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

ScTiVCr Mn FeCoNiCuZn YZrNb MoMoTcRuRhPdAgCd LaHfTaWReOsIrPtAuHg IIIBIVBVBVIBVIIBIBIIB VIIIB d-Block Transition Elements

SS CI 11.5 The d block3 1s 2s 3s 4s 2p 3p 3d Cr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 4p

Extraction of chromium Chromite, FeCr 2 O 4, 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. Cr 2 O 3(s) + 2Al (s) ==> Al 2 O 3(s) + 2Cr (s) 2Cr 2 O 3 + 3Si → 4Cr + 3SiO 2

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 CrX 3 are prepared from Cr with X 2, dehydration of CrCl 3.6H 2 O requires SOCl 2 at 650C. Cr (s) + Cl 2 (g) → CrCl 3 (s) Reduction of CrX 3 with H 2 /HX gives CrX 2. anhydrous CrCl 3 and hydrated "CrCl 3.6H 2 O",

Reactivity of chromium towards Chromium reacts with oxygen to form oxides 2Cr + 3О 2  Cr 2 О 3 (400 oC) 2Cr + 3Н 2 О  Cr 2 О 3 + 3Н 2  (600 oC) (NH 4 ) 2 Cr 2 O 7  Cr 2 O 3 + N 2 + 4H 2 O Cr(s) + S8(s) → Cr2S3(s) Nitrides: CrN, Cr 2 N; Carbydes: Cr 4 С, Cr 7 С 3, Cr 3 С 2 Intermetalides: FeCr 2, CrMn 3 oxygen and preparation chromium oxides non-metals and metals

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

Reactivity of chromium and elements of its group towards acids It dissolves readily in dil HCl but, if very pure, will often resist dil H 2 SO 4 ; again, HNO 3, whether dilute or concentrated. Cr + 2HCl  2CrCl 2 + H 2  2CrCl 2 + 1/2O 2 + 2HCl  2CrCl 3 + H 2 O. 2Сr + 6H 2 SO 4 (d)  Cr 2 (SO 4 ) 3 + 3SO 2 + 6H 2 O Cr + HNO 3  no reaction

Decomposition of chromium compounds (NH 4 ) 2 Cr 2 O 7  Cr 2 O 3 + N 2 + 4H 2 O 4Na 2 Cr 2 O 7  4Na 2 CrO 4 + 2Cr 2 O 3 + 3O 2 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 Cr 2 O 3 + 2Al  Al 2 O 3 + 2Cr Cr 2 O 3 + 3K 2 S 2 O 7  Cr 2 (SO 4 ) 3 + 3K 2 SO 4 ; Cr 2 O 3 + 3NaNO 3 + 2Na 2 CO 3  2Na 2 CrO 4 + 3NaNO 2 + 2CO 2 ; 5Cr 2 O 3 + 6NaBrO NaOH  10Na 2 CrO 4 + 3Br 2 + 7H 2 O;

Cr(OH) 3 Is Amphoteric In acidIn 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 CrO – 2Cr 3+ (aq) + 3H 2 O 2(aq) + 10OH - (aq) ==> 2CrO 4 2- (aq) + 8H 2 O (l) – Both H 2 O 2 and Cr(VI) compounds are oxidising agents but in alkaline solution H 2 O 2 is the stronger oxidising agent. When the resulting solution from above is acidified with dilute sulphuric acid, the orange dichromate(VI) ion Cr 2 O 7 2- is formed. 2CrO 4 2- (aq) + 2H + (aq) Cr 2 O 7 2- (aq) + H 2 O (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) Cr 2 O 7 2- (aq) + 14H + (aq) + 6e - 2Cr 3+ (aq) + 7H 2 O (l) orange (+6) ==> green (+3), E Ø = +1.33V – Cr(III, +3) ==> Cr(II, +2): Cr 3+ (aq) + e - Cr 2+ (aq) green (+3) ==> blue (+2), so Cr(II) is readily oxidised. Cr 2 O 7 2- (aq) + 3Zn (s) + 14H + (aq) 2Cr 3+ (aq) + 3Zn 2+ (aq) + 7H 2 O (l) 2Cr 3+ (aq) + Zn (s) 2Cr 2+ (aq) + Zn 2+ (aq)

Potassium dichromate(VI), K 2 Cr 2 O 7 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). Cr 2 O 7 2- (aq) + 14H + (aq) + 6Fe 2+ (aq) ==> 2Cr 3+ (aq) + 6Fe 3+ (aq) + 7H 2 O (l)

Potassium dichromate(VI), K 2 Cr 2 O 7 The dichromate(VI) ion is a strong oxidising agent - examples of oxidising action. It oxidises iodide ions to iodine. Cr 2 O 7 2- (aq) + 14H + (aq) + 6I - (aq) ==> 2Cr 3+ (aq) + 3I 2(aq) + 7H 2 O (l) – The released iodine can be titrated with standard sodium thiosulphate solution using starch indicator. 2S 2 O 3 2- (aq) + I 2(aq) ==> S 4 O 6 2- (aq) + 2I - (aq) (black/brown ==> colourless endpoint)

CHROMIUM(VI) oxidation state chemistry Dichromate and chromate equilibria is pH dependent: HCrO 4 - → CrO H + K= H 2 CrO 4 → HCrO H + K= Cr 2 O H 2 O → 2HCrO 4 - K= HCr 2 O 7 - → Cr 2 O H + K= Hence the variation found for solutions of CrO 3 are: pH > 8 CrO 4 2- yellow pH 2-6 HCrO 4 - and Cr 2 O 7 2- orange-red pH < 1 H 2 Cr 2 O 7

Test reaction for Chromium Cr 2 (SO 4 ) 3 + 3(NH 4 ) 2 S + 6H 2 O  2Cr(OH) 3  + 3(NH 4 ) 2 SO 4 + 3H 2 O – grey-green ppt; 3Na 2 CO 3 + 2CrCl 3 + 3H 2 O  2Cr(OH) 3  + 3CO 2 + 6NaCl Na 2 CrO 4 + AgNO 3  Ag 2 CrO 4  + 2NaNO 3 – brown-reddish ppt; Na 2 CrO 4 + (CH 3 COO) 2 Pb  PbCrO 4  + 2CH 3 COONa – 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