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Lecture 19. The d -Block Elements. IV-VI B groups

Lecture 19. The d -Block Elements. IV-VI B groups. PhD. Halina Falfushynska. VIIIB. IIIB. IVB. VB. VIB. VIIB. IB. IIB. Sc. Ti. V. Cr. Mn. Fe. Co. Ni. Cu. Zn. Y. Zr. Nb. M o. Tc. Ru. Rh. Pd. Ag. Cd. La. Hf. Ta. W. Re. Os. Ir. Pt. Au. Hg.

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Lecture 19. The d -Block Elements. IV-VI B groups

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  1. Lecture 19. The d-Block Elements.IV-VI B groups PhD. Halina Falfushynska

  2. VIIIB IIIB IVB VB VIB VIIB IB IIB Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag Cd La Hf Ta W Re Os Ir Pt Au Hg d-Block Transition Elements

  3. Energy 4p 3d 4s 3p 3s 2p 2s Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1 1s SS CI 11.5 The d block

  4. Extraction of chromium • Chromite, FeCr2O4, is the most commercially useful ore. • Chromium ore is processed and purified into chromium(III) oxide. This is reacted, very exothermically, in a thermit style reaction, with aluminium or silicon to free the chromium metal. • Cr2O3(s) + 2Al(s) ==> Al2O3(s) + 2Cr(s) • 2Cr2O3 + 3Si → 4Cr + 3SiO2

  5. Chemical reactivity and trends • Chromium reacts more readily with acids then does either molybdenum or tungsten though its reactivity depends on its purity and it can easily be rendered passive.

  6. Reactivity towards halogens CrX3 are prepared from Cr with X2, dehydration of CrCl3.6H2O requires SOCl2 at 650C. Cr (s) + Cl2 (g) → CrCl3 (s) Reduction of CrX3 with H2/HX gives CrX2. anhydrous CrCl3 and hydrated "CrCl3.6H2O",

  7. Reactivityofchromium towards oxygen and preparation chromium oxides non-metals and metals Chromium reacts with oxygen to form oxides 2Cr + 3О2 Cr2О3(400 oC) 2Cr + 3Н2О  Cr2О3 + 3Н2 (600 oC) (NH4)2Cr2O7 Cr2O3 + N2 + 4H2O Cr(s) + S8(s) → Cr2S3(s) Nitrides: CrN, Cr2N; Carbydes: Cr4С, Cr7С3, Cr3С2 Intermetalides: FeCr2, CrMn3

  8. Pb(NO3)2 and K2Cr4 reaction Sodium Hydroxide To Chromic Nitrate

  9. Reactivityofchromium and elements of its group towardsacids It dissolves readily in dilHCl but, if very pure, will often resist dil H2SO4; again, HNO3, whether dilute or concentrated. Cr + 2HCl  2CrCl2 + H2 2CrCl2 + 1/2O2 + 2HCl  2CrCl3 + H2O. 2Сr + 6H2SO4(d) Cr2(SO4)3 + 3SO2 + 6H2O Cr + HNO3 no reaction

  10. Decomposition of chromium compounds • (NH4)2Cr2O7 Cr2O3 + N2 + 4H2O • 4Na2Cr2O7 4Na2CrO4 + 2Cr2O3 + 3O2 • Cr2(CO3)3 + heat  Cr2O3 + CO2 • Cr2(NO3)3 Cr2O3 + NO2 + O2

  11. A Conceptual Example Write a plausible equation to explain the reaction shown in Figure, in which pure ammonium dichromate ignited with a match produces pure chromium(III) oxide.

  12. Chromium oxides properties • Cr2O3 + 2Al  Al2O3 + 2Cr • Cr2O3 + 3K2S2O7 Cr2(SO4)3 + 3K2SO4; • Cr2O3 + 3NaNO3 + 2Na2CO3 2Na2CrO4 + 3NaNO2 + 2CO2; • 5Cr2O3 + 6NaBrO3 + 10NaOH  10Na2CrO4 + 3Br2 + 7H2O;

