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Chapter 1: Chemical Reactions and Equations

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Chapter 1: Chemical Reactions and Equations

Questions (Page 6)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Answer: Magnesium ribbon is cleaned before burning to remove the layer of magnesium oxide (formed due to reaction with oxygen in the air) from its surface. This allows the underlying magnesium metal to come into direct contact with air and burn properly.

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 + Hydrogen

Answer:

  • (i) H2 + Cl2 → 2HCl
  • (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) 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:

  • (i) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
  • (ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Questions (Page 10)

Q1. 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.

Answer:

(i) The substance ‘X’ is Calcium oxide (also known as quick lime). Its formula is CaO.

(ii) The reaction with water is:

CaO(s) + H2O(l) → Ca(OH)2(aq)

(Note: The product formed is calcium hydroxide, also called slaked lime).

Q2. 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 water), water (H2O) decomposes into hydrogen and oxygen. As the chemical formula of water is H2O, it contains two parts hydrogen and one part oxygen. Therefore, the amount of Hydrogen gas produced is double the amount of Oxygen gas. The gas collected in double the amount is Hydrogen.

Questions (Page 13)

Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: Iron is more reactive than copper. When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution. This formation of iron sulphate (ferrous sulphate) changes the solution’s colour from blue to light green, and a brownish deposit of copper covers the nail.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: Reaction between Lead nitrate and Potassium iodide (from Activity 1.2):

Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

In this reaction, Lead iodide (PbI2) forms a yellow precipitate.

Q3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Answer:

  • (i) Oxidised: Sodium (Na) gains oxygen.
    Reduced: Oxygen (O2) is reduced.
  • (ii) Oxidised: Hydrogen (H2) gains oxygen to become water.
    Reduced: Copper oxide (CuO) loses oxygen to become copper.

Textbook Exercises (Pages 14-16)

Q1. 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

Answer: (i) (a) and (b)

Reasoning: (a) is incorrect because Lead Oxide (PbO) is reduced, not Lead. (b) is incorrect because Carbon (C) is oxidised, not Carbon dioxide.

Q2. 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.

Reasoning: Aluminium displaces iron from iron oxide because it is more reactive.

Q3. 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.

Reaction: Fe + 2HCl → FeCl2 + H2

Q4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced chemical equation is one where the number of atoms of each element is the same on both the reactant (LHS) and product (RHS) sides. Equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

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 a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:

  • (a) 3H2(g) + N2(g) → 2NH3(g)
  • (b) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
  • (c) 3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(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

Answer:

  • (a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
  • (b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
  • (c) NaCl + AgNO3 → AgCl + NaNO3 (Already balanced)
  • (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

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
Q8. 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)
    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
Q9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer:

Exothermic Reactions: Reactions in which heat is released along with the formation of products.
Example: Burning of natural gas.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Endothermic Reactions: Reactions in which energy is absorbed (in the form of heat, light, or electricity) to break down reactants.
Example: Decomposition of calcium carbonate.
CaCO3(s) + Heat → CaO(s) + CO2(g)

Q10. Why is respiration considered an exothermic reaction? Explain.

Answer: Respiration is considered exothermic because energy is released during the process. We eat food which contains carbohydrates (like rice, potatoes, bread). These carbohydrates are broken down into glucose. This glucose combines with oxygen in the cells of our body and provides energy to keep us alive.

C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + Energy

Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: In a combination reaction, two or more substances combine to form a single product. In a decomposition reaction, a single substance breaks down to give two or more simpler products. Thus, they are opposites.

  • Combination: 2H2 + O2 → 2H2O (Two reactants form one product)
  • Decomposition: 2H2O → 2H2 + O2 (One reactant forms two products via electrolysis)
Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer:

  • Heat (Thermal Decomposition): CaCO3(s) → CaO(s) + CO2(g)
  • Light (Photolytic Decomposition): 2AgCl(s) → 2Ag(s) + Cl2(g)
  • Electricity (Electrolytic Decomposition): 2H2O(l) → 2H2(g) + O2(g)
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

  • Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
    Equation: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
  • Double Displacement Reaction: There is an exchange of ions between two reactants to form new compounds.
    Equation: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer:

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

(Copper displaces silver to form Copper nitrate and Silver metal).

Q15. What do you mean by a precipitation reaction? Explain by giving examples.

Answer: Any reaction that produces an insoluble solid (called a precipitate) that separates from the solution is called a precipitation reaction.

Example: When Sodium sulphate reacts with Barium chloride, a white precipitate of Barium sulphate is formed.

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) (Precipitate) + 2NaCl(aq)

Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

Answer:

(a) Oxidation: It is the gain of oxygen during a reaction.

  • Example 1: 2Cu + O2 → 2CuO (Copper gains oxygen)
  • Example 2: C + O2 → CO2 (Carbon gains oxygen)

(b) Reduction: It is the loss of oxygen during a reaction.

  • Example 1: CuO + H2 → Cu + H2O (Copper oxide loses oxygen)
  • Example 2: ZnO + C → Zn + CO (Zinc oxide loses oxygen)
Q17. 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 shiny brown element ‘X’ is Copper (Cu). The black coloured compound formed is Copper oxide (CuO).

2Cu + O2 → 2CuO (Black)

Q18. Why do we apply paint on iron articles?

Answer: We apply paint on iron articles to prevent rusting (corrosion). Painting covers the surface of the iron and cuts off the direct contact between the iron and the atmospheric moisture/oxygen, thus preventing the oxidation of iron.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?

Answer: Oil and fat-containing food items are flushed with nitrogen to prevent rancidity. Nitrogen is an unreactive (inert) gas that surrounds the food and prevents oxygen from coming into contact with the fats and oils. This stops oxidation, preserving the smell and taste of the food.

Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

Answer:

(a) Corrosion: It is the process where metals are attacked by substances around them such as moisture or acids.
Example: Rusting of iron (reddish-brown coating on iron) or the black coating on silver.

(b) Rancidity: It is the oxidation of fats and oils in food when left for a long time, leading to a bad smell and bad taste.
Example: The change in taste and smell of chips if kept open in the air for a long time.

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