1. Chemical Reactions and Equations

CHAPTER  1.
CHEMICAL REACTIONS AND EQUATIONS
PAGE No: 6

1. Why should a magnesium ribbon be cleaned before burning in air ?

Magnesium reacts slowly with  oxygen present in the atmosphere and forms a thin layer of coating of magnesium oxide.

This has to be removed before burning , otherwise the magnesium oxide coating resist further oxidation

2. Write the balanced equation for the following chemical reaction

(i) Hydrogen  + Chlorine    à    Hydrogen chloride

       H₂ + Cl₂    à   2HCl

(ii)  Barium chloride  + Aluminium sulphate  à Barium sulphate + Aluminum chloride

      3BaCl₂   +  Al₂ (SO₄)₃   à      3BaSO₄   + 2AlCl₃

(iii)  Sodium  + Water  à  Sodium hydroxide  + Hydrogen

       2Na  +  2H₂O    à     2NaOH  + H₂

3.  Write a balanced chemical equation with state symbols for the following reactions

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride

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

(ii)  Sodium hydroxide solution ( in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water

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

PAGE No: 10

1. A solution of a substance'X' is used for white washing ?

(i) Name the substance "X" and write its formula

Calcium oxide  CaO

(ii) Write the reaction of the substance "X"  with water

CaO (s)      +      H₂O(l)   à     Ca(OH)₂

_{Quick lime                                         Slaked lime}

2. Why is the amount of gas collected in one of the testtubes in activity 1.7 double of the amount collected in the other? Name this gas

In activity 1.7, by the electrolytic decomposition of water, we get hydrogen at cathode and oxygen at anode . This can be given by the chemical equation

2H₂O(l)    à     2H₂(g)  + O₂(g)

Hydrogen combines with oxygen  in the ratio 2: 1 to form water.
Hence the volume of hydrogen produced  is double the volume of oxygen produced

There for the amount of gas collected at cathode is double the amount of gas collected in the anode

The gas produced  at the cathode which is double the other one is hydrogen

PAGE No: 13

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

When an iron nail is dipped in copper sulphate solution which is blue in colour , its colour changes to green
This is because iron which is  comparatively more reactive than copper displaces copper from its solution.

The overall reaction can be written as

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

And hence this is a displacement reaction.

2. Give an example of a double displacement reaction other than the one given in activity 1.10

Lead nitrate reacts with hydrochloric acid to give lead chloride and nitric acid

Pb(NO₃)₂ (aq)  + 2HCl(aq)   à   PbCl₂ (s)  +  2HNO₃(aq)

 The precipitate of lead chloride is formed when Pb⁺⁺   ions from lead nitrate combines with 2Cl^- ions of hydrochloric acid.

An aqueous solution of nitric acid is formed H⁺ ions combines with NO₃^- ions

3. Identify the substances that are oxidized and the substances that are reduced in the following reactions  

(i)    4Na(s)  + O₂(g)     à             2Na₂O (s)

Sodium is oxidized and oxygen is reduced 

Oxygen atom is added to sodium and hence oxidized.
From a molecule of oxygen one atom is released and hence reduced.

 (ii)  CuO (s) + H₂( g)       à              Cu (s)  + H₂O (l)

Hydrogen is oxidized and copper oxide is reduced

Copper oxide looses oxygen atom to form copper , hence reduced.
Hydrogen gains oxygen atom to form water and hence oxidized

EXERCISES

PAGE No: 14

1. Which of the statements about the reaction below are incorrect?

2PbO(s)  + C (s)  à    2Pb(s) + CO₂(g)

(a) Lead is getting reduced
(b) Carbon dioxide is getting oxidized
(c) Carbon is getting oxidised
(d) Lead oxide is getting reduced


(a) and (b) are incorrect  and both (c) and (d) are correct.
Carbon gains oxygen and hence oxidized.
Lead oxide looses hydrogen and reduced

2. Write the balanced equation for the following chemical reaction

Fe₂O₃   +  2Al    à     Al₂O₃  + 2 Fe

The above reaction is an example of  displacement reaction

Here aluminium is comparatively more reactive than iron.
In the reactivity series aluminium is above iron
Hence it displaces iron from its  solution.

