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Chemistry-Mojo, Question Practice, , Simplifying Chemistry, , d-Block Elements Q : Assignment 2, , 1. Why do transition elements show variable oxidation states? (Comptt. Delhi 2014), 2. Write the formula of an oxo-anion of Manganese (Mn) in which it shows the oxidation, state equal to its group number. (Delhi 2017), 3. What happens when (NH4)2Cr2O7 is heated?, 4. Write the formula of an oxo-anion of Chromium (Cr) in which it shows the oxidation state, equal to its group number. (Delhi 2017), 5. Explain the following observations :, (i) Generally there is an increase in density of elements from titanium (Z = 22) to copper (Z =, 29) in the first series of transition elements., (ii) Transition elements and their compounds are generally found to be good catalysts in, chemical reactions. (Delhi 2010), 6. Explain the following observations :, (i) Transition elements generally form coloured compounds., (ii) Zinc is not regarded as a transition element., 7. Assign reasons for the following :, Copper (I) ion is not known in aqueous solution., Ans. Cu2+(aq) is much more stable than Cu+(aq). This is because although second ionization, enthalpy of copper is large but Δhyd (hydration enthalpy) for Cu2+(aq) is much more negative, than that for Cu+(aq) and hence it more than compensates for the second ionization enthalpy, of copper. Therefore, many copper (I) compounds are unstable in aqueous solution and, undergo disproportionation as follows :, 2Cu+ → Cu2+ + Cu, 8. Assign reasons for each of the following :, (i) Transition metals generally form coloured compounds., (ii) Manganese exhibits the highest oxidation state of +7 among the 3d series of transition, elements. (Delhi 2011), 9. How would you account for the following :, (i) Cr2+ is reducing in nature while with the same d-orbital configuration (d4) Mn3+ is an, oxidising agent., (ii) In a transition series of metals, the metal, which exhibits the greatest number of oxidation states occurs in the middle of the series. (All, India 2011), , Tel +91 84330 83279, 98376 03674., , Page 1
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Chemistry-Mojo, Question Practice, , Simplifying Chemistry, , 10. State reasons for the following :, (i) Cu (I) ion is not stable in an aqueous solution., (ii) Unlike Cr3+, Mn2+, Fe3+ and the subsequent other M2+ ions of the 3d series of elements,, the 4d and the 5d series metals generally do not form stable cationic species. (All India 2011), Ans (ii) Because high enthalpies of atomisation of 4d and 5d series and high ionization, enthalpies, the M.P. and B.P. of heavier transition elements are greater than those of first, transition series which is due to stronger intermetallic bonding. Hence 4d and 5d series, metals generally do not form stable cationic species., 11. Explain giving a suitable reason for each of the following :, (i) Transition metals and their compounds are generally found to be good catalysts., (ii) Metal-metal bonding is more frequent for the 4d and the 5d series of transition metals, than that for the 3d series. (All India 2011), Ans. (ii) Metal-metal bonding is more frequent for the 4d and the 5d series of transition, metals than that for the 3d series as these have their electrons of outer most shell at greater, distance from the nucleus, as compared to atoms of 3d transition metals., 12. Explain giving reason why transition metals and their compounds generally exhibit a, paramagnetic behaviour., 13. Explain the following :, (a) The enthalpies of atomization of transition metals are quite high., (b) The transition metals and many of their compounds act as good catalysts. (Comptt. Delhi, 2012), 14. (a) Which metal in the first transition series (3d series) exhibits +1 oxidation state most, frequency and why?, (b) Which of the following cations are coloured in aqueous solutions and why?, Sc3+, V3+, Ti4+, Mn2+., (At. nos. Sc = 21, V = 23, Ti = 22, Mn = 25) (Delhi 2013), 15. Assign a reason for each of the following observations:, (i) The transition metals (with the exception of Zn, Cd and Hg) are hard and have high, melting and boiling points., (ii) The ionization enthalpies (first and second) in the first series of the transition elements are, found to vary irregularly. (Comptt. Delhi 2014), Ans. (i) Because of stronger metallic bonding and high enthalpies of atomization., (ii) Due to irregularities in the electronic configuration there is irregularities in the enthalpies, of atomisation. Hence there is irregular variation in I.E., 16. How would you account for the following?, The highest oxidation state of a transition metal is usually exhibited in its oxide & fluorides., , Tel +91 84330 83279, 98376 03674., , Page 2
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Chemistry-Mojo, Question Practice, , Simplifying Chemistry, , 17. Assign reason for each of the following :, (i) Transition elements exhibit paramagnetic behaviour., (ii) Co2+. is easily oxidised in the presence of a strong ligand. (Comptt. Delhi 2014), Ans., (ii) Co2+ ion is easily oxidised to Co3+ ion in presence of a strong ligand because of its higher, crystal field energy which causes pairing of electrons to give inner orbital complexes (d2sp3)., 18. Describe the general trends in the following property of the first series (3d) of the, transition elements : Number of oxidation states exhibited., 19. Assign reasons for the following :, (i) Copper(I) ion is not known to exist in aqueous solutions., (ii) Both O2 and F2 stabilize high oxidation states of transition metals but the ability of, oxygen to do so exceeds that of fluorine. (Comptt. All India 2014), 20. Assign reasons for the following :, (i) Transition metals and many of their compounds act as good catalysts., (ii) Transition metals generally form coloured compounds. (Comptt. All India 2014), Answer:, (i) The catalytic