NCERT class 10 very important frequently asked 3 mark questions of science ch-1 (part–1)

Very Important 3 mark questions of science Chemical reactions and equations (part 1): 

1. What is a chemical equation? Illustrate with an example.

Ans: The mode of communicating facts in chemical language about a chemical reaction is called chemical equation. Thus chemical equation is an expression for a given chemical change in terms of symbols aur formulae of the reactants and products. For example, the reaction of Zinc with dilute sulphuric acid to produce zinc sulphate and hydrogen is given by the following chemical equation:

          Zn + H2SO4 –—› ZnSO4 + H2.

2. list four observations that help us to determine whether a chemical reaction has taken place.

Ans: When a chemical reaction occurs, One or more of the following changes take place: 

      (i) change in state

     (ii) change in colour

    (iii) evolution of a gas

    (iv) change in temperature  or

     (v) formation of precipitate.

3. How would you justify that a chemical reaction has taken place in the following cases?

(i)   Burning of magnesium ribbon in air.

(ii)  addition of lead nitrate solution to potassium iodide solution.

(iii) addition of dilute hydrochloric acid to zinc granules.

Ans: (i) Burning of magnesium ribbon in air gives a powder of MgO. So there is a change of state and heat is produced.

         (ii) addition of colourless lead nitrate solution to potassium iodide solution gives yellow colour precipitate (lead iodide). So there is a change of colour.

       (iii) addition of dilute hydrochloric acid to zinc granules gives a gas H2 with effervescence and heat is evolved. There is change in temperature.

4. How is a chemical equation written? Illustrate with an example.

Ans: Reaction writing should be done in a systematic manner, using the conventions followed. These are illustrated with the help of an example. Let it be required to write a chemical equation that describes the reaction between magnesium and sulphuric acid to produce magnesium sulfate and hydrogen.

      (i) Write the symbols of all the substances involved in the reaction-first the reactants and then products-one after the other ON THE SAME LINE

        Reactants                        Products

     Mg       H2SO4               MgSO4      H2

    (ii) Separate the reacting substances from the products by a symbol meaning ‘produce’. The sign –—› (an arrow) or sometimes sign = (equal) is used for this purpose.

       Mg     H2SO4 –—› MgSO4      H2

    (iii) Then put (Plus) + sign between the reactants indicating that these ‘react’ and a (plus) + sign between the products which signifies ‘and’,

       Mg + H2SO4 —–› MgSO4 + H2

5. What is meant by a skeletal chemical equation? Using suitable chemical reaction, differentiate between a skeletal chemical equation and a balanced chemical equation.

Ans:If the number of atoms of any elements in a chemical equation is not equal on both the sides, then it is a skeletal equation. For example,

  Mg + HCl —–› MgCl2 + H2

        Here, the number of Chlorine and Hydrogen atoms are not equal on both the sides. 

         In a balanced equation, the number of atoms of different elements on both sides of a chemical equation are equal. For example,

   Mg + 2HCl –—› MgCl2 + H2

6. (a) Why do we need to balance a chemical equation? 

 (b) Using the chemical formulae write balanced chemical equations for the following reactions:

       (i) Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide.

      (ii) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans: (a) The no. and kind of atoms on both sides of the chemical equation are made equal, i.e., equation is balanced since atoms cannot be created or destroyed.

  (b)  (i) CS2 + O2 –—› CO2 + SO2

              CS2 + 3O2 —–› CO2 + 2SO2

        (ii) K + H2O —–› KOH + H2

             2k + 2H2O —–› 2KOH + H2

7. What does a balanced chemical equation convey to a chemist?

Ans: The following information is conveyed by a balanced chemical equation:

      (i) Formulae of substance taking part in chemical reaction i.e., reactants.

     (ii) Formulae of substances produced in the reaction i.e., products.

    (iii) The relative no. of molecules of reactants and products.

    (iv) The relative masses of reactants and products.

     (v) The relative volumes of gaseous substances involved in the reaction.

8. What are the essentials of a chemical equation?

 

Ans: (i) It must refer to an actual chemical reaction.

     (ii) It should be Molecular. For example; H,O,N should be written as H2, O2, N2.              (iii) It should be balanced and conform to the law of Conservation of Mass.

