Chromium A case study. Chromium A case study. Electrolytic chromium-plating

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1 H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Chromium H A case study He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Fr Ba Ra La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Inorganic Chemistry 1 1 Chromium A case study Element no. 24 of the periodic table Atomic symbol Cr Chromium from colour κρωμα: skin, colour d-block metal Many co-ordination compounds Appears most often as Cr(III) [CrCl 2(H 2O) 4]Cl 2H 2O Inorganic Chemistry 2 Electrolytic chromium-plating In acidic chromate(vi)-baths chromium metal is deposited on the cathode CrO 3 +6H + + 6e - Cr + 3H 2 O Inorganic Chemistry 3 1

2 Chromium ore Chromite (FeCr 2 O 4 ) - large deposits in South Africa Inorganic Chemistry 4 Isolation of chromium metal FeCr 2 O 4 +NaOH + O 2 Na 2 CrO 4 + Fe 2 O 3 +H 2 O Na 2 CrO 4 (aq) + C Cr 2 O 3 + Na 2 CO 3 (aq) Cr 2 O 3 + Al Cr + Al 2 O Inorganic Chemistry 5 Chromium nuggets Hard, gray metal Density 7.2 g ml -1 Melting point 1890 o C Corrosion resistant Inorganic Chemistry 6 2

3 Metal + acid Cr + 2H + Cr aq 2+ + H 2 (g) E o (Cr 2+ /Cr) = -0.91V light blue solution in hydrochloric acid 2Cr aq H + 2Cr aq + H 2 (g) E o (Cr /Cr 2+ ) = -0.41V green solution in hydrochloric acid green solution:(λ max = 635 nm ; 445 nm) Inorganic Chemistry 7 [CrCl 2 (H 2 O) 4 ]Cl 2H 2 O Composition of green chromium(iii) chloride CrCl 3 (H 2 O) 6 1. Chromium analysis 2Cr aq + 3S 2 O 8 + 7H 2 O Cr 2 O 7 + 6HSO H + Excess 2S 2 O 8 + 2H 2 O heated 4HSO 4- + O 2 Cr 2 O 7 + 6Fe H + 2Cr + 6Fe + 7H 2 O Excess Fe 2+ determined by standard Cr 2 O Inorganic Chemistry 8 [CrCl 2 (H 2 O) 4 ]Cl 2H 2 O Composition of green chromium(iii) chloride 2. Chloride analysis a. Dissolution of solid in nitric acid and addition of silver nitrate; weighing dry silver chloride : M 1 =m 1 /n 1 b. Dissolution of solid in nitric acid, boiling of solution and addition of silver nitrate; weighing dry silver chloride M 2 =m 2 /n 2 M 1 =3 M Inorganic Chemistry 9 3

4 [CrCl 2 (H 2 O) 4 ]Cl 2H 2 O Composition of green chromium(iii) chloride 3. Thermogravitric analysis Loss of mass at 10X o C Water: n(h 2 O)/n(Cr) = 2.0 Stoichiometry: CrCl a 2Cl b (H 2 O a ) 4 (H 2 O b ) 2 4 water molecules and 2 Cl - mark stronger bonds to Cr than 1 Cl - and 2 water molecules Inorganic Chemistry 10 Safety: Hazard symbols for K 2 Cr 2 O 7 New symbols in EU since 2010 Old symbols (UK, DK) Inorganic Chemistry 11 [CrCl 2 (H 2 O) 4 ]Cl 2H 2 O Stability Cr(H 2 O) 6 + ncl - Cr(H 2 O) 6-n Cl n (3-n)+ logk 1 = -1.20, logβ 2 = -3.27, log β 3 = (logk 2 = -2.07, logk 3 = -3.16) Kinetics slow Cr(H 2 O) 6 + H 2 O Cr(H 2 O) 5 H 2 O + H 2 O k = ~10-5 sec Inorganic Chemistry 12 4

5 Synthesis of new compound from green chromium(iii) chloride green solution in DMSO destillation of water + DMSO azeotropic mixture to give a violet solution of [CrCl 3 (DMSO) 3 ] added to an excess liquid 1,ethanediamine gives a brown mass mixture dissolves in dilute hydrochloric acid with a yellowish brown colour Inorganic Chemistry 13 New yellow compound Recrystallised from dilute HCl (separation by filtration) 1,ethanediamine abbreviated as en Formula: [Cren 3 ]Cl 3 3H 2 O Name: tris(1,ethanediamine)chromium(iii) chloride trihydrate Two properties Light sensitive (darkens and degrades) Soluble in water Inorganic Chemistry 14 Absorption spectrum aqueous solution tris(1, ethanediamine)chromium(iii) Molar absorptivity Wavelength (λ, ε) max = (457, 75.0), (351, 60.5) og (λ, ε) min = (394, 14.5) Inorganic Chemistry 15 5

6 Octahedral Geometry A regular octahedron with Cr(III) ion (the central atom) in the centre and Three 1,ethanediamines as bidentate ligands 6 N-atoms in the 6 corners of the octahedron with equal distances from the centre Inorganic Chemistry 16 Ligand field model ν 1 ( 4 A 2g (F) 4 T 2g (F)= cm-1 ν 2 ( 4 A 2g (F) 4 T 1g (F)= cm-1 =10Dq = cm -1 B = 650 cm Inorganic Chemistry 17 Symmetry Inorganic Chemistry 18 6

7 Quantum chemical calculation Inorganic Chemistry 19 Thermodynamics Equilibrium: [Cr(H 2 O) 6 ] + 3en [Cren 3 ] + 6H 2 O ] [ Cren 3 [ Cr( H O) ] [ en] 3 logβ 3 = 19.4 (24 o C; 1.00M NaCl) ; logk 1 = Inorganic Chemistry 20 Other properties Magnetic behaviour Crystal form Distances and angles between atoms IR spectrum Reactivity Redox behaviour and so on Inorganic Chemistry 21 7