  13. Chromate and Dichromate Ions In acid In base Cr(OH)3 Is Amphoteric

  14. CHROMIUM(VI) oxidationstatechemistry • Whenhydrogenperoxideisaddedtoanalkalinechromium(III) solution, oxidationoccurstogivetheyellowchromate(VI) ion CrO42- .  • 2Cr3+(aq) + 3H2O2(aq) + 10OH-(aq)==> 2CrO42-(aq) + 8H2O(l) • Both H2O2andCr(VI) compoundsareoxidisingagentsbutinalkalinesolution H2O2isthestrongeroxidisingagent. • Whentheresultingsolutionfromaboveisacidifiedwithdilutesulphuricacid, theorangedichromate(VI) ion Cr2O72-  isformed. • 2CrO42-(aq) + 2H+(aq) Cr2O72-(aq) + H2O(l) (nochangeinox. state)

  15. CHROMIUM(VI) oxidationstatechemistry • The dichromate(VI) ion is reduced in two stages by a zinc/dilute sulphuric acid mixture.Cr(VI, +6) ==> Cr(III, +3) Cr2O72-(aq) + 14H+(aq) + 6e- 2Cr3+(aq) + 7H2O(l) • orange (+6) ==> green (+3),  EØ = +1.33V  • Cr(III, +3) ==> Cr(II, +2): Cr3+(aq) + e- Cr2+(aq) • green (+3) ==> blue (+2), so Cr(II) is readily oxidised. Cr2O72-(aq) + 3Zn(s) + 14H+(aq) 2Cr3+(aq) + 3Zn2+(aq) + 7H2O(l) 2Cr3+(aq) + Zn(s) 2Cr2+(aq) + Zn2+(aq)

  16. Potassiumdichromate(VI), K2Cr2O7 • It can be crystallised to high purity standard without water of crystallisation, and is a valuable 'standard' redox volumetric reagent. • It can used to titrate iron(II) ions in solution acidified with dilute sulphuric acid, using a redox indicator like barium diphenylamine sulphonate(blue colour). Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq)==> 2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l)

  17. Potassium dichromate(VI), K2Cr2O7 Thedichromate(VI) ionisastrongoxidisingagent - examplesofoxidisingaction. Itoxidisesiodideionstoiodine. • Cr2O72-(aq) + 14H+(aq) + 6I-(aq)==> 2Cr3+(aq) + 3I2(aq) + 7H2O(l) • Thereleasediodinecanbetitratedwithstandardsodiumthiosulphatesolutionusingstarchindicator. • 2S2O32-(aq)  +  I2(aq)  ==> S4O62-(aq) + 2I-(aq) (black/brown ==> colourlessendpoint)

  18. CHROMIUM(VI) oxidationstatechemistry • Dichromate and chromate equilibria is pH dependent: • HCrO4- → CrO42- + H+     K=10-5.9H2CrO4 → HCrO4- + H+     K=10+0.26Cr2O72- + H2O → 2HCrO4-     K=10-2.2HCr2O7- → Cr2O72- + H+     K=10+0.85 • Hence the variation found for solutions of CrO3 are: • pH > 8     CrO42- yellow pH 2-6     HCrO4- and Cr2O72- orange-red pH < 1     H2Cr2O7

  19. Test reaction for Chromium • Cr2(SO4)3 + 3(NH4)2S + 6H2O 2Cr(OH)3+ 3(NH4)2SO4 + 3H2O – grey-green ppt; • 3Na2CO3 + 2CrCl3 + 3H2O 2Cr(OH)3 + 3CO2 + 6NaCl • Na2CrO4 + AgNO3Ag2CrO4+ 2NaNO3 – brown-reddish ppt; • Na2CrO4 + (CH3COO)2Pb PbCrO4 + 2CH3COONa – yellow ppt

  20. Chromium usage

  21. Biological role of chromium • Chromium is an essential trace element in mammalian metabolism. In addition to insulin, it is responsible for reducing blood glucose levels, and is used to control certain cases of diabetes. It has also been found to reduce blood cholesterol levels by diminishing the concentration of (bad) low density lipoproteins "LDLs" in the blood.

  22. Biological role of chromium and its uses in pharmacy • Chromium is an essential trace element. Chromium plays a role in the metabolism of glucose, and is necessary for energy production • Chromium picolinate or Chromium Nicotinate Complexinfluences carbohydrate, fat and protein metabolism. It assists in the management of fluctuating blood sugar levels in healthy people. A higher dietary intake of refined carbohydrates needs more chromium.

  23. Source of chromium

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