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

2. d-Block Transition ElementsSc 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

3. Energy Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1 4p 3d 4s 3p 3s 2p 2s 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. 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

8. 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

9. 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

10. 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.

11. 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;

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

13. 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)

14. 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)

15. 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)

16. 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)

17. CHROMIUM(VI) oxidation state chemistry
Dichromate and chromate equilibria is pH dependent:
HCrO4- → CrO42- + H+     K= H2CrO4 → HCrO4- + H+     K= Cr2O72- + H2O → 2HCrO4-     K= HCr2O7- → Cr2O72- + H+     K=
Hence the variation found for solutions of CrO3 are:
pH > 8     CrO42- yellow pH 2-6     HCrO4- and Cr2O72- orange-red pH < 1     H2Cr2O7

18. 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

19. Chromium usage

20. 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.

21. 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.

22. Source of chromium

23. 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.

24. 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.

25. 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.