Class 10th Science Chemical Reaction And equation. Notes 2021-2022 ( latest NCERT Syllabus…. )

CHEMICAL REACTION AND EQUATIONS .

Questions: 1. What is chemical combination reaction? [SSLC July, 2018]

Ans: When two or more substances combine to form a single product, the reactions are called combination reactions.

2. Is burning of a candle, a physical change or a chemical change? [NCERTExemplar]

Ans: Both, chemical change and physical change.

3. Write a balanced chemical equation for the following reaction. Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H 2SO 4. [NCERT Exemplar]

Ans: C2H5OH + CH3COOH Conc. H2 SO4 Esterification CH3COOC2H5 + H2O

4. In test-tubes A and B, zinc sulphate solution and silver nitrate solution are taken respectively. Copper turnings are added to both test-tubes. In which of the two test-tubes do you observe the reaction. Justify your answer with scientific reason. [SSLC June, 2018]

Ans: Test-tube B containing silver nitrate solution. Because copper being more reactive than silver, displaces silver from silver nitrate solution.

5. Write the balanced chemical equations for the following chemical reactions.

(i) Potassium bromide reacts with Barium iodide.

(ii) Zinc carbonate is heated. [SSLC July, 2016]

Ans: (i) 2KBr + Bal2 → 2Kl + BaBr2 (ii) ZnCO3 → ZnO + C O 2

6. Which coloured fumes are obtained when lead nitrate is heated? Write the balanced chemical equation for this reaction. Name the type of this chemical reaction. [SSLC July, 2016]

Ans: Brown coloured fumes 2Pb(NO3)2(s) (Lead) → H eat 2PbO(s) + 4NO2 (g) + O− 2 (g) (Lead (Nitrogen (Oxygen) oxide) oxide)

7. A copper plate was dipped into a solution of silver nitrate. After sometime, a black layer was observed on the surface of copper plate. State the reason for it and write chemical equation of the reaction involved.

Ans: Black layer was deposited due to coating of silver, because copper being more reactive than silver, displaced silver from silver nitrate solution. 2AgNO3(aq) + Cu → Cu (NO 3)2(aq) + 2Ag(s)

8. Give reactions of calcium and magnesium with dilute nitric acid.

Ans: Ca + 2HNO 3 → Ca(NO 3)2 + H2 Mg + 2HNO3 → Mg(NO 3)2 + H2

9. Convert the following statements into balanced chemical equations:

(i) Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas.

(ii) Magnesium burns in oxygen to form magnesium oxide.

Ans: (i) Zn + H2SO4 → ZnS O 4 + H2 (ii) 2Mg + O 2 → 2MgO

10. Define the term decomposition reaction. Give one example each of thermal decomposition and electrolytic decomposition.

Ans: In a decomposition reaction, a single reactant breaks down into two or more simpler products. When a decomposition reaction is carried out by heating, it is called thermal decomposition reaction. CaCO3(s) → (Calcium carbonate) CaO(s) + CO2(g) (Calcium oxide) (Carbon dioxide) (Calcium carbonate) (Calcium oxide) (Carbon dioxide) When a decomposition reaction is carried out with the help of electric current, the process is called electrolysis. 2H2O(l) (W ater) → E l e c t r i c c u r r e n t 2H2(g) + O2(g) (Hydrogen gas ) (Oxygen gas )

11. Write the steps for balancing the chemical equation for the formation of ammonia by the combination of nitrogen and hydrogen.

Ans: N 2 + H2 → N H3 (Unbalanced equation).

(i) Examine the number of atoms of different elements present in the unbalanced equation.

(ii) In the above reaction, N and H of both the sides are unbalanced.

(iii) To balance hydr oge n, H 2 is multiplie d by 3. It ma ke s 6H – a toms on the left ha nd side.

(iv) Now to balance hydrogen atoms on the right hand side, NH3 should be multiplied by 2. It makes 6H-atoms on this side.

(v) Now to balance nitrogen atoms, they are counted separately for both the sides and we will find that, nitrogen atoms are 2 on both the sides.

(vi) Balanced chemical equation will be: N 2 + 3 H2 → 2NH3

12. (a) Mention the four informations given by an equation. (b) State the law of conservation of mass as applicable in a chemical reaction.