9. What information is available from the following equation? 

  CaCO3 + 2HCl —–› CaCl2 + H2O + CO2

Ans: The following information can be derived from the given equation: 

        (i) Calcium carbonate and Hydrochloric acid are reactants and calcium chloride, water and Carbon dioxide are products.

       (ii) 1 molecule of Calcium carbonate reacts with 2 molecules of hydrochloric acid to produce 1 molecule each of calcium chloride, water and Carbon dioxide.

      (iii) 1g molecule, i.e., 100g CaCO3 reacts with 2g molecules, i.e., 73g HCl and produces, 1g molecules, i.e., 111g CaCl2, 1g molecule, i.e., 18g H2O and 1g molecule, i.e., 44g CO2.

       (iv) 22.4 litres of CO2 is formed at S.T.P.

10. What is a thermochemical equation? Give two examples.

Ans: Most chemical reactions are accompanied by either evolution or absorption of heat. These reactions are known as exothermic and endothermic reactions respectively. Such an equation in which information about heat change is included is called a thermochemical equation. It is very important in such a case to indicate the physical state of the various species involved.

Examples are :

    (i) C + O2 —–› CO2         ∆H = - 393.5 KJ

                                        (Exothermic reaction)

   (ii) C + H2 —–› CH2        ∆H = + 74.25 KJ

                                     (Endothermic Reaction)

11. How can a chemical reaction be made more informative?

Ans: (i) The physical condition of the various species May be indicated by sign (s), (l), and (g) for solid, liquid and gas respectively after the formula of a substance, e.g.,

   2P(s) + 3Cl2(g) -—–› 2PCl3(l)

         (ii) The precipitate formed in the reaction is shown by an arrow pointing downward (↓) e.g.,

   BaCl2 + H2SO4 —–› BaSO4 ↓ + 2HCl

          (iii) (↑) sign is used for gas evolve.              Zn + 2HCl —–› ZnCl2 + H2 ↑

           (iv) If a substance dissolved in water is used or product, sign (aq) is used.

   NaOH(aq) + HCl(aq) —–› NaCl(aq) + H2O

            (v) Heat changes occurring during a chemical reaction may also be indicated such as

  2H2(g) + O2(g) —–› 2H2O(l); ∆H = +571.6KJ

                                                      Endothermic

H2(g) + Cl2(g) —–› 2HCl(g) ;   ∆H = - 184.7 KJ

                                                        Exothermic

12. (i) Differentiate between an exothermic and endothermic Reaction. Write one example for each of these reactions in the form of balanced chemical equation. (1.5m)

      (ii) Classify the following reactions into Exothermic and Endothermic reactions.

(a) 2C + H2 —–› C2H2 - 222.2 KJ

(b) H2 + Cl2 —–› 2HCl + 184.7KJ.

Ans: (i) Exothermic reactions: Chemical reactions in which energy is evolved (or given out) are known as Exothermic reactions. For example,

C(s) + O2(g) —–› CO2(g) + 393.7 KJ

                                        { ∆H = -393.7 KJ }

This shows 1 mole of carbon reacts with 1 mole of oxygen and liberates 393.7 KJ of heat.

        Endothermic reactions: Reactions in which energy is absorbed are called endothermic Reactions.For example,

When carbon reacts with Sulphur, 92 KJ of heat is absorbed during the process.

C(s) + 2S(s) —–› CS2(l) - 92 KJ

                            (∆H = + 92 KJ)

(ii) (a) Endothermic Reaction.

       (b) Exothermic Reaction.

13. Name three Chemical reactions in which heat is evolved and three chemical reactions in which heat is absorbed? 

Ans: Exothermic Reactions (heat evolved)

(i) Oxidation of methane in presence of air

      CH4(g) + 2O2(g) –—› CO2(g) + 2H2O(l)

(ii) Formation of Sulphur trioxide 

      2SO2 + O2 —–› 2SO3 

(iii) Oxidation of ammonia

       4NH3 + 5O2 —–› 4NO + 6H2O

Endothermic Reactions: 

(i) Formation of hydrogen iodide

     H2(g) + I2(g) —–› 2HI(g) 

(ii) Decomposition of calcium carbonate

     CaCO3 (s) —–› CaO(s) + CO2(g)

(iii) Formation of carbon disulphide

     C(s) + 2S(s) —–› CS2(l) 

14. A housewife wanted her house to be white-washed. She bought 10 kg of quick lime from the market and dissolved It in 30 litres of water. On adding lime to water she noticed that the water started boiling even when it was not being heated. Give reason for her observation. Write the corresponding equation and name the product formed.

