Chapter - 1, Chemical Reactions and Equations NCERT Solution

PAGE No – 6

Q1 – Why should a magnesium ribbon be cleaned before burning in air ?
Ans – Magnesium ribbon is cleaned before burning to remove the protective layer of basic magnesium oxide form the surface of ribbon. The layer is stable and prevent reaction so it removed by sand paper.

Q2 – Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine    →  Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate →  Barium sulphate + Aluminium chloride
           (iii) Sodium +  Water    →     Sodium hydroxide + water
Ans – The balanced chemical equation are as follows :-
        (i) H2 + Cl2     →    2 HCl.
(ii) 3BaCl2 + Al2 (SO4)3   →    3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O    →  2NaOH + H2

Q3 – Write a balanced chemical equation with state symbols for the following reactions.
(i) Solution of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii)Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans – Balanced chemical reaction with state symbols are as follows :-
(i)  BaCl2 (aq) + Na2SO4 (aq)    →    BaSO4 (s) + 2NaCl (aq)
(ii)  NaOH (aq) + HCl (aq)       →    NaCl (aq) + H2O

PAGE NO – 10

Q1 – A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Ans – (i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) CaO (s) + H2O   →   Ca(OH)2 (aq)

Q2 – Why the amount of gas collected in one of the tubes in Activity 1.7 double of the amount collected in the other ?
Ans – The gas which is collected in double the amount in electrolysis of water experiment is hydrogen. This is because water contain 2 hydrogen element as compare to oxygen.

PAGE NO – 13

Q1 – Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Ans – When iron nail is dipped in copper sulphate solution, than iron sulphate solution and copper metal are foamed. In this reaction iron displaces copper from copper sulphate solution.
CuSo (aq) + Fe (s)    →      FeSO4 (aq) + Cu (s)

Q2 – Give an example of a double displacement reaction other than the one given in Activity 1.10.
Ans – Another example of double displacement reaction.
Pb(NO3)2 (aq) + 2KI (aq)     →   PbI2 (s) + 2KNO3 (aq)

Q3 – Identify the substances that are oxidized and the substances that are reduced in the following reactions.
(i)  4Na (s) + O2 (g)     →        2Na2O (s)
(ii)  CuO (s) + H2    →     Cu (s) + H2O (l)
Ans - (i)  4Na (s) + O2 (g)   →     2Na2O (s)
In this reaction Na is oxidized because it combines with O2 to form Na2O. And O2 is reduced because it is converted into Na2O.

(ii)  CuO (s) + H2     
 →   Cu (s) + H2O (l)
In this reaction, CuO is reduced because it loses oxygen. H2 is oxidized because it combines with Oxygen.


Q1 – Which of the statement about the reaction below are incorrect ?
2PbO (s) + C (s)      →    2Pb (s) + CO2 (g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized
(c) Lead oxide is getting oxidized
(d) Lead is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All.
Ans – As statement (a) and (b) are incorrect, answer (i) is correct.

Q2 – Fe2O3 + 2Al     →
   Al2O3 + 3Fe
The above reaction is an example of a
(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction
Ans – This is an example of displacement reaction because Fe in FeOhas displaced by Al. Then the correct answer is (d)

Q3 – What happens when dilute hydrochloric acid is added to iron filling ? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced
(c) No reaction takes place
(d) Iron salt and water are produced
Ans – Answer (a) is correct.

Q4 – What is balanced chemical equation ? Why should chemical equation be balanced ?
Ans – A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.
The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in chemical reaction the total mass of reactants should be equal to the total mass of products.

Q5 – Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and Sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans –
(a) 3H2 (g) + N2 (g)   →  2NH3 (g)
(b) 2H2S (g) + 3O2 (g)    →  2H2O (l) + 2SO2 (g)
(c) 3 BaCl(aq) + Al2(SO4)3 (aq)   →   2AlCl (aq) + 3 BaSO4 (s)
(d) 2K (s) + 2H2O (l)   →   2KOH (aq) + H2 (g)

Q6 – Balance the following chemical equations .
(a) HNO3 + Ca(OH)2   →   Ca(NO3)2 + H2O
(b) NaOH + H2SO4   →   Na2So4 + H2O
(c) NaCl + AgNO3  →  AgCl + NaNO3
(d) BaCl2 + H2SO4   →    BaSO4 + HCl
Ans –
(a) 2HNO3 + Ca(OH)2   →  Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO →  Na2So4 + 2H2O
(c) NaCl + AgNO →  AgCl + NaNO3
(d) BaCl2 + H2SO4   →  BaSO4 + 2HCl

Q7 – Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water.
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium Sulphate → Barium Sulphate + Potassium chloride
Ans –
(a) Ca(OH)2 + CO2  →   CaCO3 + H2O
(b) Zn + 2AgNO3   →  Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2  →  2AlCl + 3Cu
(d) BaCl2 + K2SO4  →   BaSO4 + 2KCl

