in which two or more substance combine to form a single substance, are called combination reactions.

(In a combination reaction, two or more element can combine to form a compound; two or more compounds can combine to form a new compound; or an element and a compound can combine to form a new compound.)

Example:

1.      Magnesium and Oxygen combine, when heated, to form magnesium oxide:

            2Mg(s)                       +          O₂(g)        2MgO (s)

          Magnesium               Oxygen                                             Magnesium oxide

2.      Hydrogen burns in oxygen to form water:

            2H₂     +          O₂             2H₂O

          Hydrogen                  Oxygen                                               water

3.      Carbon (Coal) burns in air to form carbon dioxide:

C(s)    +          O₂(g)        CO₂(g)

                           Carbon                Oxygen          Carbon dioxide

4.      Hydrogen combines with chlorine to form hydrogen chloride:

            H₂ (g) +          Cl₂(g)       2HCl (g)

        Hydrogen         Chlorine                                hydrogen Chloride

5.      Sodium metal burns in chlorine to form sodium chloride:

            2Na(s)                       +          Cl₂(g)                   2NaCl (s)

            Sodium                                  Chlorine                                Sodium Chloride

6.      When iron powder is heated with sulphur, iron sulphide is formed:

Fe (s)              +          S (s)          FeS (s)

Iron                            Sulphur                                 Iron Sulphide

7.      Calcium oxide (lime or quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime)

CaO                +          H₂O          Ca(OH)₂

Calcium oxide                     water                         Calcium Hydroxide

(Lime or quicklime)                                               (Slaked Hydroxide)

8.      The calcium hydroxide solution, when applied to the walls, reacts slowly with the carbon dioxide gas present in air to form a thin, shining layer of calcium on the walls of the house:

Ca(OH)₂ (aq)     +          CO₂(g)    CaCO₃ (s)     + H₂O (l)

Calcium Hydroxide           Carbon dioxide                   Calcium Carbonate

9.      Ammonia reacts with hydrogen chloride to form ammonium chloride. This can be written as:

NH₃ (g)                     +          HCl(g)                 NH₄Cl(s)

Ammonia                 Hydrogen chloride             ammonia chloride

10. Carbon monoxide reacts with oxygen to form carbon dioxide:

2CO (g)                     +          O₂                2CO₂

Carbon monoxide                oxygen                                        carbon dioxide

11. Sulphur dioxide reacts with oxygen to produce sulphur trioxide. This reaction can be written as:

2SO₂(g)                      +          O₂             2SO₃ 

Sulphur dioxide                  oxygen                                   sulphur trioxide

2.      Decomposition Reactions:

Those reaction in which a compound splits up into two or more simpler substances are known as decomposition reactions.

A decomposition reaction is just the opposite of a combination reaction.

1.      When Calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.

CaCO₃ (s)             CaO(s)                       +          CO₂ (g)

Calcium carbonate                              Calcium Oxide              Carbon Dioxide

(Limestone)                                                  (Lime)

2.      When Potassium chlorate is heated in the presence of manganese dioxide catalyst, it decompose to give potassium chloride and oxygen.

2KClO₃ (s)       2KCl (s)              +          3O₂ (g)

Potassium chlorate                                          Potassium chloride            Oxygen

3.      When ferrous sulphate is heated strongly, it decomposes to form ferric oxide, sulphur dioxide and sulphur trioxide.

2Fe₂SO₄          Fe₂O₃ +         SO₂     +         SO₃

Ferrous sulphate                                               ferric oxide       sulphur     sulphur

(Green Colour)                                      (Brown Colour)           Dioxide     trioxide

4.      When lead nitrate is heated strongly, it breaks down to form simple substances like lead monoxide, nitrogen dioxide and oxygen.

2Pb(NO₃)₂  (s)        2PbO(s)    +  4NO₂  +     O₂

Lead nitrate                                               lead Monoxide       Nitrogen dioxide

Colourless                                                          Yellow                       Brown fumes

5.      When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

2H₂O (l)                            2H₂ (g)              +       O₂ (g)

Water                                                                            Hydrogen             Oxygen

6.      When electric current is passed through molten sodium chloride, it decomposes to give sodium metal and chlorine gas.

2NaCl (l)              2Na (s)            +      Cl₂ (g)

Sodium chloride                                                    Sodium              Chlorine gas

7.      When electric current is passed through molten aluminium oxide, it decomposes to give aluminium metal and oxygen gas.