Ans: The housewife noticed that the water boiling because Reaction between quick lime (CaO) and water is Exothermic. The large amount of heat evolved made the water boiling.

    CaO(s) + H2O —–› Ca(OH)2 + heat

The product is calcium hydroxide.

15. You might have noted that when copper powder is heated in a China dish, the surface of Copper powder becomes coated with a black colour substance.

     (i) How has this black coloured substance formed?

     (ii) What is that black substance?

    (iii) Write the chemical equation of the reaction that takes place.

Ans: (i) The black coloured substance is formed due to Oxidation of copper.

        (ii) Copper Oxide.

       (iii) 2Cu + O2 —–› 2CuO

16. What is the need to balance a chemical equation with respect to mass as well as charge?

Ans: The chemical equation must be balanced with respect to mass because matter can neither be created nor destroyed. The equation must be balanced with respect to charge because the electrons lost by atoms to form cations are equal to the electrons gained by other atoms to form anions. Thus, as the reaction mixture does not contain any free electrons, the total positive charge should be equal to the total negative charge.

17. A student dropped few pieces of marble in dil. Hydrochloric acid contained in a test tube. The evolved gas was passed through lime water. What change would be observed in lime water? Write balanced chemical equation for both the changes observed.

Ans: lime water would turn milky because of formation of Calcium carbonate by evolved CO2 gas.

Chemical equations: 

      (i) CaCO3 + 2HCl —–› CaCl2 + H2O + CO2

      (ii) Ca(OH)2 + CO2 —–› CaCO3 + H2O

18. Here is an ionic equation:

             Al  +  2H+  —–›  Al3+  +  H2

 Is this a balanced equation? Explain.

Ans: Al + 2H+ —–› Al3+  + H2   is not a balanced equation. An ionic equation should be balanced with respect to atoms (or ions) as well as charges. An ionic equation must be balance in 3 steps.

    (i) First balance the equation with respect to atoms. Thus equation is balanced in respect of the number of atoms.

   (ii) Then balance the equation in respect of charges. There are 2 positive charges (in 2H+) on the left side but we have 3 positive charges (in Al3+) on the right side. In order to make equal charges on both sides, multiply 2H+ by 3 and Al3+ by 2, so that we get 6 positive charges on both sides.

               Al + 6H+   —–›  2Al3+   + H2

  (iii) Then again balance the equation in terms of atoms if it is disturbed. There is one Al atom on the left but 2Al3+ on the right. Thus, we should multiply Al by 2 or write 2Al on the  left. Again, there are 6H on the right side, we should multiply H2 by 3 or write 3H2. Now, the above equation becomes 

       2Al + 6H+ —–›  2Al3+  + 3H2

19. How are chemical changes classified based on there special features?

Ans: Chemical changes can be classified into the following categories depending on their specific characteristics.

(i) Combination reactions

(ii) Decomposition reaction

(iii) Displacement reaction

(iv) Double Displacement reaction

(v) Oxidation reaction

(vi) Reduction reaction

(vii) Redox Reactions

20. What happens to the food that we take daily?

Ans: It is decomposed by the chemical reactions taking place in the body. Bread contains starch, which gets converted into simple sugar in our body. Sugar gets hydrolysed to glucose and fructose. These are Oxidised to produce CO2, H2O and energy. 

 C6H12O6 + 6O2 —–› 6CO2 + 6H2O + energy

Proteins etc., are changed into simple substances called amino acids which help in cell formation.

21. A light sensitive compound X of silver is used in black and white photography. On exposure to sunlight it's colour changes to grey:

(a) Identify X

(b) Write a chemical equation to express the above change.

(c) Identify the type of chemical reaction.

