Redox Reactions in Daily Life - UNSOLVED PRACTICE SET

Q7. Explain the process of rusting of iron as a redox reaction. Write the chemical equation and identify what is oxidised and what is reduced.

Q8. Describe the role of redox reactions in photosynthesis. Identify the substance oxidised and the substance reduced.

Q9. Explain how a dry cell (Leclanché cell) works. Identify the oxidation and reduction half-reactions.

Q10. Why do we apply paint or grease on iron objects to prevent rusting? Explain the principle behind this method of corrosion prevention.

Q11. Explain why antioxidants are added to food products. What redox process do they prevent?

Q12. A student notices that a cut apple turns brown after some time. Explain this observation as a redox reaction.

Q13. (a) Explain the process of corrosion of iron (rusting) as an electrochemical redox process. Write the half-reactions occurring at the anode and cathode.

(b) Explain any three methods of preventing corrosion of iron with the underlying redox principles.

(c) In coastal areas of India, iron corrodes faster than in dry inland areas. Explain this observation in terms of redox reactions.

Q14. (a) Explain the role of redox reactions in:

(i) Respiration in living organisms

(ii) Photosynthesis in plants

(iii) Combustion of fuels

(b) For each process in (a), write the overall chemical equation and identify the oxidising agent and the reducing agent.

(c) A student argues that photosynthesis and respiration are reverse processes. Is the student correct? Explain with reference to the redox nature of both processes.

Q15. (a) Explain the working of a lead-acid storage battery used in automobiles. Write the discharge and charge reactions, identifying the oxidation and reduction processes.

(b) Explain why lithium-ion batteries are preferred in mobile phones and laptops compared to nickel-cadmium batteries, with reference to redox chemistry.

(c) In the Indian context, explain the role of redox reactions in:

(i) The use of tincture of iodine as an antiseptic

(ii) The use of potassium permanganate (KMnO₄) for water purification

(iii) The browning of chapatis when heated on a tawa

Q16. Analyse the following everyday redox reactions:

(a) Burning of LPG (mainly propane, C₃H₈) in a kitchen stove:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

(i) Identify the element oxidised and the element reduced

(ii) Calculate the change in oxidation number of carbon

(iii) If a household uses 2 kg of LPG per day, calculate the mass of CO₂ produced (M of C₃H₈ = 44 g/mol, M of CO₂ = 44 g/mol)

(b) The thermite reaction used for welding railway tracks:

2Al + Fe₂O₃ → 2Fe + Al₂O₃

(i) Identify the oxidising agent and reducing agent

(ii) Calculate the mass of aluminium required to produce 500 g of iron

(iii) Calculate the heat released using ΔH°f values: Al₂O₃ = –1675 kJ/mol, Fe₂O₃ = –824 kJ/mol

(c) The reaction in a hydrogen fuel cell:

2H₂ + O₂ → 2H₂O

(i) Identify the oxidation and reduction half-reactions

(ii) Calculate the volume of H₂ required at STP to produce 1 kWh of energy (1 kWh = 3.6 × 10⁶ J, ΔH°f of H₂O = –286 kJ/mol)

(d) The reaction of antacid tablets (containing NaHCO₃) with stomach acid (HCl):

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Is this a redox reaction? Justify your answer by calculating oxidation numbers.

Q17. Consider the following scenarios from Indian daily life:

(a) A farmer uses urea (CO(NH₂)₂) as a nitrogen fertiliser. In the soil, bacteria convert urea to ammonium ions, which are then oxidised to nitrate ions by nitrifying bacteria:

NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O

(i) Identify the oxidation and reduction processes

(ii) Calculate the change in oxidation number of nitrogen

(iii) If a field requires 50 kg of nitrogen, calculate the mass of urea needed (M of urea = 60 g/mol, N = 14 g/mol)

(b) In a village, water is purified by adding bleaching powder (CaOCl₂). The active component is hypochlorous acid (HOCl), which kills bacteria:

HOCl + H⁺ + 2e⁻ → Cl⁻ + H₂O

(i) Identify this as an oxidation or reduction half-reaction

(ii) Calculate the oxidation number of chlorine in HOCl and Cl⁻

(iii) If 1 ppm (1 mg/L) of available chlorine is needed, calculate the mass of bleaching powder (containing 35% available chlorine) required for 1000 L of water

(c) A mechanic uses oxalic acid (H₂C₂O₄) to remove rust (Fe₂O₃) from iron parts:

Fe₂O₃ + 3H₂C₂O₄ → 2Fe(C₂O₄)₃³⁻ + 6H⁺ + 3H₂O

(i) Identify the redox nature of this reaction by calculating oxidation numbers

(ii) Explain why oxalic acid acts as a reducing agent

(iii) Calculate the mass of oxalic acid required to remove 10 g of rust (M of Fe₂O₃ = 160 g/mol, M of H₂C₂O₄ = 90 g/mol)

Q18. In India, redox reactions are central to many traditional and modern practices:

(a) In Ayurveda, turmeric (containing curcumin) is used for its antioxidant properties. Antioxidants prevent oxidation of cells by free radicals. Explain how vitamin C (ascorbic acid, C₆H₈O₆) acts as an antioxidant by being oxidised to dehydroascorbic acid (C₆H₆O₆). Write the half-reaction and calculate the change in oxidation number of carbon.

(b) In the textile industry, hydrogen peroxide is used for bleaching cotton. The reaction is:

H₂O₂ → H₂O + ½O₂

(i) Identify this as a disproportionation reaction

(ii) Calculate the volume of O₂ produced at STP when 100 mL of 0.5 M H₂O₂ decomposes

(iii) Explain why the bleaching action is due to the oxidising nature of H₂O₂

(c) In photography, the developer solution contains hydroquinone (C₆H₄(OH)₂), which reduces exposed silver bromide to metallic silver:

2AgBr + C₆H₄(OH)₂ → 2Ag + 2HBr + C₆H₄O₂

(i) Identify the oxidation and reduction processes

(ii) Calculate the mass of silver produced when 5.0 g of hydroquinone is used (M of hydroquinone = 110 g/mol, M of Ag = 108 g/mol)

(iii) Explain why only exposed AgBr grains are reduced (hint: light creates latent image sites)

(d) A student sets up a simple galvanic cell using zinc and copper electrodes dipped in their respective sulphate solutions. The cell produces 1.1 V.

(i) Write the oxidation half-reaction at the anode

(ii) Write the reduction half-reaction at the cathode

(iii) Calculate the mass of zinc that dissolves when the cell produces 0.5 A current for 2 hours (1 F = 96500 C/mol, M of Zn = 65 g/mol)

(iv) Explain why this cell is called a Daniel cell and its historical significance in India