* Advanced Inorganic Chemistry Transition metals 10.6 Chromium Chemistry Doc Brown's

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 Doc Brown's Chemistry  Periodic Table Revision Notes 10.6

Part 10. Transition Metals 3d-block:  10.6 Chromium Chemistry

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses)  GCSE Periodic Table * GCSE notes Transition Metals

INORGANIC Part 10 3d block TRANSITION METALS sub-index: 10.1-10.2 Introduction 3d-block Transition Metals * 10.3 Scandium * 10.4 Titanium * 10.5 Vanadium * 10.6 Chromium * 10.7 Manganese * 10.8 Iron * 10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc * 10.13 Other Transition Metals e.g. Ag and Pt * Appendix 1. Hydrated salts, acidity of hexa-aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, Eø * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block - extended data * Appendix 11 Some 3d-block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots * All 11 Parts have their own sub-indexes near the top of the pages

10.6. Chemistry of Chromium Cr, Z=24, 1s22s22p63s23p63d54s1 

  • Cr data table 1 summary * extended chromium data table 2 * Chromium & electrode potential chart of 3d-block

  • Summary of some complexes-compounds & oxidation states of chromium compared to other 3d-block elements

  • Chromium is a hard white metal that is extremely resistant to chemical attack at room temperature. It is used in the production of extremely hard steel alloys e.g. ball bearings and coating steel articles for corrosion protection ('chrome plating') and chromium metal is an important component in 'stainless steel'.

  • Extraction of chromium

    • Chromium ore is processed and purified into chromium(III) oxide. This is reacted, very exothermically, in a thermit style reaction, with aluminium (see reactions of aluminium) to free the chromium metal.

    • Cr2O3(s) + 2Al(s) ==> Al2O3(s) + 2Cr(s) 

    • The chromium(III) oxide is reduced to chromium by O loss, the aluminium is oxidised to aluminium oxide by O gain, and the aluminium is the reducing agent i.e. the O remover.

  • These are examples of metal displacement reactions e.g. the less reactive chromium or titanium are displaced by the more reactive sodium, magnesium or aluminium.

Advanced Inorganic Chemistry Page Index and Links

CHROMIUM(III) chemistry

  • Chromium forms the stable green (greyish green almost violet sometimes?) chromium(III) ion, [Cr(H2O)6]3+(aq).

  • With aqueous ammonia (alkaline) or sodium hydroxide colour green chromium(III) hydroxide is precipitated.

    • Cr3+(aq) + 3OH-(aq) ==> Cr(OH)3(s) (but the structures can be quite complex)

    • or [Cr(H2O)6]3+(aq)  + 3OH-(aq) ==> [Cr(OH)3(H2O)3](s) + 3H2O(l) 

      • The hydroxide readily dissolves in acids to form salts,

      • Cr(OH)3(s) + 3H+(aq) ==> Cr3+(aq) + 3H2O(l) 

        • or more elaborately: [Cr(OH)3(H2O)3](s) + 3H3O+(aq) rev [Cr(H2O)6]3+(aq)  + 3H2O(l)

          • or more simply Cr(OH)3(s) + 3H+(aq) rev Cr3+(aq)  + 3H2O(l)

        • thus showing amphoteric behaviour, since the hydroxide ppt. also dissolves in excess strong alkali to give a dark green solution and the hydroxide ppt. does not dissolve in the weak base aqueous sodium carbonate. However, it will dissolve in excess ammonia because a new green complex ion is formed. (more details on these reactions below)

    • VIEW ppts. with OH-, NH3 and CO32-, & complexes, if any, with excess reagent.