2Al₂O₃(l)                4Al (l)               +       3O₂ (g)

Aluminium oxide                                 aluminium metal   oxyge  (Molten)

8.      When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

2AgCl (s)                      2Ag (s)           +          Cl₂ (g)

Silver chloride                                                            Silver                Chlorine

(White)                                             (Greyish white)       (Yellowish-Green)

9.      Silver bromide also behaves in the same way as silver chloride with light energy.

      2AgBr (s)                            2Ag (s)           +          Br₂ (g)

Silver Bromide                                                           Silver                            Bromine

(Pale Yellow)                                     (Greyish white)                 (Red-Brown)

3.      Displacement Reactions:

Those reaction in which one element takes the place of another element in a compound, are known as displacement reaction.

1.   When of zinc metal is placed in copper sulphate solution, then zinc sulphate and copper are obtained.

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

Copper Sulphate          Zinc                            Zinc Sulphate                      Copper

This displacement reaction takes place because zinc is more reactive than copper.

2.   When a piece of iron metal (say, an iron nail) is placed in copper sulphate solution, then iron sulphate solution and copper metal are formed.

    CuSO₄ (aq)     +          Fe(s)         FeSO₄ (aq)    +          Cu (s)

Copper Sulphate          iron                            Iron Sulphate                      Copper

(Blue Solution)              (Grey)                (Greenish Solution)  (Red-Brown)

3.   When a piece of magnesium metal is placed in copper sulphate solution, then magnesium sulphate solution and copper metal are formed :

CuSO₄ (aq)         +          Mg (s)                   MgSO₄ (aq)  +          Cu (s)

Copper Sulphate          Magnesium       Magnesium Sulphate         Copper

4.   When a copper strip is placed in a solution of silver nitrate, then copper nitrate solution and silver metal are formed.

2AgNO₃ (aq)     +          Cu (s)        Cu(NO₃)₂(aq)          +          2Ag (s)

Silver nitrate                 copper                       Copper Nitrate                   silver

(Colourless solution)   (Red-Brown)   (Blue Solution)             (Greyish White)

5.   When a strip of lead metal is placed in a solution of copper chloride, then lead chloride solution and copper metal are formed.

CuCl₂ (aq)             +          Pb (s)                    PbCl₂ (aq)     +          Cu (s)

Copper chloride            Lead                           Lead Chloride                     Copper

(Green Solution) (Bluish Grey)        (Colourless Solution)         (Red-Brown)

6.   Iron metal reacts with dilute hydrochloric acid to form iron (II) chloride and hydrogen gas.

Fe (s)                    +          2HCl (aq)            FeCl₂ (aq)     +          H₂ (g)

Iron                            Hydrochloric                 Iron (II) Chloride      Hydrogen

(As iron Filings)                        acid                            (Ferrous Chloride)

7.   Magnesium Metal Reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.

Mg(s)                   +          2HCl (aq)            MgCl₂ (aq)   +          H₂

Magnesium                    Hydrochloric  Magnesium Chloride            Hydrogen

8.   Sodium Metal Reacts with water to form sodium hydroxide solution and hydrogen gas.

2Na (s)                +          2H₂O (l)                           2NaOH (aq) +          H₂

Sodium                    Water                              Sodium Hydroxide     Hydrogen

9.   Chlorine gas reacts with potassium iodide solution to form potassium chloride and iodine.

Cl_2­            (g)       +          2KI (aq)                           2KCl (aq)      +          I₂

Chlorine              Potassium Iodide                Potassium chloride            Iodine

10.                     When Copper oxide is heated with magnesium powder, then magnesium oxide and copper are formed.

CuO (s)    +          Mg (s)                   MgO(s)                      +          Cu (s)

Copper Oxide    Magnesium              Magnesium Oxide           Copper

11.                     When iron (III) Oxide is heated with aluminium powder, then aluminium oxide and iron metal are formed.

Fe2O3 (s) +          2Al (s)                   Al2O3 (s)      +          2Fe (L)

Iron (III) Oxide   Aluminium                      Aluminium Oxide Iron

(Ferric Oxide)                                                                (Molten)

4.      Double Displacement Reactions

Those Reactions in which two compounds react by an exchange of iron to form two new compounds are called double displacement reactions.

A double displacement reaction usually occurs in solution and one of the products, being insoluble, and precipitation out (separates as a solid). Some of the examples of double displacement reaction are given below.

1.      When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver chloride is formed along with sodium nitrate solution.

AgNO₃ (aq0)     + NaCl (aq)        Agcl(s)           +          NaNO₃ (aq)

Silver Nitrate          Sodium Chloride                Silver Chloride        Sodium Nitrate

                                                                                    (White ppt)

2.      When barium chloride solution is added to sodium sulphate solution, then a white precipitate of barium sulphate is formed along with sodium chloride solution.

BaCl₂(aq)   +   Na₂SO₄(aq)       BaSO₄(s)        +  2NaCl (aq)

Barium Chloride     sodium sulphate           Barium Sulphate     Sodium Chloride

                                                                                    (White ppt)

3.      If barium chloride solution is added to copper sulphate solution, then a white precipitate of barium sulphate is produced along with copper chloride solution.