properties of the transition elements are due to the presence of unpaired, electrons in their incomplete d-orbitals and variable oxidation states., (ii) Because of presence of unpaired electrons in d-orbital, which undergoes d-d transition by, absorption of energy from visible region and then the emitted light shows complementary, colours., 21. What are the transition elements? Write two characteristics of the transition elements., (Delhi 2015), 22. Suggest reasons for the following features of transition metal chemistry :, (i) The transition metals and their compounds are usually paramagnetic., (ii) The transition metals exhibit variable oxidation states. (Comptt. Delhi 2015), Ans. (ii) The transition metals exhibit variable oxidation states because of very close energies, of incompletely filled (n – l)d orbitals and ns orbitals due to which both can participate in, bonding., 23. Give reasons :, (i) Zn is not regarded as a transition element., (ii) Cr2+ is a strong reducing agent. (Comptt All India 2016), Ans. (ii) Cr+2 (3d4) after loosing one electron forms Cr+3 (d3). It has t32ge0g configuration., Hence more stable. Hence Cr+2 acts as reducing agent., 24. Explain the following observations :, (i) Copper atom has completely filled d orbitals (3d10) in its ground state, yet it is regarded as, Tel +91 84330 83279, 98376 03674., , Page 3
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Chemistry-Mojo, Question Practice, , Simplifying Chemistry, , a transition element., (ii) Cr2+ is a stronger reducing agent than Fe2+ in aqueous solutions. (Comptt. All India 2017), Ans., (i) Copper atom has completely filled d orbitals (3d10) in its ground state, yet it is regarded as, a transition element due to incompletely filled d-orbital in its ionic states i.e. Cu2+ (3d9)., (ii) The highest oxidation state for Cr is +6, therefore it can loose 3 more electrons, whereas, Fe needs to loose only 1 electron to achieve its highest oxidation state of +3. Thus, Cr3+ is, more reducing than Fe2+., 25. Explain the following observations :, (a) Silver atom has completely filled d-orbitals (4d10) in its ground state, yet it is regarded as, a transition element., (b) E0 value for Mn3+ /Mn2+ couple is much more positive than Cr3+/Cr2+. (Comptt. All India, 2017), Ans. (a) Because silver has incomplete d-orbital (4d9) in its +2 oxidation state (seen in rare, compounds), hence it is a transition element., (b) The large positive E° value for Mn3+/Mn2+ shows that Mn2+ is much more stable than, Mn+3 due to stable half filled configuration (3d5). Therefore the 3rd ionisation energy of Mn, will be very high and Mn3+ is unstable and can be easily reduced to Mn2+. E° value for Cr3+ |, Cr2+ is positive but small i.e. Cr3+ can also be reduced to Cr2+ but less easily. Thus Cr3+ is, more stable than Mn3+., 26. Explain the following observations :, (i) Zn2+ salts are colourless., (ii) Copper has exceptionally positive Eº M2+/M value. (Comptt. All India 2017), Ans. (ii) Copper has high enthalpy of atomization (i.e. energy absorbed and low enthalpy of, hydration (i.e. energy released). Hence E0M2+/M for copper is positive. The high energy, required to transform Cu(s) to Cu2+(aq) is not balanced by its hydration enthalpy., 27. How would you account for the following :, (i) Many of the transition elements and their compounds can act as good catalysts., (ii) The metallic radii of the third (5d) series of transition elements are virtually the same as, those of the corresponding members of the second series., 28. How would you account for the following?, (i) With the same d-orbital configuration (d4) Cr2+ is a reducing agent while Mn3+ is an, oxidizing agent., (ii) Most of the transition metal ions exhibit characteristic in colours in aqueous solutions., (Delhi 2012), 29. Explain the following observations giving an appropriate reason for each., (i) The enthalpies of atomization of transition elements are quite high., (ii) There occurs much more frequent metal- metal bonding in compounds of heavy transition, Tel +91 84330 83279, 98376 03674., , Page 4
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Chemistry-Mojo, Question Practice, , Simplifying Chemistry, , metals (i.e. 3rd series)., (iii) Mn2+ is much more resistant than Fe2+ towards oxidation. (Delhi 2012), Ans. (i) In transition elements, there are large number of unpaired electrons in their atoms,, thus they have a stronger inter atomic interaction and thereby stronger bonding between the, atoms. Due to this they have high enthalpies of atomization., (ii) Because of high enthalpy of atomisation of 3rd series, there occurs much more frequent, metal-metal bonding in compounds of heavy transition metals., (iii) The 3d orbital in Mn2+ is half-filled and is more stable compared to Fe2+ has 6 electrons, in the 3d orbital. Mn2+ prefer to lose an electron or get oxidised whereas Fe2+ will readily, loose one electron or get oxidised. Therefore, Mn2+ is much more resistant than Fe2+ towards, oxidation., 30. Explain why :, (i) There is a general increase in density from titanium (Z = 22) to copper (Z = 29)., (ii) With the same d-orbital configuration (d4), Cr2+ is a reducing agent while Mn3+ is an, oxidising agent., (iii) There is hardly any increase in atomic size with increasing atomic numbers in a series of, transition metals. (All India 2012), Ans. (ii) Cr+2 is reducing agent as its configuration changes from d4 to d3 which is having half, filled t2g level whereas Mn+3 to Mn+2 results in stable half filled d5 configuration hence it is, oxidising., (iii) Along a transition series, the nuclear charge increases which tends to decrease the size, but the addition of electrons in the penultimate d-subshell increases the screening effect, which counter balances the effect of increased nuclear charge. Thus, atomic radius does not, change much., , Tel +91 84330 83279, 98376 03674., , Page 5
