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

d-Block Transition Elements 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 IIIB IVB VB VIB VIIB IB IIB VIIIB

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

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

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.

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",

Reactivity of chromium 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

Reactivity of chromium and elements of its group towards acids It dissolves readily in dil HCl 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

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

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.

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;

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

CHROMIUM(VI) oxidation state chemistry When hydrogen peroxide is added to an alkaline chromium(III) solution, oxidation occurs to give the yellow chromate(VI) ion CrO42- .  2Cr3+(aq) + 3H2O2(aq) + 10OH-(aq) ==> 2CrO42-(aq) + 8H2O(l) Both H2O2 and Cr(VI) compounds are oxidising agents but in alkaline solution H2O2 is the stronger oxidising agent. When the resulting solution from above is acidified with dilute sulphuric acid, the orange dichromate(VI) ion Cr2O72-  is formed. 2CrO42-(aq) + 2H+(aq) Cr2O72-(aq) + H2O(l) (no change in ox. state)

CHROMIUM(VI) oxidation state chemistry 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)

Potassium dichromate(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)

Potassium dichromate(VI), K2Cr2O7 The dichromate(VI) ion is a strong oxidising agent - examples of oxidising action. It oxidises iodide ions to iodine. Cr2O72-(aq) + 14H+(aq) + 6I-(aq) ==> 2Cr3+(aq) + 3I2(aq) + 7H2O(l) The released iodine can be titrated with standard sodium thiosulphate solution using starch indicator. 2S2O32-(aq)  +  I2(aq)  ==>  S4O62-(aq) + 2I-(aq) (black/brown ==> colourless endpoint)

CHROMIUM(VI) oxidation state chemistry Dichromate and chromate equilibria is pH dependent: HCrO4- → CrO42- + H+     K=10-5.9 H2CrO4 → HCrO4- + H+     K=10+0.26 Cr2O72- + H2O → 2HCrO4-     K=10-2.2 HCr2O7- → 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

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

Chromium usage

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.

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 Complex influences 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.

Source of chromium

Biological role of Manganese Manganese is required for the metabolism of proteins and fats.  A partial list of manganese-dependent enzyme families includes oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.  Manganese is involved in the function of numerous organ systems and is needed for normal immune function, regulation of blood sugars, production of cellular energy, reproduction, digestion, and bone growth.  Manganese works with vitamin K to support clotting of the blood.   The National Research Council has recommended an Estimated Safe and Adequate Daily Dietary Intake (ESADDI) for Mn of 2 to 5 mg per day for adults.

Manganese in pharmacy Manganese has been examined as a treatment for a variety of conditions, including osteoarthritis and wound healing. However, manganese is often used in combination with other vitamins and/or minerals. Therefore, the effects of manganese alone are difficult to determine. Manganese in combination with calcium and zinc may be beneficial in patients with chronic wounds. A combination of calcium and manganese may alleviate symptoms associated with premenstrual syndrome.

Source of manganese Excellent food sources of manganese include mustard greens, kale, chard, raspberries, pineapple, strawberries, romaine lettuce, collard greens, spinach, garlic, summer squash, grapes, turnip greens, eggplant, brown rice, blackstrap molasses, maple syrup, cloves, cinnamon, thyme, black pepper, and turmeric.