Chemical Reactions And Equations (Question Answer)

Chemical Reactions And Equations 

(Question Answer)

QUESTIONS

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

Answer: A magnesium ribbon has to be cleaned before burning in the air as magnesium reacts with atmospheric oxygen to form a layer of magnesium oxide (MgO). It needs to be cleaned to prevent subsequent reactions with oxygen.

2. 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 + Hydrogen 

Answer:         (i) H₂ + Cl₂ → 2HCl

                (ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃

                (iii) 2Na + 2H₂O → 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. 

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

Answer:         (ii) BaCl₂(aq) + Na₂SO₄(aq) ---→ BaSO₄(s) + 2NaCl(aq)

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

4. 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. 

Answer:     (i) Name of the substance X is Calcium Oxide and its formula is CaO.

                    (ii) Reaction of substance X with water is 

                            

                        CaO (s) + H₂O (l) → Ca(OH)₂(aq)

                        (limestone)                    (slake lime)

5. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer: In activity 1.7 electrolysis of acidic water is done. When electricity is passed through water, water molecules dissociate and produce hydrogen and oxygen. These gases are collected at the cathode and anode respectively. When two water molecules dissociate, two molecules of hydrogen and one molecule of oxygen are formed. Therefore, in one test, the amount of gas collected is double the amount of gas collected in the other test tube.

    Name of this gas is  hydrogen.

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

Answer: Copper sulphate solution changes color when a iron nail is immersed because, iron being more reactive than copper, it replaces copper from the copper sulphate solution. Therefore, the deep green color of copper sulphate becomes pale.

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

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

Answer: Barium chloride reacts with aluminium sulphate to form barium sulphate and aluminium chloride.

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

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

         (i)            4Na(s) + 0₂(g) → 2Na₂O(s)

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

Answer: (i) Na is oxidized.

                (ii) H2 is oxidised and CuO is reduced

EXERCISES 

1. Which of the statements 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) Carbon is getting oxidized. 

(d) Lead oxide is getting reduced. 

(i) (a) and (b)

 (ii) (a) and (c)

 (iii) (a), (b) and (c) 

(iv) all. 

Answer:     (ii) (a) and (c)

2. Fe2O3 + 2Al → Al2O3 + 2Fe 

The above reaction is an example of a

 (a) combination reaction. 

(b) double displacement reaction.

(c) decomposition reaction. 

(d) displacement reaction. 

Answer:     (d) displacement reaction. 

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

Answer:     (a) Hydrogen gas and iron chloride are produced. 

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

Answer: A chemical equation in which the number of atoms in each element is the same before and after the reaction is called a balanced chemical equation.

            According to the law of  conservation of mass, mass can neither be created nor destroyed  in a chemical reaction. In other words, in a chemical reaction, the total mass of the reactants must be equal to the mass of the product. Thus, the number of atoms in each element must be the same before and after a chemical reaction. Therefore chemical equations have to be balanced.

5. 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 sulpur dioxide. 

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

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

Answer:                  (a) 3H₂ + N₂ → 2NH₃

                               (b) 2H₂S + 3O₂ → 2H₂O + 2SO₂

                (c) 3BaCl₂ + Al₂ (SO₄)₃ → 3BaSO₄ + 2AlCl₃

                (d) 2K + 2H₂O → 2KOH + H₂

6. 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 

Answer:           (a) 2HNO₃+ Ca (OH)₂ → Ca (NO₃)₂ + 2H₂O

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

        (c) NaCl+ AgNO₃ → AgCl + NaNO₃

         (d) BaCl₂+ H₂SO₄ → BaSO₄ + 2HCI 

7. 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 

Answer:    (a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

                (b) Zn + 2AgNO₃ → Zn(NO₃)₂ +  2Ag

                (c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

                (d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

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

(a) Potassium bromide(aq) + 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) 

Answer:         (a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) ------- Double displacement reaction