  • With aqueous sodium carbonate the hydroxide ppt. is formed (as above) and carbon dioxide because of the acidity of the hexaaquachromium(III) ion (see Appendix 1.):

    • *initially 2[Cr(H2O)6]3+(aq) + CO32-(aq) ==> 2[Cr(H2O)5(OH)]2+(aq) + H2O(l) + CO2(g)   

    • this process of proton donation (deprotonation) continues until [Cr(OH)3(H2O)3](s) is formed

    • No Cr2(CO3)3 is formed because of the acid-base reaction above, due to the acidity of the chromium(III) ion. Note the similarly highly charged small ions Al3+ and Fe3+ behave in the same way.

    • * the acidity of a the hexa-aquachromium(III) ion can be expressed as ...

      • [Cr(H2O)6]3+(aq) + H2O(l) rev [Cr(H2O)5(OH)]2+(aq) + H3O+(aq)

  • With excess sodium hydroxide or ammonia, further complex ions are formed by ligand replacement reactions:

    • [Cr(H2O)6]3+(aq) + 6OH-(aq) ==> [Cr(OH)6]3-(aq) + 6H2O(l)  (from original hexa-aqua ion)

      • or [Cr(OH)3(H2O)3](s) + 3OH-(aq) ==> [Cr(OH)6]3-(aq) + 3H2O(l) (from hydroxide ppt.)

        • or more simply Cr(OH)3(s) + 3OH-(aq) ==> [Cr(OH)6]3-(aq)

        • showing amphoteric behaviour, since the hydroxide ppt. also dissolves in acid.

    • [Cr(H2O)6]3+(aq) + 6NH3(aq) ==> [Cr(NH3)6]3+(aq) + 6H2O(l)   (from original hexa-aqua ion)

      • or [Cr(OH)3(H2O)3](s) + 6NH3(aq) ==> [Cr(NH3)6]3+(aq) + 3OH-(aq) + 3H2O(l) (from hydroxide ppt.)

        • or more simply Cr(OH)3(s) + 6NH3(aq) ==> [Cr(NH3)6]3+(aq) + 3OH-(aq)

    • The uncharged ligand molecules ammonia NH3 and water H2O are similar in size and ligand exchange occurs without change in co-ordination number. They all octahedral complexes with a co-ordination number of 6.

  • Chromium(III) complexes are extremely numerous and varied, including many examples of isomerism. (see Appendix 2 and Appendix 3 for an introduction to complexes)

  • Ionisation isomerism in chromium(III) chloride based on Cr3+, 3Cl- and 6H2O 

    • [Cr(H2O)6]3+(Cl-)3  (violet or grey-blue?)

    • [CrCl(H2O)5]2+(Cl-)2.H2O  (pale green)

    • [CrCl2(H2O)4]+ Cl-.2H2O  (dark green)

    • [CrCl3(H2O)3]0*.3H2O ?  (brown?, this I found reference to on a Russian site, doesn't seem to be in textbooks? *the 0 to signify an overall electrically neutral complex can be omitted)

    • and this is not all, the 3rd one down with two chloride ligands can exist as cis (1) or trans (2) geometric isomers (Z/E isomers) illustrated below, and also serve as models for representing the other octahedral complexes which exhibit cis/trans or Z/E isomerism.

    • (c) doc b

    • With excess chloride ion you get the formation of the tetrahedral tetrachlorochromate(III) ion

      • [Cr(H2O)6]3+(aq) + 4Cl-(aq) ==> [CrCl4]-(aq) + 6H2O(l)

    • You also get geometrical cis/trans isomers (Z/E) with tetraamminedichlorochromium(III) complexes.

    • (c) doc b

  • A similar case of isomerism occurs with the chromium(III) complexes with ammonia and chloride ligands shown above. All the complex ions above have a plane of symmetry and cannot exhibit optical isomerism.

    • Again, these are all octahedral complexes with a coordination number of 6.

    • [Cr(H2NCH2CH2NH2)3]3+, H2NCH2CH2NH2, ethane-1,2-diamine (ethylenediamine), is often represented in shorthand by en,

      • and the complex simply written as [Cr(en)3]3+.

      • This complex has mirror image forms i.e. enantiomers of optical isomers.