3. What happens when dilute hydrochloric acid is added to iron filings?

Hydrogen gas and iron chloride are produced

It is a displacement reaction.

In the reactivity series chlorine is above hydrogen
Hence it displaces hydrogen from its  solution.

4. What is a balanced chemical equation? Why should chemical equation be balanced?

A chemical equation in which the number of atoms of each element in the reactant and the product side are equal , then the chemical equation is said to be balanced.

Principle of balancing a chemical equation is based on the law of conservation of mass

The number of atoms of each element remains the same before and after a chemical reaction, so we need to balance a skeletal chemical equation

5. Translate the following statements into chemical equations and then balance them

(a)  Hydrogen gas combines with nitrogen gas to form ammonia

       3H₂(g)  + N₂ (g)   à   2NH₃(g)

(b)  Hydrogen sulphide gas burns in air to give water and sulpur dioxide

       2H₂S(g) + 3O₂(g)   à 2H₂O(l)  + 2SO₂(g)

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of          barium sulphate

        3BaCl₂ (aq)  +  Al₂ (SO₄)₃ (aq)  à     3BaSO₄ (s) + 2AlCl₃(aq)

(d) Potassium metal react with water to give potassium hydroxide and hydrogen gas

      2K (s)  +  2H₂O(l)      à      2KOH (aq) + H₂(g)

6. Balance the following chemical equations?

(a)   2HNO₃  + Ca(OH)₂   à  Ca(NO₃)₂  + 2H₂O


(b) 2NaOH  + H₂SO₄    à Na₂SO₄  + 2H₂O


(c) NaCl  + AgNO₃  à AgCl  + NaNO₃



(d) BaCl₂  + H₂SO₄   à BaSO₄ + 2HCl

7. Write the balanced chemical equations for the following reactions?

(a) Calcium hydroxide  +  Carbon dioxide  à   Calcium carbonate + Water

      Ca(OH)₂   + CO₂     à  CaCO₃  + H₂O

(b)  Zinc   +  Silver nitrate à Zinc nitrate  + Silver

      Zn  + 2AgNO₃   à   Zn(NO₃)₂  + 2Ag

(c)  Aluminium + Copper chloride  à  Aluminium chloride  + Copper

       2Al + 3CuCl₂  à 2AlCl₃ + 3Cu

(d)  Barium chloride + Potassium sulphate  à  Barium Sulphate + Potassium chloride

       BaCl₂ + K₂SO₄    à BaSO₄  + 2KCl

8. Write the balanced chemical equations for the following and identify the type of reaction in each case

(a) Potassium bromide (aq) + Barium iodide (aq) à Potassium iodide (aq) + Barium bromide(s)

      2KBr (aq)  + BaI₂ (aq) à  2KI (aq)  + BaBr₂(s)

       Double displacement reaction

(b) Zinc carbonates(s)   à Zinc Oxide (s) +Carbon dioxide  (g)

      ZnCO₃ (s)  à ZnO(s) + CO₂(g)

      Decomposition reaction

(c) Hydrogen (g) + Chlorine (g)     à    Hydrogen chloride (g)

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

     Combination Reaction

(d) Magnesium (s) + Hydrochloric acid (aq)   à  Magnesium chloride (aq) + Hydrogen (g)

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

     Displacement reaction

9. What does one mean by exothermic and endothermic reactions ? Give examples.

Exothermic Reaction:

The reaction in which heat is liberated along with the formation of products is known as exothermic reaction
Burning of natural gas

CH₄  + 2O₂  à CO₂ + 2H₂O  + heat

Endothermic Reaction:

The reaction in which heat is absorbed during the formation of products is known as exothermic reaction 

