Balancing Redox Reactions - Oxidation Number Method - UNSOLVED PRACTICE SET

Q7. Outline the steps involved in balancing a redox reaction by the oxidation number method.

Q8. Balance the following redox reaction by the oxidation number method:

Cu + HNO₃ → Cu(NO₃)₂ + NO₂ + H₂O

Q9. In the reaction KMnO₄ + H₂SO₄ + FeSO₄ → K₂SO₄ + MnSO₄ + Fe₂(SO₄)₃ + H₂O, identify the elements that change oxidation number and calculate the total change for each.

Q10. Balance the following equation in acidic medium by the oxidation number method:

Cr₂O₇²⁻ + Fe²⁺ → Cr³⁺ + Fe³⁺

Q11. Explain why H⁺ ions are added when balancing redox reactions in acidic medium, while OH⁻ ions are added in basic medium.

Q12. A student tries to balance the reaction H₂S + HNO₃ → S + NO + H₂O but forgets to balance hydrogen atoms at the end. What mistake will this lead to in the final balanced equation?

Q13. (a) Describe the step-by-step procedure for balancing redox reactions by the oxidation number method.

(b) Balance the following reaction by the oxidation number method:

MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O

(c) Identify the oxidising agent and the reducing agent in the above reaction. Calculate the equivalent weight of MnO₂ in this reaction.

Q14. (a) Balance the following redox reaction in acidic medium by the oxidation number method:

K₂Cr₂O₇ + H₂SO₄ + SO₂ → K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(b) Balance the following redox reaction in basic medium by the oxidation number method:

MnO₄⁻ + I⁻ → MnO₂ + IO₃⁻

(c) A student claims that the oxidation number method cannot be used for reactions where the same element undergoes both oxidation and reduction. Is the student correct? Explain with an example.

Q15. (a) Balance the following complex redox reaction by the oxidation number method:

As₂S₃ + HNO₃ → H₃AsO₄ + H₂SO₄ + NO₂ + H₂O

(b) Balance the following reaction and identify the oxidising and reducing agents:

P₄ + NaOH + H₂O → PH₃ + NaH₂PO₂

(c) In the Indian context, the reaction of copper with concentrated nitric acid is used in etching printed circuit boards. Balance this reaction and calculate the volume of NO₂ produced at STP when 6.35 g of Cu reacts with excess HNO₃. (M of Cu = 63.5 g/mol)

Q16. Balance the following redox reactions by the oxidation number method. Show all steps clearly:

(a) Zn + HNO₃ → Zn(NO₃)₂ + NH₄NO₃ + H₂O (dilute HNO₃)

(b) Cu + HNO₃ → Cu(NO₃)₂ + NO + H₂O (dilute HNO₃)

(c) KMnO₄ + H₂C₂O₄ + H₂SO₄ → K₂SO₄ + MnSO₄ + CO₂ + H₂O

(d) FeS₂ + O₂ → Fe₂O₃ + SO₂ (roasting of pyrites)

For each reaction, identify:

(i) The oxidising agent and the reducing agent

(ii) The element oxidised and the element reduced

(iii) The total number of electrons transferred

Q17. A student performs a redox titration experiment:

(a) 25.0 mL of 0.1 M FeSO₄ solution is acidified with H₂SO₄ and titrated against KMnO₄ solution. The reaction is:

MnO₄⁻ + Fe²⁺ + H⁺ → Mn²⁺ + Fe³⁺ + H₂O

(i) Balance this reaction by the oxidation number method

(ii) Calculate the volume of 0.02 M KMnO₄ required to reach the endpoint

(iii) What colour change indicates the endpoint?

(b) In another experiment, 20.0 mL of 0.1 M Na₂C₂O₄ (sodium oxalate) is titrated against acidified KMnO₄:

MnO₄⁻ + C₂O₄²⁻ + H⁺ → Mn²⁺ + CO₂ + H₂O

(i) Balance this reaction by the oxidation number method

(ii) Calculate the molarity of KMnO₄ if 16.0 mL is required to reach the endpoint

Q18. In Indian industry and agriculture, redox reactions are balanced for practical applications:

(a) In the contact process for sulphuric acid, the reaction is:

SO₂ + O₂ → SO₃

(i) Balance this reaction by the oxidation number method

(ii) Calculate the mass of SO₃ produced from 100 kg of SO₂

(iii) If the conversion is only 95% efficient, what mass of SO₂ is actually converted?

(b) In the extraction of manganese from pyrolusite (MnO₂), the reaction with aluminium is:

MnO₂ + Al → Mn + Al₂O₃

(i) Balance this reaction by the oxidation number method

(ii) Calculate the mass of aluminium required to produce 1 kg of manganese

(iii) This reaction is highly exothermic and is used in thermite welding of railway tracks. Calculate the heat released if ΔH°f(Al₂O₃) = –1675 kJ/mol and ΔH°f(MnO₂) = –520 kJ/mol

(c) In water treatment, chlorine dioxide (ClO₂) is used as a disinfectant. It is produced by the reaction:

NaClO₃ + HCl → ClO₂ + Cl₂ + NaCl + H₂O

(i) Balance this reaction by the oxidation number method

(ii) Identify which chlorine species undergoes oxidation and which undergoes reduction

(iii) Calculate the mass of NaClO₃ required to produce 100 g of ClO₂