Chapter 1 Chemical Reactions and Equations

NCERT Questions

1. Why should a magnesium ribbon be cleaned before burning in air?
Solution : Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air.

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
Solution :
(i) H2+Cl2⟶2HCl
(ii)  3BaCl+ Al2(SO4)→ 3BaSO4  +  2A∣Cl3
(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.
Solution :
(i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O(j)

4. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Solution :
(i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
CaO(s)+H2O(l) ⟶ Ca(OH)2(oH)

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.
Solution : The gas which is collected in double the amount in the electrolysis of water experiment is hydrogen. This is because water contains 2 parts of hydrogen element as compared to only 1 part of oxygen element.
In Activity 1.7, water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode.
2H2O(l) → 2H2(g) + O2(g)
Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.

6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Solution : When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.
Fe(s) + CuSO(ay) ⟶ FeSO4(ay) + Cu(s)

7. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Solution : Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.
Na2CO3(aq) + CaCl2(oq) ⟶ CaCO3(s) + 2NaCl(oq)

8. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na+O2 → 2Na2O
(ii) CuO+H2 → Cu(s)+H2O

Solution :
(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen(H2)  gets oxidised to waterH2O

9. 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 oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Solution : As statement (a) and (b) are incorrect, answer (i) is correct.

10. Fe2Oa + 2Al → Al2Os + 2Fe
The above reaction is an example of a

(a) combination reaction.
(b) double displacement reaction. Chemical Reactions and Equations 15
(c) decomposition reaction.
(d) displacement reaction.
Solution : This is an example of displacement reaction because Fe in FeO3 has been displaced by Al. Hence correct answer is (d).

11. 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.
Solution : (a) Hydrogen gas and iron chloride are produced. The reaction is as follows:
Fe(s) + 2HCl(aq) ⟶ FeCl2(αx) + H2

12. What is a balanced chemical equation? Why should chemical equations be balanced?
Solution : The reaction in which the number of atoms of each element is equal on the reactant side and product side is called balanced equation.
Chemical reaction should be balanced because only a balanced equation tells us the relative quantities of different reactants and products involved in the reaction.

13. 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 a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Solution :
(i) 3H2 + N2 ⟶ 2NH3
(ii) 2H2S + 3O2 ⟶ 2H2O + 2SO2
(iii)3BaCl2 + Al2(SO4)3  → 2AlCl3 + 3BaSO4
(iv) 2K+2H2O→ 2KOH+H2

14. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)+ H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl+AgNOa → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl

Solution :
(a) 2HNO+ Ca(OH)2 ⟶ Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 ⟶ Na2SO+ 2H2O
(c)  NaCl + AgNO3 ⟶ AgCl + NaNO3
(d) BaCl2 + H2SO4 ⟶ BaSO4 + 2HCl

15. 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
Solution :
(a) Ca(OH)2 + CO2 ⟶ CaCO3 + H2O
(b) Zn + 2AgNO3 ⟶ Zn(NO3)2 + 2Ag
(c)  2Al + 3CuCl2 ⟶ 2AlCl3 + 3Cu
(d) BaCl2 + K2SO4 ⟶ BaSO4 + 2KCl

16. 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)
Solution :
(a) 2KBr (aq) + Bal2(aq)  →  2Kl(aq)  +  BaBr2(s)
Type : Double displacement reaction
(b) ZnCO3 (s)  →  ZnO (s) + CO2 (g)
Type : Decomposition reaction
(c) H2 (g) +  Cl2 (g)  →  2HCl(g)
Type : Combination reaction
(d) Mg (s)  +  2HCl (aq)  →  MgCl2 (aq)  +  H2 (g)
Type : Displacement reaction

17. What does one mean by exothermic and endothermic reactions? Give examples?
Solution :
Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions. An exothermic reaction is indicated by writing “+ Heat”on the products side of an equation.
Example :
(i) C (s) + O2 (g)  →  CO(g)  +  Heat
(ii) N2 (g)  +  3H2 (g)  →  2NH3 (g) + Heat

Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions. An endothermic reaction is usually indicated by writing “Heat” on the product side of a chemical equation.
Examples :
(i) C (s)   +  2S (s) → CS2 (l) – Heat
(ii) N2 (g)  +  O2 (g)  →  2NO(g)  –  Heat

18. Why is respiration considered an exothermic reaction? Explain?
Solution : Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration.
Since energy is released in the whole process, it is an exothermic process.
C6H12O6 (aq) + 6O2(g)

19. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions?
Solution : I

Combination reaction is said to be the reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction.