Ans: (a) Silver chloride

         (b) 2AgCl2 + light —–› Ag + Cl2

         (c) Decomposition reaction.

22. (a) Give an example of a Combination reaction which is also an exothermic reaction.

(b) Complete the following chemical equation and balance it:

           C6H12O6 + O2 —–› 

(c) Which two gases are evolved on heating ferrous sulphate?

Ans: (a) Magnesium ribbon burnt in air.

         (b) C6H12O6 + 6O2 —–› 6CO2 + 6H2O

         (c) SO2 and SO3

23. When is a chemical reaction categorised as a Precipitation reaction? Explain with two examples.

Ans: When two Aqueous solutions are mixed, and a new substance is formed with precipitation, this is categorised as precipitation reaction. Two examples are:

(a) BaCl2 + Na2SO4 –—› BaSO4(s) ↓ + 2NaCl

(b) AgNO3 + KCl —–› AgCl(s) ↓ + KNO3

These are also called double displacement reaction.

24. Explain, why?

(a) Respiration is an exothermic reaction.

(b) All decomposition Reactions are endothermic Reactions.

(c) When blue salt of copper sulphate is heated it becomes colourless.

Ans: (a) During respiration, food is converted into glucose which oxidised with liberation of energy.

(b) Decomposition Reactions take place when heat is supplied in the form of heat, light or electricity.

 (c) Copper sulphate (CuSO4.5H2O) is blue in colour. On heating it loses water of crystallization and gives colourless copper sulphate (CuSO4).

25. When electric current is passed through molten aluminium oxide, the following reaction take place: 

     2Al2O3 —–› 4Al + 3O2

Name the type of Reaction. To what use such Reactions are made in indusrtry?

 

Ans: This is a decomposition Reaction. Such Reactions are used to extract metals from their naturally occurring compounds like oxides or chlorides.   

26. Identify the type of each of the following Reactions: 

(a) A Reaction in which a single product is formed from two or more reactants.

(b) The Reaction mixture becomes warm.

(c) An insoluble substance is formed.

(d) External surface of the container in which Reaction takes place becomes freezing cold.         {a+b =1.5; c+d =1.5} Tot=3

Ans: (a) Combination reaction

         (b) Exothermic reaction

         (c) precipitation reaction

         (d) Endothermic Reaction

27. Identify the type of reaction from the following equations: 

(i) CH4 + 2O2 —–› CO2 + 2H2O

(ii) Pb(NO3)2 + 2KI —–› PbI2 + KNO3

(iii) CaO + H2O —–› Ca(OH)2

(iv) CuSO2 + Zn –—› ZnSO4 + Cu

Ans: (i) Oxidation Reaction

         (ii) Double Displacement reaction

         (iii) Combination reaction

         (iv) Displacement reaction

28. What are the types of Combination Reactions? Give examples of each type.

Ans: Combination Reactions are of three types. These are:

(i) Combination b/w two elements: 

E.g.,  C + O2 —–› CO2

(ii) Combination b/w an element and a compound: 

E.g., 2CO + O2 —–› 2CO2

(iii) Combination b/w two compounds:

  E.g., CaO + CO2 —–› CaCO3

29. How will you show that heating of copper carbonate is a decomposition reaction?

Ans: Take 5g green coloured copper carbonate in a hard glass tube. Heat it strongly and pass the gas evolved through lime water. It is a Decomposition reaction, it can be explained as: 

              CuCO3 —–› CuO + CO2

The black residue left is of copper oxide and the evolved gas is CO2, can be confirmed because if it is passed through lime water it turns milky due to the formation of copper carbonate.

            Ca(OH)2 + CO2 —–› CaCO3 + H2O

30. A student has mixed the solutions of lead (II) nitrate and potassium Iodide.

(a) State the colour of the precipitate formed.

(b) Write a balanced chemical equation for the reaction.

(c) Suggest an alternative name for the above precipitation Reaction. Give justification for your answer.

Ans: (a) Yellow coloured precipitate

         (b) Pb(NO3)2 + 2KI —–› PbI2 + 2KNO3

         (c) Double Displacement Reaction.

Here two metals exchange their partner ions, e.g., lead and potassium.