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

PhD. Halina Falfushynska

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 IIBVIIIB

d-Block Transition Elements

SS CI 11.5 The d block 3

1s

2s

3s

4s

2p

3p

3d

Cr

1s2 2s2 2p6 3s2 3p6 3d5

4s1

4p

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

Chromium reacts with oxygen to form oxides2Cr + 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

oxygen and preparation chromium oxides

non-metals and metals

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 + H22CrCl2 + 1/2O2 + 2HCl 2CrCl3 + H2O.2Сr + 6H2SO4 (d) Cr2(SO4)3 + 3SO2 + 6H2OCr + 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 ExampleWrite 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;

Cr(OH)3 Is Amphoteric

In acid In base

Chromate and Dichromate Ions

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 CrO4

2- . – 2Cr3+

(aq) + 3H2O2(aq) + 10OH-(aq) ==> 2CrO4

2-(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 Cr2O7

2- is formed.• 2CrO4

2-(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) Cr2O7

2-(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.Cr2O7

2-(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 Cr2O7

2- 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 usageChromium 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 chromiumSource of chromium

Biological role of ManganeseBiological 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 pharmacyManganese 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 manganeseSource 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.