Chemical Reactions and Equations
Q1.. Why should a magnesium ribbon be cleaned before burning in air?
Answer:
A magnesium ribbon is cleaned before burning in air to remove the layer of magnesium oxide (MgO) that forms on its surface due to reaction with atmospheric oxygen. This layer is a white, powdery coating that prevents further contact between the magnesium metal and oxygen in the air, inhibiting the burning process. Cleaning with sandpaper exposes the fresh, reactive magnesium metal underneath, allowing the ribbon to burn readily with a dazzling white flame.
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(g) + Cl₂(g) → 2HCl(g)
(ii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(iii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Explanation: Balancing ensures the number of atoms of each element is equal on both sides of the equation, following the law of conservation of mass.
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)
Explanation: Barium chloride and sodium sulphate (both aqueous) react to form barium sulphate (insoluble solid, indicated by (s)) and sodium chloride (aqueous).
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Explanation: Sodium hydroxide (aqueous) reacts with hydrochloric acid (aqueous) to produce sodium chloride (aqueous) and water (liquid). This is a neutralization reaction.
Questions within chapter
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) Substance X is calcium hydroxide (slaked lime). Its formula is Ca(OH)2.
(ii) Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
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 the electrolysis of water, water (H2O) decomposes to give hydrogen (H2) and oxygen (O2) in a 2:1 volume ratio. The gas collected in double the amount is hydrogen.
Q1 (second set). Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer:
Iron displaces copper from copper sulphate solution, forming iron sulphate (greenish) and copper metal. The blue colour of copper sulphate fades due to the formation of iron sulphate.
Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer:
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) (precipitation of yellow lead iodide).
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) Sodium (Na) is oxidised (gains oxygen). Oxygen (O2) is reduced (gains electrons).
(ii) Copper oxide (CuO) is reduced (loses oxygen). Hydrogen (H2) is oxidised (gains oxygen).
Exercises
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:
Option (i) (a) and (b) is incorrect. Statement (b) is incorrect because carbon dioxide is a product, not a reactant being oxidised. Statement (a) is correct: lead is reduced. Statement (c) is correct: carbon is oxidised. Statement (d) is correct: lead oxide is reduced.
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:
Option (d) displacement reaction. Aluminium displaces iron from iron oxide.
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:
Option (a) Hydrogen gas and iron chloride are produced.
Fe + 2HCl → FeCl2 + H2
Q4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer:
A balanced chemical equation has the same number of atoms of each element on both sides of the equation. It must be balanced to obey the law of conservation of mass (mass is neither 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(aq) + CO2(g) → CaCO3(s) + H2O(l)
(b) Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)
(d) BaCl2(aq) + K2SO4(aq) → BaSO4(s) + 2KCl(aq)
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); 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
Q9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer:
Exothermic reactions release heat energy. Example: Burning of magnesium (2Mg + O2 → 2MgO + heat).
Endothermic reactions absorb heat energy. Example: Decomposition of calcium carbonate (CaCO3 → CaO + CO2, heat required).
Q10. Why is respiration considered an exothermic reaction? Explain.
Answer:
Respiration involves the oxidation of glucose (C6H12O6) to produce carbon dioxide, water, and energy. The energy released is used for cellular activities, making it an exothermic process.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer:
In combination reactions, two or more substances combine to form a single product. In decomposition reactions, a single compound breaks down into two or more simpler substances. They are opposite processes.
Combination: 2H2 + O2 → 2H2O
Decomposition: 2H2O → 2H2 + O2
Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
Heat: CaCO3(s) → CaO(s) + CO2(g) (thermal decomposition)
Light: 2AgCl(s) → 2Ag(s) + Cl2(g) (photolysis)
Electricity: 2H2O(l) → 2H2(g) + O2(g) (electrolysis)
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:
In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Example: Zn + CuSO4 → ZnSO4 + Cu.
In a double displacement reaction, two compounds exchange ions to form two new compounds. Example: Na2SO4 + BaCl2 → BaSO4 + 2NaCl.
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:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
Q15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer:
A precipitation reaction is a reaction in which an insoluble solid (precipitate) is formed from the reaction of two soluble substances. Example: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) (white precipitate of barium sulphate).
Q16. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction
Answer:
(a) Oxidation is the gain of oxygen. Examples: 2Mg + O2 → 2MgO (magnesium gains oxygen); 2Cu + O2 → 2CuO (copper gains oxygen).
(b) Reduction is the loss of oxygen. Examples: CuO + H2 → Cu + H2O (copper oxide loses oxygen); 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:
Element X is copper (Cu). The black compound formed is copper(II) oxide (CuO).
Q18. Why do we apply paint on iron articles?
Answer:
Paint is applied to iron articles to prevent them from coming into contact with air and moisture, which cause rusting (corrosion). This layer acts as a barrier and slows down oxidation.
Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer:
Nitrogen is an inert gas that prevents oxidation (rancidity) of oils and fats. Flushing with nitrogen removes oxygen, thus slowing down the oxidation process and keeping the food fresh for longer.
Q20. Explain the following terms with one example each. (a) Corrosion (b) Rancidity
Answer:
(a) Corrosion is the deterioration of metals due to reaction with moisture, air, or chemicals. Example: Rusting of iron (formation of Fe2O3·xH2O).
(b) Rancidity is the oxidation of fats and oils, causing unpleasant smell and taste. Example: Stale chips or butter left open develop a rancid odor.
Recall Questions
QActivity 1.2 recall. Recall Activity 1.2, where you have mixed the solutions of lead(II) nitrate and potassium iodide. (i) What was the colour of the precipitate formed? Can you name the compound precipitated? (ii) Write the balanced chemical equation for this reaction. (iii) Is this also a double displacement reaction?
Answer:
(i) Yellow precipitate; lead iodide (PbI2).
(ii) Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(iii) Yes, it is a double displacement reaction.
QActivity 1.1 recall. Recall Activity 1.1, where a magnesium ribbon burns with a dazzling flame in air (oxygen) and changes into a white substance, magnesium oxide. Is magnesium being oxidised or reduced in this reaction?
Answer:
Magnesium is being oxidised because it gains oxygen to form magnesium oxide (2Mg + O2 → 2MgO).