Heating of calcium carbonite

CaCO₃  + heat   à CaO + CO₂

10. Why is respiration considered an exothermic reaction? Explain

During respiration carbohydrates are converted into glucose.
And further glucose gets oxidized to simpler substances such as carbon dioxide and water along with the liberation of energy in the form of heat

Hence respiration is considered as an exothermic reaction

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions

Decomposition reaction: 

Here the compound splits into two or more simpler compounds or elements
Eg: While heating calcium carbonate decomposes to form calcium oxide and carbondioxide

CaCO₃  + heat   à CaO + CO₂

Combination Reaction:
Here two or more reactants combines to form a single product
Eg: Carbon combines with oxygen to form carbondioxide

C + O₂ à CO₂

12. Write one equation each for decomposition reaction where energy is supplied in the form of        heat, light or electricity

Heat :
Calcium carbonite when heated decomposes to form calcium oxide and carbondioxide.
CaCO₃  + heat   à CaO + CO₂

Here energy is supplied in the form of heat

Sunlight :
Silver chloride in the presence of sunlight turns gray . This is due to the decomposition of silver chloride to silver and chlorine. This reaction was utilized in was used in old black and white photography

2AgCl   + sunlight    à 2Ag + Cl₂

Electricity:

Electrolysis of water. Here electricity is used to decompose  water molecule into hydrogen and oxygen

2H₂O  à H₂  + O₂

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions

Displacement Reaction:

The reaction in which a highly reactive metal displaces a less reactive from its solutions

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

Double displacement Reaction:

The reaction in which there is an exchange of ions between the reactants

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

14. In the refining of silver , the recovery of silver from silver nitrate solution involved                      displacement by copper metal. Write down the reactions involved

Here copper displaces silver from its nitrate solution and the products are copper nitrate   and silver. Hence silver get refined

Cu + 2AgNO₃   à   Cu(NO₃)₂  + 2Ag

15. What do you mean by a precipitation reactions?

Those reactions in which the reactants combines to form an insoluble substance called precipitate 

Eg: BaCl₂ (aq)  + Na₂SO₄ (aq)  à    BaSO₄ (s)  + 2NaCl  (aq)
Here is the BaSO₄  precipitate

16. Explain the following in  terms of gain or loss of oxygen with two examples each?

(a) Oxidation

The process by which a molecule gains  oxygen atom or atoms

C + O₂ à CO₂

2Mg + O₂   à   2MgO

(b)  Reduction

The process by which a molecule lose  oxygen atom or atoms
2H₂O(l)    à     2H₂(g)  + O₂(g)    
2MgO   à   2Mg + O₂

17. A shiny brown colored element "X" on heating in air becomes black in colour. Name the element "X" and the black coloured compound formed

Element "X" is Copper
Black coloured compound is copperoxide

2Cu  +   O₂   à   2CuO

18. Why do we apply paint on iron articles?

When iron is exposed to air which is humid, the iron reacts with oxygen to form iron oxide.
This is a brown flaky substance. Hence it  corrodes.

So iron articles can be protected by applying paint, to prevent direct contact with atmosphere

19. Oil and fat containing items are flushed with nitrogen. Why?

The food containing oil and fat get rancid.
This is due to the oxidation of food in the presence of oxygen.
Rancid food is not good for consumption
Hence oil and fat containing items are flushed with nitrogen.

20. Explain the following terms with one example each

(a) Corrosion

Metals when exposed to air which is humid, reacts with substances of atmosphere  to form various surface compounds. Corrosion causes damage to the metal articles like car bodies, iron bridges, ships , iron railings etc
Chemical formula of rust is  Fe₂O₃.2H₂O  

(b) Rancidity

Rancidity is due to the oxidation of food containing oil and fat  in the presence of oxygen.
Rancid food is not good for consumption because the smell and taste changes

It can be reduced by

 Keeping in air tight containers
 Refrigerating the food.
 Flushing packets of foods with nitrogen.
Storing food in vacuum packets
Antioxidants such as Butylated hydroxy anisole ( BHA)  and  Butylated hydroxy toluene ( BHT)