Chapter 8 Redox Reactions

Question 1. Which of the following is not an example of redox reaction?

(i) $CuO + H_2 \rightarrow Cu + H_2O$

(ii) $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$

(iii) $2K + F_2 \rightarrow 2KF$

(iv) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + 2HCl$

Question 2. The more positive the value of $E^\ominus$, the greater is the tendency of the species to get reduced. Using the standard electrode potential of redox couples given below find out which of the following is the strongest oxidising agent.

$E^\ominus$ values: $Fe^{3+}/Fe^{2+} = + 0.77$; $I_2(s)/I^– = + 0.54$; $Cu^{2+}/Cu = + 0.34$; $Ag^+/Ag = + 0.80V$

(i) $Fe^{3+}$

(ii) $I_2(s)$

(iii) $Cu^{2+}$

(iv) $Ag^+$

Question 3. $E^\ominus$ values of some redox couples are given below. On the basis of these values choose the correct option.

$E^\ominus$ values : $Br_2/Br^– = + 1.90$; $Ag^+ /Ag(s) = + 0.80$; $Cu^{2+}/Cu(s) = + 0.34$; $I_2(s)/I^– = + 0.54$

(i) Cu will reduce $Br^–$

(ii) Cu will reduce Ag

(iii) Cu will reduce $I^–$

(iv) Cu will reduce $Br_2$

Question 4. Using the standard electrode potential, find out the pair between which redox reaction is not feasible.

$E^\ominus$ values : $Fe^{3+}/Fe^{2+} = + 0.77$; $I_2/I^– = + 0.54$; $Cu^{2+}/Cu = + 0.34$; $Ag^+/Ag = + 0.80 V$

(i) $Fe^{3+}$ and $I^–$

(ii) $Ag^+$ and Cu

(iii) $Fe^{3+}$ and Cu

(iv) Ag and $Fe^{3+}$

Question 5. Thiosulphate reacts differently with iodine and bromine in the reactions given below:

$2S_2O_3^{2–} + I_2 \rightarrow S_4O_6^{2–} + 2I^–$

$S_2O_3^{2–} + 2Br_2 + 5H_2O \rightarrow 2SO_4^{2–} + 2Br^– + 10 H^+$

Which of the following statements justifies the above dual behaviour of thiosulphate?

(i) Bromine is a stronger oxidant than iodine.

(ii) Bromine is a weaker oxidant than iodine.

(iii) Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions.

(iv) Bromine undergoes oxidation and iodine undergoes reduction in these reactions.

Question 6. The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct in this respect?

(i) The oxidation number of hydrogen is always +1.

(ii) The algebraic sum of all the oxidation numbers in a compound is zero.

(iii) An element in the free or the uncombined state bears oxidation number zero.

(iv) In all its compounds, the oxidation number of fluorine is – 1.

Question 7. In which of the following compounds, an element exhibits two different oxidation states.

(i) $NH_2OH$

(ii) $NH_4NO_3$

(iii) $N_2H_4$

(iv) $N_3H$

Question 8. Which of the following arrangements represent increasing oxidation number of the central atom?

(i) $CrO_2^–, ClO_3^–, CrO_4^{2–}, MnO_4^–$

(ii) $ClO_3^–, CrO_4^{2–}, MnO_4^–, CrO_2^–$

(iii) $CrO_2^–, ClO_3^–, MnO_4^–, CrO_4^{2–}$

(iv) $CrO_4^{2–}, MnO_4^–, CrO_2^–, ClO_3^–$

Question 9. The largest oxidation number exhibited by an element depends on its outer electronic configuration. With which of the following outer electronic configurations the element will exhibit largest oxidation number?

(i) $3d^14s^2$

(ii) $3d^34s^2$

(iii) $3d^54s^1$

(iv) $3d^54s^2$

Question 10. Identify disproportionation reaction

(i) $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

(ii) $CH_4 + 4Cl_2 \rightarrow CCl_4 + 4HCl$

(iii) $2F_2 + 2OH^– \rightarrow 2F^– + OF_2 + H_2O$

(iv) $2NO_2 + 2OH^– \rightarrow NO_2^– + NO_3^– + H_2O$

Question 11. Which of the following elements does not show disproportionation tendency?

