Oxidation and Reduction Redox

Q7. Define oxidation and reduction in terms of: (i) oxygen, (ii) hydrogen, (iii) electrons, and (iv) oxidation number.

Q8. In the reaction ZnO + C → Zn + CO, identify: (i) the substance oxidized, (ii) the substance reduced, (iii) the oxidizing agent, and (iv) the reducing agent.

Q9. Your mother applies lemon juice (citric acid) to cut apples to prevent them from turning brown. Explain the oxidation reaction that causes browning and how lemon juice prevents it.

Q10. Why is the reaction between zinc and dilute sulphuric acid called a redox reaction? Identify the oxidizing agent and reducing agent in this reaction.

Q11. Differentiate between an oxidizing agent and a reducing agent with one example of each.

Q12. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, identify which substance is oxidized and which is reduced. Justify your answer.

Q13. Define oxidation and reduction. Explain the four different ways of defining these processes (in terms of oxygen, hydrogen, electrons, and oxidation number). Illustrate with the reaction: CuO + H₂ → Cu + H₂O. Identify the oxidizing agent and reducing agent in this reaction.

Q14. A student is confused about why oxidation and reduction always occur together. Help the student by:

(a) Explaining why electrons lost by one substance must be gained by another.

(b) Analysing the reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂ to identify what is oxidized and what is reduced.

(c) Identifying the oxidizing agent and reducing agent in the above reaction.

(d) Explaining why the term "redox" is used for such reactions.

(e) Giving one more example of a redox reaction from everyday life.

Q15. "Redox reactions are the driving force behind many natural processes and industrial applications that we take for granted." Discuss this statement by explaining: (i) the role of redox reactions in respiration and photosynthesis, (ii) the extraction of metals from their ores, (iii) the working of batteries and fuel cells, and (iv) the corrosion of metals and its prevention.

Q16. In a chemistry laboratory, students study redox reactions by performing the following experiments:

Table

Reaction Observation

CuO + H₂ → Cu + H₂O Black to reddish-brown, water droplets

Zn + CuSO₄ → ZnSO₄ + Cu Blue to colourless, brown deposit

2Na + Cl₂ → 2NaCl Bright flame, white solid

Fe₂O₃ + 2Al → 2Fe + Al₂O₃ Intense heat, molten iron

(a) Identify the substance oxidized and the substance reduced in each reaction.

(b) Identify the oxidizing agent and reducing agent in each reaction.

(c) For Reaction 4 (thermite), if 160 g of Fe₂O₃ reacts with excess aluminium, calculate the mass of aluminium oxide produced. (Molar masses: Fe₂O₃ = 160 g/mol, Al₂O₃ = 102 g/mol)

(d) Calculate the oxidation number of iron in Fe₂O₃ and in elemental Fe. What change occurs?

(e) Why is the thermite reaction considered both a redox reaction and a displacement reaction?

Q17. A dry cell (Leclanché cell) used in torches and radios works on redox principles:

Zn + 2MnO₂ + 2NH₄Cl → ZnCl₂ + Mn₂O₃ + 2NH₃ + H₂O

(a) Identify the substance oxidized and the substance reduced.

(b) Calculate the oxidation number of manganese in MnO₂ and in Mn₂O₃.

(c) If a dry cell contains 6.5 g of zinc, calculate the mass of MnO₂ required for complete reaction. (Molar masses: Zn = 65 g/mol, MnO₂ = 87 g/mol)

(d) Why is the redox reaction in a dry cell useful? What energy conversion takes place?

(e) If India produces 1 billion dry cells annually, and each cell contains 3.25 g of zinc, calculate the total mass of zinc used per year.

Q18. In India, iron structures like bridges, railway tracks, and pipelines are prone to rusting due to the monsoon climate. Rusting is a redox process:

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (which dehydrates to Fe₂O₃·xH₂O)

(a) Identify what is oxidized and what is reduced in the rusting of iron.

(b) If a railway bridge has 50 tonnes of iron and 5% of it rusts over 10 years, calculate the mass of rust (Fe₂O₃·xH₂O, assume x = 3) formed. (Molar masses: Fe = 56 g/mol, Fe₂O₃·3H₂O = 214 g/mol)

(c) Painting, galvanization, and alloying are methods to prevent rusting. Explain the redox principle behind galvanization.

(d) The Konkan Railway along India's west coast faces severe corrosion. Why is this region particularly vulnerable, and what special measures are taken?

(e) Calculate the annual economic loss if 1% of India's 100 million tonnes of iron infrastructure rusts each year, at a replacement cost of ₹50,000 per tonne.