Ans: (a) (i) Physical state of reactants and products

. (ii) Conditions such as temperature, pressure, heat etc.

(iii) Catalyst involved.

(iv) Change in state.

(b) Total mass of the elements present in the products in a chemical reaction has to be equal to the total mass of elements present in the reactants or Mass can neither be created nor destroyed in a chemical reaction.

13. When a copper wire was left in silver nitrate solution for sometime, it was observed that the solution turned bluish green.

(i) Explain the observation.

(ii) Write the balanced chemical equation to represent the change taking place.

Ans: (i) Copper is more reactive than silver. Hence, when copper wire is dipped in silver nitrate solution, it displaces silver from AgN O 3 solution forming copper nitrate which is bluish green in colour.

(ii) Cu + 2AgNO3 → Cu(NO3)2 (Copper (II) nitrate + 2Ag (Silver) Bluis h green)

14. What do you observe in activity given in the figure? What are the causes of these observations, explain giving reaction?

Ans: Due to reaction of sodium carbonate with dilute hydrochloric acid, carbon dioxide gas is produced. L ime water turned milk y w he n C O 2 is passed through it, due to the formation of calcium carbonate. Na2CO3 + 2HCl → 2NaCl + CO2 Ca(OH)2 + CO2 → C aC O 3 + H2O

15.(i) 2PbO + C → 2Pb + C O 2 (ii) MnO2 + 4HC1 → MnCl2 + 2 H2O + Cl2 What is redox reaction? Identify the substance oxidised and the substance reduced in the above reactions.

Ans: Those reactions in which both oxidation and reduction occur simultaneously.

(i) PbO is reduced, C is oxidised.

(ii) HCl is oxidised, MnO 2 is reduced.

16. Identify the type of reaction from the following equation and define it. CH + 2O → C O 2 + 2 H2O + heat

Ans: Exothermic reaction: It is a reaction in which heat energy is released along with the formation of products.

17. Write the balanced chemical equation for the following reaction and identify the type of reaction and define it. Iron III oxide reacts with aluminium and gives molten iron and aluminium oxide. [Board Term-I Set -15, 2012]

Ans: Fe2O 3 + 2Al→ Al2O 3 + 2Fe Displacement reaction is the reaction in which one element displaces another element.

18. A silver article generally turns black when kept in the open for a few days. The articles when rubbed with toothpaste again starts shining.

(i) Why do they turn black? Name the phenomenon involved.

(ii) Name the black substance formed and write its formula. [NCERT Exemplar]

Ans : (i) Silver article reacts w ith sulphur c ompounds s uc h a s H 2S present in air. The phe nome non called corrosion. For silver particula r ly, it is called tarnishing of silver.

(ii) The black substance is silver sulphide (Ag2S).

19. What is rancidity? ‘Mention any two ways by which rancidity can be prevented.

Ans: The oxidation of oils or fats in a food resulting into bad smell and bad taste is called rancidity. It can be prevented by —

(i) Adding anti-oxidants.

(ii) Flushing with nitrogen gas.

20. Name the term used to indicate the development of unpleasant smell and taste in fat and oil containing food due to oxidation. What are anti-oxidants? Why are they added to fat and oil containing food.

Ans: Rancidity. Antioxidants are substances that inhibit oxidation, especially one used to counter act the deterioration of stored food products. Antioxidants are added to the food materials containing fats and oils to prevent their oxidation. Oxidation of food materials containing fats and oils is known as rancidity because of which the food becomes unfit for consumption and develop bad odour.

21. Name two metals which do not corrode easily. Give an example in each of the following case to support that:

(i) Corrosion of some metals is an advantage. (ii) Corrosion of a metal is a serious problem.

Ans: Gold and platinum. (i) Corrosion of aluminium is useful. A protective layer of aluminium oxide is formed on the surface of the metal which renders the metal passive and prevents it from further corrosion. (ii) Corrosion of iron is a serious problem. Every year large amount of money is spent to replace damaged iron and steel structures. Here corrosion is a serious problem.