BaCl₂(aq)  +     CuSO₄(aq)           BaSO₄(s)        +          CuCl₂ (aq)

Barium Chloride     Copper sulphate   Barium Sulphate Copper chloride

4.      When hydrogen sulphate gas is passed through copper sulphate solution, then a black precipitate of copper sulphate is formed along with sulphuric acid solution.

CuSO₄ (aq)  +  H₂S (g)                CuS (s)      +       H₂SO₄(aq)

Copper Sulphate    Hydrogen sulphide copper Sulphide         sulphuric acid

5.       When ammonium hydroxide solution is added to aluminium chloride solution, then a white precipitate of aluminium hydroxide is formed along with ammonium chloride solution.

AlCl₃(aq)            +       3NH₄OH(aq)            Al(OH)₃ (s)+    3NH₄Cl (aq)

Aluminium       Ammonium            Aluminium         Ammonium

chloride                                 Hydroxide                Hydroxide                Chloride

6.      When potassium iodide solution is added to lead nitrate solution, then a yellow precipitate of lead iodide is produced along with potassium nitrate solution.

Pb(NO₃)₂ (aq)    +          KI (aq)                  PbI₂ (s)           +          2KNO₃

Lead Nitrate      Potassium Iodide                lead iodide     potassium nitrate

                                                                              (Yellow ppt)

7.      The reactions between acids and bases to form salts and water are also double displacement reaction.

For example: sodium hydroxide and hydrochloride acid react to form solution chloride and water.

NaOH (aq)               +          HCl (aq)               NaCl (aq)     +          H₂O (l)

Sodium Hydroxide            Hydrochloric Acid             Sodium chloride     Water

5.      Oxidation and Reduction Reactions

Oxidation:

i.)                 The addition of oxygen to a substance is called oxidation.

ii.)              The removal of hydrogen from a substance is also called oxidation.

Reduction:

i.)                 The addition of hydrogen to a substance is called reduction.

ii.)              The removal of oxygen from a substance is also called reduction.

Oxidising Agent:

a.)   The substance which gives oxygen for oxidation is called an oxidising agent.

b.)   The substance which removes hydrogen is also called an oxidising agent.

Reducing Agent:

a.)    The substance which gives hydrogen for reduction is called a reducing agent.

b.)   The substance which removes oxygen is also called a reducing agent.

The oxidation and reduction reactions are also called redox reaction (In the name ‘redox’, the term “red” stands for ‘reduction’ and “ox” stands for ‘oxidation’).

Ex: when copper oxide is heated with hydrogen, then copper metal and water are formed.

CuO               +          H₂                     Cu       +          H₂O

Copper oxide     Hydrogen                 copper     Water

In this reaction, CuO is changing into Cu. That is, oxygen is being removed from copper oxide. Now, by definition, removal of oxygen from a substance is called reduction, so we can say that copper oxide is being reduced to copper.

Removal of oxygen: reduction

In this reaction H₂ is changing into H₂O. That is, oxygen is being added to hydrogen. Now, by definition, addition of oxygen to a substance is called oxidation, so we can say that hydrogen is being oxidised to water.

CuO       +             H2                                                           Cu           +             H2O

Heat

Addition of oxygen: Oxidation

 

6.      Corrosion:

Corrosion is the process in which metal are eaten up gradually by the action of air, moisture or a chemical (such as acid) on their surface. Corrosion is caused mainly by the oxidation of metals by the oxygen of air. Rusting of iron metal is the most common form of corrosion. When an iron object is left in damp air for a considerable time, it gets covered with a red-brown flaky substance called “Rust”. This is called rusting of iron.

During the corrosion of iron (or rusting of iron), iron metal is oxidised by the oxygen of air in the presence of water (moisture) to form hydrated iron(III) oxide called rust.

4Fe      +          3O₂     +          2x H₂O                      à        2Fe₂O.xH₂O

Iron                Oxygen                      water             hydrated iron (III) oxide (Rust)

Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc., and cuts short their life.

7.      Rancidity:

The condition produced by aerial oxidation of fats and oils in food marked by unpleasant and taste is called rancidity.

Rancidity spoils the food materials prepared in fats and oils which have been kept for considerable time and makes them unfit for eating. The characteristics of a rancid food are that it gives out unpleasant smell and also has an unpleasant taste. Rancidity is called “vikritgandhita” in Hindi.

The development of rancidity of food can be prevented or retarded (slowed down) in the following ways:

1.      Rancidity can be prevented by adding anti-oxidants to food containing fats and oils.

2.      Rancidity can be prevented by packaging fat and oil containing food in nitrogen gas.

3.      Rancidity can be retarded by keeping in a refrigerator

Rancidity can be retarded by storing food in air-tight containers.