                    (b) ZnCO₃(s) → ZnO(s) + CO₂(g) ---------- Decomposition reaction

                    (c) H₂(g) + Cl₂(g) → 2HCl(g) ------------ Combination reaction

                    (d) Mg(s) + 2HCl(aq) → MgCl₂ (aq) + H₂(g) ----------Displacement reaction

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

Answer: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Example of exothermic reaction is –  Burning of natural gas

                 CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)

    

                On the other hand, reactions in which energy is absorbed are known as endothermic reactions. The absorbed energy in endothermic reaction may be in the form of heat, light or electricity. Example of one endothermic reaction is-

                2AgCl (s) -----------→ 2Ag(s) + Cl₂(g) (in presence of sunlight)

10. Why is respiration considered an exothermic reaction? Explain. 

Answer: The energy which we need to survive comes from food. In the process of digestion, food breaks down into simpler substances. For example, rice, potatoes, and bread contain carbohydrates. These carbohydrates are broken down into glucose. During respiration, this glucose attaches to the oxygen in our body cells and supplies energy. Since energy is generated in the process of respiration, respiration is considered to be a exothermic reaction.

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

Answer: In a combination reaction, two or more substances (elements or compounds) combine together to form a single product, whereas in a decomposition reaction only one substance breaks down into more than one simpler substances. Therefore, the decomposition reaction can be said to be the opposite of the combination reaction.

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

Answer: Calcium carbonate can be decomposed into calcium oxide and carbon di oxide by the application of heat.

            CaCO₃(s) ---^তাপ----→ CaO(s) + CO₂(g)

            Example of a decomposition reaction in presence of light is-

            2AgCl (s) -----------→ 2Ag(s) + Cl₂(g) (in presence of sunlight)

  Example of a decomposition reaction in presence of electricity is the electrolysis of water

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

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

Answer: When an element displaces another element from its compound, a displacement reaction occurs. 

                    E.g.         Fe(s) + CuSO₄(aq) ---→FeSO₄ (aq) + Cu(s)

            On the other hand, two different atoms or groups of atoms (ions) are exchanged in double displacement reactions

                     E.g.           Na₂SO₄(aq) + BaCl₂(aq) ---→ 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 reaction involved. 

Answer: Reaction involved in refining of silver is-

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

15. What do you mean by a precipitation reaction? Explain by giving examples. 

Answer: If in a reaction a substance which is insoluble in water (precipitate) is formed, then the reaction is called a precipitation reaction

E.g.             Na₂SO₄(aq) + BaCl₂(aq) ---→ BaSO₄(s) + 2NaCl(aq)

                                                         (Precipitate)

16. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction 

Answer: (a) Oxidation: If in a reaction a substance gains oxygen then the reaction is called oxidation. 

E.g.                     2Cu(s) + O₂(g) ---→ 2CuO(s)

            (b) Reduction:  If in a reaction a substance loses oxygen, then the reaction is said to be reduction reaction.

E.g.                 CuO(s) + H₂(g) ---→Cu + H₂O(l)

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

Answer: The name of the element X is Copper (Cu) and the black coloured compound formed is copper oxide (CuO)

18. Why do we apply paint on iron articles? 

Answer: When iron articles are attacked by substances around it such as moisture, acids, etc., then a reddish brown coloured layer is formed in its surface.  This process is called corrosion. Corrosion damages iron articles. Hence, to prevent iron articles from corrosion we apply paint on iron articles.

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

Answer: Oil and fat containing food items suffer from rancidity when kept for several days. To prevent rancidity such food items are flushed with nitrogen.

20. Explain the following terms with one example each. (a) Corrosion (b) Rancidity

Answer: 

Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc. then its shine is reduced and a coloured coating is formed on it. This is called corrosion of metals. Reddish brown coating on iron (rusting), black coating on silver and the green coating on copper are some examples of corrosion. Corrosion causes damage to the metal.

Rancidity:  When fat/oil containing food materials are kept for a long time, it develop some sort of smell. It is because, when fats and oils are open in air they are oxidized and their smell and taste change. This is called rancidity.