22. The following diagram displays a chemical reaction. Observe carefully and answer the following questions:

(i) Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?

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

(iii) Mention one commercial use of this salt.

Ans: (i) Photochemical decomposition: A single reactant breaks down to give simpler products. White silver chloride changes to grey, as it decomposes to silver and chlorine in presence of sunlight.

(ii) 2AgCl(s) Su n li gh t → 2Ag(s) + Cl2 (g)

(iii) Black and white photography

23. (i) Define corrosion. (ii) What is corrosion of iron called? (iii) How will you recognise the corrosion of silver? (iv) Why corrosion of iron is a serious problem?

Ans : (i) C orrosion is a process in which metals are deteriorated b y action o f air, moistur e, chemicals, etc. (ii) Corrosion of iron is called Rusting.

(iii) Silver turns black as it reacts with H 2S present in air and form a layer of Ag2S.

(iv) Corrosion of iron is a serious problem because it leads to wastage of tonnes of iron every year and lot of money is spent to repair or replace it:

24. (i) Account for the following: (a) White silver chloride turns grey in sunlight. (b) Brown coloured

copper pow de r on heating in air turns into black colour e d substa333nc e

(ii) What d o you mean by: (a) Displacement reaction (b) Reduction reaction Write balanced chemical equation.

Ans : (i) (a) Due to the decomposition of silver chloride into silver and chlorine b y sunlight. 2AgCl Su n li gh t → 2Ag + Cl2

(b) Due to the oxidation o f copper powder to copper oxide, brown colour turns into b lack 2Cu(s) + O2(g) → 2CuO (s)

(ii) (a) A chemical reaction in which the more reactive element displaces the less reactive element from its compound is called displacement reaction. CuSO4(aq) + Zn (s) → ZnSO4(aq) + Cu(s)

(b) A chemical reaction in which hydrogen is added or oxygen is removed is called reduction reaction. ZnO + C → Zn + CO

25. (i) Balance the following chemical equation MnO2 + HCl → MnCl2 + Cl2 + H2O (ii) What is decomposition reaction? Explain it with suitable example.

Ans: (i) MnO2 + 4HCl → MnCl2 + 2 H2O + Cl2

(ii) A simple reactant breaks down to give simpler products and the process is known as decomposition reaction.

Example: C aC O 3(s) → H eat CaO(s) + C O 2(g) (Quicklime)

TEXTBOOK EXERCISES:

1. Which o f 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.

(d) Lead oxide is getting reduced

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) All

Ans: (i) (a) and (b) are correct.

2. Fe2O 3 + 2Al → Al2O 3 + 2F e The reaction is an example of:

(a) C ombination reaction.

(b) Double displacement reaction.

Ans : (d) Displacement reaction.

3 . What happens when dilute hydrochlor ic 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.

Ans: The reaction can be written as Fe + 2HCl → FeCl2 + H2 ∴ The correct option is (a)

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

Ans: Balanced chemical equation is that equation in which

(a) N umber o f different atoms in reactants and products are equal.

(b) The mass of the reactants and products is equal. The chemical equation should b e balanced to follow the law of conservation o f mass.

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 sulphur dioxide.

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

Ans: (a) 3H2 + N2 → 2NH3

(b) 2H2S + 3O2 → 2H2O + 2SO2

(d) 2K + 2H2O → 2KOH + H2

6. Balance the following chemical equation.

(a) HNO3 + Ca(OH)2 → Ca (N O 3)2 + H2O

(b) NaOH + H2SO4→ N a 2S O 4 + H2O

(d) BaCl2 + H2SO 4 → BaS O 4 + HC l

Ans: (a) 2HNO3 + Ca(OH)2 → Ca (N O 3)2 + 2 H2O

(b) 2NaOH + H2SO 4 → N a 2S O 4 + 2 H2O

(d) BaCl2 + H2SO 4 → BaS O 4 + 2HCl

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

(a) Calcium hydroxide + Carbon dioxide → C alcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(d) Barium chloride + Potassium sulphate → Barium sulphate + P otassium chloride