In most of the cases the decomposition reaction is endothermic since heat from the surrounding or induced heat is used to break the bonds of the larger molecule. Few examples of decomposition reactions are:

ZnCO3 → ZnO + CO2

CaCO3 + Energy → CaO + CO2

2HgO → 2Hg + O2

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

Solution :
(a) Decomposition reaction where heat is supplied for energy: Thermal decomposition
CaCO3 (Heat) ⟶ CaO + CO2

A thermal decomposition reaction can be defined as a decomposition reaction which is activated by thermal energy.

(b) Decomposition reaction where electricity is supplied for energy: Electrolytic decomposition
2H2O(l) ⟶ 2H2(g) + O2(g)

An electrolytic decomposition reaction is a type of decomposition reaction that in the form of electrical energy, provides the activation energy for decomposition

(c) Decomposition reaction where light is supplied for energy: Photo decomposition

2H2O2⟶ 2H2O + O2

Photolysis or photodecomposition reaction is defined as the decomposition of substances on exposure to light, while photo-oxidation is the oxidation of a substance on exposure to light. Irradiation of tryptophan with UV light results in the formation of a tryptophan radical and a hydrated electron.

21. What is the difference between displacement and double displacement reactions? Write equations for these reactions?
Solution :

A displacement reaction is the one when a more reactive substance displaces a less reactive one from its salt solution whereas a double displacement reaction is the one where a mutual exchange of ions happens between two compounds.

In a displacement reaction, only a single displacement takes place whereas in the double displacement reaction, as the name suggests two displacement takes place between the molecules.


Displacement reaction

Mg + 2HCl → MgCl+ H2

Double displacement reaction

2KBr + BaI2 → 2KI + BaBr2

22. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal.
Write down the reaction involved?

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

23. What do you mean by a precipitation reaction? Explain by giving examples.
Solution :

When two solutions containing soluble salts are combined, a double displacement reaction takes place in which the ions are exchanged between the compounds. When one of such compounds formed is in solid form (that is insoluble in aqua) then it settles down at the bottom of the container. This solid is known as the precipitate and the respective reaction is termed as the precipitation reaction. Few examples of precipitation reactions are:

CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)

2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)

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

Solution :
(a) Oxidation is the gain of oxygen. For example:
(i) CO2  + H2 ⟶ CO + H2O
(ii) 2Cu + O2 ⟶ 2CuO
In equation (i), H2 is oxidized to H20 and in equation (ii), Cu is oxidised to CuO.
(b) Reduction : Reduction- addition of hydrogen or removal of oxygen in a chemical reaction is called oxidation reaction. For example
ZnO + C → Zn + CO, ZnO is reduced to Zn.

CuO + H2 → Cu + H2O, CuO is reduced to Cu.

25. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Solution : The brown coloured element ‘X ‘ is copper. On heating in air it forms copper oxide, which is black in colour.
2Cu(s)   +    O2 →  2CuO

Brown      (from air)            Black

26. Why do we apply paint on iron articles?
Solution : 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.

27. Oil and fat containing food items are flushed with nitrogen. Why?
Solution : 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. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

28. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Solution :
(a) Corrosion : Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.
For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.
4Fe + 3O2 + nH2O ⟶ 2Fe2O3, nH2O

(b) Rancidity : The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and makes them unfit for eating.
Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils. It can also be prevented by flushing fat and oil containing foods with nitrogen before sealing.

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