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Q1. The electrode potential of four metallic elements (A, B, C and D) are +0.80V, -0.76V, +0.12V and +0.34V respectively. Arrange them in order of decreasing electropositive character.

Solution

Higher the electrode potential (E⁰) lower is the tendency of the metal to lose electrons and hence lower is the electropositive character of the metal.

Q2. The oxidation number of Fe in Fe₃O₄ is fractional. Why?

Solution

Fe₃O₄ contains Fe atoms of both +2 and +3 oxidation number. It is a stoichiometric mixture of Ferrous (FeO) and Ferric (Fe₂O₃) oxides combined  as FeO.Fe₂O₃. Therefore, the oxidation number found in such cases is average oxidation number.

Q3. In which compounds is the oxidation number of Oxygen -1 and +2?

Solution

The oxidation number in peroxides (Na₂O₂) is -1 and in OF₂ its oxidation number is +2.

Q4. Define oxidation in terms of oxidation numbers.

Solution

Oxidation may be defined as a chemical change in which there occurs an increase in the oxidation number of an atom or atoms.

Q5. Which of the following species do not show disproportionation reaction and why? ClO^-, ClO₂^-, ClO₃^- and ClO₄^-

Solution

Out of the above species, ClO₄^- does not undergo disproportionation since in this oxoanion chlorine is already present in the highest oxidation state of +7 and hence cannot be further oxidised.

Q6. Write the net ionic equation for the reaction of Potassium Dichromate (VI), K₂Cr₂O₇ with Sodium Sulphite, Na₂SO₃ in acid solution to give Chromium (III) ion and Sulphate ion.

Solution

Cr₂O₇^2-_(aq) + 3SO₃^2-_(aq) + 8H⁺_(aq) →  2Cr³⁺_(aq) + 3SO₄^2-_(aq) + 4H₂O_(liq)

Q7. Is the sum of oxidation numbers of all atoms in an ion is equal to 0?

Solution

No, the sum of oxidation number of all atoms in an ion is equal to the charge on the ion.

Q8. What are the steps involved in balancing a Redox equation by Ion Electron method?

Solution

Step - I Write the skeletal equation and indicate the Oxidation number of all the elements which appear in the skeletal equation above their respective symbols. Step – II Find out the species which are oxidized and which are reduced. Step – III Split the skeletal equation into two half reactions, i.e. oxidation half reaction and reduction half equation. Step – IV Balance the two half equations separately by the rules described below: (a) In each half reaction, first balance the atoms of the elements which have undergone a change in oxidation number. (b) Add electrons to whatever side is necessary to make up the difference in oxidation number in each half reaction. (c) Balance charge by adding H⁺ ions if the reaction occurs in the acidic medium and by adding OH^- ions if the reaction occurs in basic medium. (d) Balance oxygen atoms by adding required number of H₂O molecules to the side deficient in O atoms. (e) In the acidic medium, H atoms are balanced by adding H⁺ ions to the side deficient in H atoms. However, in the basic medium H atoms are balanced by adding H₂O molecules equal in number to the deficiency of H atoms and equal OH^- ions are included in the opposite side of the equation. Remove the duplication, if any. Step – V The two half reactions are then multiplied by suitable integers so that the total number of electrons gained in one half reaction is equal to the number of electrons lost in the other half reaction. The two half reactions are then added up.

Q9. What is a disproportionation reaction?

Solution

A reaction in which the same species is simultaneously oxidised as well as reduced is called a disproportionation reaction.

Q10. The standard electrode potentials of a few metals are given below: AI(-0.66V), Cu(+0.34V), Li(-3.05V), Ag(+0.80V) and Zn(-0.76V) Which of these will behave as the strongest oxidizing agent and which as the strongest reducing agent?

Solution

Li is the strongest reducing agent while Ag+ is the strongest oxidizing agent.

Q11. Calculate the Oxidation number of Sulphur in S₂O₈^2- ion.

Solution

The Oxidation number of Sulphur in this ion is 6

Q12. Define Reduction in terms of oxidation numbers.

Solution

Reduction may be defined as a chemical change in which there occurs a decrease in the oxidation number of an atom or atoms.

Q13. Define Oxidation number.

Solution

The charge which an atom appears to have when all other atoms are removed from it as ions.

Q14. Justify that the reaction 2Cu₂O_(s)+Cu₂S_(s)   →   6Cu_(s)+SO_2(g) is a redox reaction.

Solution

Writing the oxidation number of each atom above its symbol, we have                                  _{+1  -2         +1  -2                  0            +4 -2}                         2Cu₂O_(s)  + Cu₂S_(s) →   6Cu_(s)   +   SO_2(g) Here in the reaction, the oxidation number of copper decreases from +1 in Cu₂O or Cu₂S to 0 in copper metal, therefore copper is reduced. Further the oxidation number of S increases from -2 in Cu₂S to +4 in SO₂, therefore S is oxidised. Therefore, this reaction is a redox reaction.

Q15. Suggest a scheme of classification of the following redox reaction: 2NO_2(g) + 2OH^-_(aq) → NO^-_2(aq) + NO^-_3(aq) + H₂O_(liq)

Solution

This is a disproportionation reaction since here the oxidation state of nitrogen decreases from +4 in NO₂ to +3 in NO₂^- ion, as well as increases from +4 in NO₂ to +5 in NO₃^-ion.

Q16. What are the two ways by which a Redox equation is balanced?

Solution

The two methods of balancing the redox equation are:a) Oxidation number method b) Ion electron method or half equation method.

Q17. What is Standard Electrode Potential?

Solution

Electrode potential are generally measured under standard conditions in 1 mol L^-1 and a temperature of 298 K and are called Standard Electrode potential denoted by E^o.

Q18. Can the oxidation number of an element be zero?

Solution

Yes, the oxidation number of an element can be zero. For example, the oxidation number of carbon in CH₂Cl₂ is zero.

Q19. What is the condition necessary for a disproportionation redox reaction?

Solution

For such redox reaction to occur, the reacting species must contain an element which has at least three oxidation states. The element in the reacting species is present in the intermediate oxidation state while higher and lower oxidation states are available for reduction and oxidation to occur.

Q20. What is fractional oxidation state?

Solution

Fractional oxidation state is the average oxidation of an element when two or more of its atoms are present in different oxidation states in a given compound.