Ans: (a) Ca(OH)2 + C O 2— CaC O 3 + H2O

(b) Zn + 2AgNO3 → Zn (NO3)2 + 2Ag

(d) BaCl2 + K 25O 4 → BaS O 4 + 2K Cl

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

(a) P otassium bromide (aq) + Barium iodide (aq) → P otassium iodide (aq) + Barium bromide (s)

(b) Zinc carbonate (s) → Zinc oxide (s) + Carbondioxide(g)

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

Ans: (a) 2KBr + Bal2 → 2 K l + BaBr 2 (Double displacement reaction)

(b) ZnCO3 → ZnO + C O 2 (Decomposition reaction)

(d) Mg + 2HCl → MgC l2 + H2 (Displacement reaction)

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

Ans: Exothermic reactions are those reactions during which heat energy is released. E.g.

(a) 2Mg + O2 → 2MgO + Heat (b) CaO + H2O → C a(O H) 2 + Heat Those chemical reactions in which heat energy is required to carry out the reactions are called endothermic reactions. E.g. (a) CaCO3 → H e a t C aO + C O 2 (b) H2 + S → H e a t H2S

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

Ans : Heat released during the process o f respiration therefore, respiration is considered as an exothermic reaction. C6H1206 + 6O 2 → 6 C O 2 + 6 H2O + Heat

11. W hy are decomposition reactions called opposite o f combination reactions? Write equations for these reactions.

Ans : In decomposition reactions, larger molecules break to give smaller molecules while in combination reactions, two or smaller units combine to give larger molecules.

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

Ans: Decomposition by heat: CH4 → H eat C + 2 H2 Methane Carbon Hydrogen Decomposition by light: 2 H2O 2 → Light 2 H2O + O2 Hydrogen peroxide W ater Oxygen Decomposition by electricity: 2NaCl (molten) → E l e c t r i c i t y 2N a + Cl2 Sodium chloride Sodium Chlorine

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

Ans : In displacement reactions, more reactive metal displaces less reactive metal from its aqueous salt solutions. Ex: C uS O 4 + Fe→ F eS O 4 + C u In double displacement reactions, exchange o f ions takes place when the aqueous solutions o f two salts get mixed together. Ex: NaCl + AgNO3 → AgCl(↓) + NaNO3

14. In the refining of silver, the recovery o f silver from silver nitrate solution involves displacement b y copper metal. Write down the reaction involved.

Ans: Cu + 2AgNO3 → Cu(NO 3)2 + 2Ag

15. What d o you mean b y precipitation reaction? Explain b y giving examples.

Ans: The chemical reactions in which a solid product o r precipitate is formed are called precipitation reactions. Examples are. BaCl2 + Na2S O 4 → BaS O (↓) + 2NaCl FeCl3 + 3NH4OH → Fe(OH)3 (↓) + 3NH4Cl

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

Ans: (a) Oxidation : The gain of oxygen is called oxidation. E.g.: (i) 2Zn + O 2 → 2ZnO (ii) 4Al + 3O2 → 2Al2O3 (b) Reduction The loss of oxygen is called reduction. E.g.: (i) Fe2O 3 + 3C → 2F e + 3 C O (ii) PbO2 + C → P b + C O 2

17. A shiny brown coloured element ‘X ’ o n heating in air becomes black in colour. N ame the element ‘X ’ and the black coloured compound formed.

Ans : Element ‘X’ is copper and black coloured compound is copper oxide. 2Cu + O 2→ 2C uO

18. Why d o we apply paint o n iron articles?

Ans : To prevent the exposure o f iron articles to air and moisture, thereby preventing corrosion.

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

Ans : As nitrogen is inert in nature and checks the growth of decomposing microbes and also protect from oxygen, so oil and fat containing food items are flushed with nitrogen.

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

Ans : (a) C orrosion : The slow eating up o f metals b y the reaction o f some atmospheric gases like oxygen, carbon-dioxide or water vapour is known as corrosion. E.g.: The copper articles get coated with green colour which is due to the formation o f basic copper carbonate.

(b) Rancidit y : Rancidit y ma y be defined as the natural oxidation o f the oils and fats, which are present in food items, giving bad smell and taste.

E.g.: F ood items, containing fats and oils, give bad smell when they exposed to air for a long time.

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