 Q8 – Write the balanced chemical equation for the following and identify the type of reaction in case.
(a) Potassium bromide (s) + Barium iodide (aq)   →   Potassium iodide (aq) + Barium bromide (s)
(b) Zinc carbonate (s)    →   Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g)   →   Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq)   →  Magnesium chloride (aq) + Hydrogen (g)
Ans – Balanced Chemical equations are
(a) 2KBr (aq) + BaI2 (aq)   →  2 KI (aq) + BaBr2 (aq)
Double Displacement Reaction
(b) ZnCO3 (s)     →   ZnO (s) + CO(g)
Decomposition Reaction
(c) H2 (g) + Cl2 (g)    →    2HCl
Combination Reaction
(d) Mg (s) + 2HCl (aq)   →   MgCl2 (aq) + H2 (g)
Displacement Reaction

Q9 – What does one mean by exothermic and endothermic reactions ? Give Examples
Ans – A reaction in which energy is released in the form of heat or light is called exothermic reaction. Ex -    CH+ 2O2   →   Co2 + 2H2O + Heat
A reaction in which energy is absorbed or require energy to proceed are called endothermic reaction. Ex -  N2 + O  →   2NO

Q10 – Why is respiration considered an exothermic reaction ? Explain
Ans – During respiration we inhale oxygen form atmosphere which reacts with glucose in our body cells to produce carbon dioxide and water.
C6H12O6 (aq) + 6O2   →    6CO2 (g) + 6H2O (l) + heat
In this reaction heat also release during chemical reaction, hence respiration is considered an exothermic reaction.

Q11 – Why decomposition reaction are called the opposite of combination reactions ? Write equations for these reactions.
Ans – In combination reaction, two or more substances react to produce single substance while in a decomposition reaction, a single substance breaks down into two more substances.
Example of combination reaction : -
CaO (s) + CO2 (g)    →    CaCO3 (s)
Example of decomposition reaction : -
2H2O (l)    →    2H2 (g) + O2 (g)

Q12 – Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, electricity.
Ans – (i) Decomposition is carried by heat is called thermal decomposition
Ex – CaCO(s) + heat   →   CaO (s) + CO2 (g)
(ii) Decomposition is carried by electricity passing is called Elecctrolytic decomposition
Ex - 2H2O (l) + Electric current    →    2H2 (g) + O2 (g)
(iii) Decomposition is carried out by light is called photolysis decomposition
Ex – 2AgCl (s) + Sunlight  →  2Ag (s) + Cl2

Q13 – What is the difference between displacement and double displacement reactions ? Write equations for these reactions.
Ans -  In Displacement reaction, more reactive element displaces (replace) a less reactive element form a compound.
Ex – CuSO4 (aq) + Zn (s)   →   ZnSO4  (aq) + Cu (s)
But In Double displacement reaction, exchange of ions takes place and new compound foamed.
Ex – HCl (aq) + AgNO3 (aq)   →   AgCl (s) +HNO3

Q14 – In refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Ans – Silver Nitrate + Copper   →  Copper nitrate + Silver
AgNO3  (aq) + Cu (s)   →   Cu(NO3)2 (aq) + 2 Ag (s)

Q15 - What do you mean by a precipitation reaction ? Explain by giving examples.
Ans – A Reaction in which an insoluble solid is formed is called a precipitation reaction.
Ex- Na2CO3 (aq) + CaCl2 (aq)   →   CaCO3 (s) + 2NaCl (aq)

Q16 – Explain  the following in terms of gain or loss of oxygen with two examples each ?
a. Oxidation
b. Reduction
Ans – Oxidation – Addition of oxygen or removal of hydrogen in a chemical reaction is called oxidation reaction. For example
2Cu + O2   →  2CuO
4Al  + 3O2   →   2Al2O3
Reduction – Addition of hydrogen or removal of oxygen in a chemical reaction is called oxidation reaction. For example
CuO + Cl2   →   Cu + H2O
H2 + Cl2  →  2HCl

Q17 – A shiny brown coloured element ‘X’ one heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Ans – The brown coloured element ‘X’ is copper on heating in air it forms copper oxide, which is black in colour. The chemical reaction is as follows : -
2Cu + O →  2CuO

Q18 – Why do we apply paint on iron articles ?
Ans – We apply paint on iron articles to prevent rusting. Iron articles do not come in contact of atmospheric oxygen and moisture and thus the rusting is prevented.

Q19 – Oil and fat containing food items are flushed with nitrogen. Why ?
Ans – Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack.

Q20 – Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans – (a) Corrosion – Action of air, moisture, water and acid on metal surfaces to form oxides is called corrosion.
Ex – Rusting of Iron, Black coating on silver
(b) Rancidity – The oxidation of fats and oils when exposed to air is known as rancidity.
Ex – Bed smell and bed taste of food

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