(i) Cl

(ii) Br

(iii) F

(iv) I

Question 12. Which of the following statement(s) is/are not true about the following decomposition reaction.

$2KClO_3 \rightarrow 2KCl + 3O_2$

(i) Potassium is undergoing oxidation

(ii) Chlorine is undergoing oxidation

(iii) Oxygen is reduced

(iv) None of the species are undergoing oxidation or reduction

Question 13. Identify the correct statement (s) in relation to the following reaction:

$Zn + 2HCl \rightarrow ZnCl_2 + H_2$

(i) Zinc is acting as an oxidant

(ii) Chlorine is acting as a reductant

(iii) Hydrogen ion is acting as an oxidant

(iv) Zinc is acting as a reductant

Question 14. The exhibition of various oxidation states by an element is also related to the outer orbital electronic configuration of its atom. Atom(s) having which of the following outermost electronic configurations will exhibit more than one oxidation state in its compounds.

(i) $3s^1$

(ii) $3d^14s^2$

(iii) $3d^24s^2$

(iv) $3s^23p^3$

Question 15. Identify the correct statements with reference to the given reaction

$P_4 + 3OH^– + 3H_2O \rightarrow PH_3 + 3H_2PO_2^–$

(i) Phosphorus is undergoing reduction only.

(ii) Phosphorus is undergoing oxidation only.

(iii) Phosphorus is undergoing oxidation as well as reduction.

(iv) Hydrogen is undergoing neither oxidation nor reduction.

Question 16. Which of the following electrodes will act as anodes, when connected to Standard Hydrogen Electrode?

(i) $Al/Al^{3+} \quad E^\ominus = –1.66$

(ii) $Fe/Fe^{2+} \quad E^\ominus = –0.44$

(iii) $Cu/Cu^{2+} \quad E^\ominus = +0.34$

(iv) $F_2(g)/2F^–(aq) \quad E^\ominus = +2.87$

Question 17. The reaction

$Cl_2(g) + 2OH^–(aq) \rightarrow ClO^–(aq) + Cl^–(aq) + H_2O(l)$

represents the process of bleaching. Identify and name the species that bleaches the substances due to its oxidising action.

Question 18. $MnO_4^{2–}$ undergoes disproportionation reaction in acidic medium but $MnO_4^–$ does not. Give reason.

Question 19. PbO and $PbO_2$ react with HCl according to following chemical equations :

$2PbO + 4HCl \rightarrow 2PbCl_2 + 2H_2O$

$PbO_2 + 4HCl \rightarrow PbCl_2 + Cl_2 + 2H_2O$

Why do these compounds differ in their reactivity?

Question 20. Nitric acid is an oxidising agent and reacts with PbO but it does not react with $PbO_2$. Explain why?

Question 21. Write balanced chemical equation for the following reactions:

(i) Permanganate ion ($MnO_4^–$) reacts with sulphur dioxide gas in acidic medium to produce $Mn^{2+}$ and hydrogensulphate ion. (Balance by ion electron method)

(ii) Reaction of liquid hydrazine ($N_2H_4$) with chlorate ion ($ClO_3^–$) in basic medium produces nitric oxide gas and chloride ion in gaseous state. (Balance by oxidation number method)

(iii) Dichlorine heptaoxide ($Cl_2O_7$) in gaseous state combines with an aqueous solution of hydrogen peroxide in acidic medium to give chlorite ion ($ClO_2^–$) and oxygen gas. (Balance by ion electron method)

Question 22. Calculate the oxidation number of phosphorus in the following species.

(a) $HPO_3^{2–}$ and (b) $PO_4^{3–}$

Question 23. Calculate the oxidation number of each sulphur atom in the following compounds:

(a) $Na_2S_2O_3$ (b) $Na_2S_4O_6$ (c) $Na_2SO_3$ (d) $Na_2SO_4$

Question 24. Balance the following equations by the oxidation number method.

(i) $Fe^{2+} + H^+ + Cr_2O_7^{2–} \rightarrow Cr^{3+} + Fe^{3+} + H_2O$

(ii) $I_2 + NO_3^– \rightarrow NO_2 + IO_3^–$

(iii) $I_2 + S_2O_3^{2–} \rightarrow I^– + S_4O_6^{2–}$

(iv) $MnO_2 + C_2O_4^{2–} \rightarrow Mn^{2+} + CO_2$

Question 25. Identify the redox reactions out of the following reactions and identify the oxidising and reducing agents in them.

(i) $3HCl(aq) + HNO_3(aq) \rightarrow Cl_2(g) + NOCl(g) + 2H_2O(l)$

(ii) $HgCl_2(aq) + 2KI(aq) \rightarrow HgI_2(s) + 2KCl(aq)$

(iii) $Fe_2O_3(s) + 3CO(g) \xrightarrow{\Delta} 2Fe(s) + 3CO_2(g)$

(iv) $PCl_3(l) + 3H_2O(l) \rightarrow 3HCl(aq) + H_3PO_3(aq)$

(v) $4NH_3 + 3O_2(g) \rightarrow 2N_2(g) + 6H_2O(g)$

Question 26. Balance the following ionic equations

(i) $Cr_2O_7^{2–} + H^+ + I^– \rightarrow Cr^{3+} + I_2 + H_2O$

(ii) $Cr_2O_7^{2–} + Fe^{2+} + H^+ \rightarrow Cr^{3+} + Fe^{3+} + H_2O$

(iii) $MnO_4^– + S_2O_3^{2–} + H^+ \rightarrow Mn^{2+} + S_4O_6^{2–} + H_2O$

(iv) $MnO_4^– + H^+ + Br^– \rightarrow Mn^{2+} + Br_2 + H_2O$

Question 27. Match Column I with Column II for the oxidation states of the central atoms.

Column I

(i) $Cr_2O_7^{2–}$

(ii) $MnO_4^–$

(iii) $VO_3^–$

(iv) $FeF_6^{3–}$

Column II

(a) + 3

(b) + 4

(c) + 5

(d) + 6

(e) + 7

Question 28. Match the items in Column I with relevant items in Column II.

Column I

(i) Ions having positive charge

(ii) The sum of oxidation number of all atoms in a neutral molecule

(iii) Oxidation number of hydrogen ion ($H^+$)

(iv) Oxidation number of fluorine in NaF

(v) Ions having negative charge

Column II

(a) +7

(b) –1

(c) +1

(d) 0

(e) Cation

(f) Anion

Question 29. Assertion (A) : Among halogens fluorine is the best oxidant.

Reason (R) : Fluorine is the most electronegative atom.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

Question 30. Assertion (A): In the reaction between potassium permanganate and potassium iodide, permanganate ions act as oxidising agent.

Reason (R) : Oxidation state of manganese changes from +2 to +7 during the reaction.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

Question 31. Assertion (A) : The decomposition of hydrogen peroxide to form water and oxygen is an example of disproportionation reaction.

Reason (R) : The oxygen of peroxide is in –1 oxidation state and it is converted to zero oxidation state in $O_2$ and –2 oxidation state in $H_2O$.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

Question 32. Assertion (A) : Redox couple is the combination of oxidised and reduced form of a substance involved in an oxidation or reduction half cell.

Reason (R) : In the representation $Fe^{3+}/Fe^{2+}$ and $Cu^{2+}/Cu$, $Fe^{3+}/Fe^{2+}$ and $Cu^{2+}/Cu$ are redox couples.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

Question 33. Explain redox reactions on the basis of electron transfer. Give suitable examples.

Question 34. On the basis of standard electrode potential values, suggest which of the following reactions would take place? (Consult the book for $E^\ominus$ value).

(i) $Cu + Zn^{2+} \rightarrow Cu^{2+} + Zn$

(ii) $Mg + Fe^{2+} \rightarrow Mg^{2+} + Fe$

(iii) $Br_2 + 2Cl^– \rightarrow Cl_2 + 2Br^–$

(iv) $Fe + Cd^{2+} \rightarrow Cd + Fe^{2+}$

Question 35. Why does fluorine not show disporportionation reaction?

Question 36. Write redox couples involved in the reactions (i) to (iv) given in question 34.

Question 37. Find out the oxidation number of chlorine in the following compounds and arrange them in increasing order of oxidation number of chlorine.

$NaClO_4, NaClO_3, NaClO, KClO_2, Cl_2O_7, ClO_3, Cl_2O, NaCl, Cl_2, ClO_2$.

Which oxidation state is not present in any of the above compounds?

Question 38. Which method can be used to find out strength of reductant/oxidant in a solution? Explain with an example.