        • This optical isomerism can be illustrated thus

        • where L-L represents H2NCH2CH2NH2

        • The ligand bonds via the lone pairs of electrons on the nitrogen which are donated to form the metal-ligand dative covalent bonds.

  • Both the hexa-aqua ions of chromium(II) and chromium(III) readily complex with EDTA

    • [Cr(H2O)6]2+(aq) + EDTA4-(aq) ===> [Cr(EDTA)]2-(aq) + 6H2O(l)

      • Kstab = {[Cr(EDTA)3]2-(aq)} / {[Cr(H2O)6]2+(aq)} [EDTA4-(aq)]

      • Kstab = 1.0 x 1013 mol-1 dm3 [lg(Kstab) = 13.0]

    • [Cr(H2O)6]3+(aq) + EDTA4-(aq) ===> [Cr(EDTA)]-(aq) + 6H2O(l)

      • Kstab = {[Cr(EDTA)3]-(aq)} / {[Cr(H2O)6]3+(aq)} [EDTA4-(aq)]

      • Kstab = 1.0 x 1024 mol-1 dm3 [lg(Kstab) = 24.0]

    • From the Kstab values, you can see that the more highly charged Cr3+(aq) ion complexes more strongly than the Cr2+(aq) ion.

  • -

Advanced Inorganic Chemistry Page Index and Links

CHROMIUM(VI) 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)

    • Redox changes: oxidation 2Cr(+3) ==> 2Cr(+6), reduction 6 O(-1) in 3H2O2 ==> 6(-2) in 6 of the 8H2O, total of 6 'units' oxidation state change.

    • Both H2O2 and Cr(VI) compounds are oxidising agents but in alkaline solution H2O2 is the stronger oxidising agent.

      • EØ = +?V details to add???

    • When the resulting solution from above is acidified with dilute sulphuric acid, the orange dichromate(VI) ion Cr2O72-  is formed.

    • The equilibrium is pH dependent. From 'Le Chatelier's Principle':

      • in more acidic solution, more H+, decrease pH ==> more orange (net change L to R) or in

      • more alkaline, less H+ (removed by OH-), increase pH <= more yellow (net change R to L).

    • 2CrO42-(aq) + 2H+(aq) rev Cr2O72-(aq) + H2O(l) (no change in ox. state)

  • 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- rev 2Cr3+(aq) + 7H2O(l)

      • orange (+6) ==> green (+3),  EØ = +1.33V 

    • Cr(III, +3) ==> Cr(II, +2): Cr3+(aq) + e- rev Cr2+(aq)

      • green (+3) ==> blue (+2), EØ = -0.41V, so Cr(II) is readily oxidised by dissolved oxygen and can only be retained in an inert atmosphere.

    • Note the  EØZn(s)/Zn2+(aq) is -0.76V, so the reducing power of zinc is sufficient to effect either of the two chromium oxidation state reduction changes.

      • The full redox equations for the reactions which happen on the surface of the zinc are:

      • Cr2O72-(aq) + 3Zn(s) + 14H+(aq) rev 2Cr3+(aq) + 3Zn2+(aq) + 7H2O(l)

      • 2Cr3+(aq) + Zn(s) rev 2Cr2+(aq) + Zn2+(aq)

      • You will see hydrogen formed as a by-product of the zinc-acid reaction but the reductions take place on the surface of the zinc.

  • Potassium dichromate(VI), K2Cr2O7,  can be crystallised to high purity standard without water of crystallisation, and is a valuable 'standard' redox volumetric reagent.

    • E.g. It can used to titrate iron(II) ions in solution acidified with dilute sulphuric acid, using a redox indicator like barium diphenylamine sulphonate which is less readily oxidised than iron(II) ions, but once all the iron(II) ions are oxidised the indicator is oxidised to a blue colour. The iron(III) ions formed affect the indicator to give an inaccurate end point so phosphoric(V) acid is also added at the start to complex the Fe3+ ions as they form.

    • Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) ==> 2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l)

    • See also fully worked examples of redox volumetric titration calculation questions.

  • The dichromate(VI) ion is a strong oxidising agent - examples of oxidising action:

  • See above for oxidation of iron(II) ions.

  • 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)

    • This reaction between the released iodine and sodium thiosulfate can be used to estimate oxidising agents like dichromate(VI) ions. The iodine is titrated with standardised sodium thiosulphate (e.g. 0.10 mol dm-3) using a few drops of starch solution as an indicator. Iodine gives a blue colour with starch, so, the end-point is very sharp change from the last hint of blue to colourless.

  • Soluble chromate(VI) salts give yellow solutions, but lead(II) ions give a yellow ppt. of lead(II) chromate(VI) and silver ions a dark red ppt. of silver chromate(VI).

  • Pb2+(aq) + CrO42-(aq) ==> PbCrO4(s) and 2Ag+(aq) +  CrO42-(aq) ==> Ag2CrO4(s)

    • A few drops of silver chromate is used as an indicator when titrating chloride solutions with silver nitrate solution in neutral solution. The solubility product for the white ppt. of silver chloride

    • Ksp = [Ag+(aq)][Cl-(aq)] = 2 x 10-10 mole2dm-6

    • is exceeded before the solubility product of silver chromate(VI)

    • Ksp = [Ag+(aq)]2[CrO42-(aq)] = 3 x 10-12 mole3dm-9, until all the chloride is precipitated. The next drop of silver nitrate causes the precipitation of brownish-red silver chromate, so the end point is the formation of the dark red ppt

    • CHROMIUM(II) chemistry: The blue hexaaquachromium(II) ion, [Cr(H2O)6]2+(aq), can be formed by reducing chromium(III) salt solutions with zinc and hydrochloric acid but it is rapidly oxidised back to green chromium(III) ions by dissolved oxygen unless protected by an inert atmosphere.V3+/V2+ EØ = -0.26V, O2+H+/H2O EØ = +1.23V , see Redox Electrode Potential Chart

Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

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(indonesia) Doc Brown Kimia Tabel Periodik Catatan Revisi 10,6 Bagian 10. Logam Transisi 3d-blok: 10 0,6 Kromium Kimia ANORGANIK Bagian 10 3d blok Transisi LOGAM sub-indeks: 10,1-10,2 Pendahuluan 3d-blok Logam Transisi * 10,6 Kromium 10,13 Logam transisi lain seperti Ag dan Pt * Lampiran 1. Terhidrasi garam, keasaman-aqua ion hexa * Lampiran 2. Kompleks & ligan * Lampiran 3. Kompleks dan Isomerisme * Lampiran 4. Elektron konfigurasi & teori warna * Lampiran 5. Redoks persamaan, kelayakan, E ø * Lampiran 6. Katalisis * Lampiran 7. Redoks persamaan * Lampiran 8. Stabilitas Konstanta dan perubahan entropi * Lampiran 9. Kolorimetri analisis dan kompleks ion formula * Lampiran 10 3d blok - data diperpanjang * Lampiran 11 Beberapa 3d- blok senyawa, kompleks, keadaan oksidasi & potensi elektroda * Lampiran 12 Hidroksida endapan kompleks 'foto', rumus dan persamaan * (portuguese) Doc Brown Química Revisão Tabela Periódica Notes 10,6 Parte 10. Metais de transição 3d bloco: 10 .6 Chromium Chemistry 10 Cromo 0,6 Química Revisão de notas Subsidiária Nível Nível Avançado Rever Química revisão cursos para estudantes pré-universitários (igual E.U. grau 11 e grau 12 e Títulos / nível de cursos de honras) * (chinese) 督布朗的化学 元素周期表修订债券第10部分。过渡金属三维块 铬 化学 铬 数据表1总结* 延长铬数据表2 * 铬及电极块潜在的三维图 铬总结一些配合化合物的氧化态及相对于其他三维块元素 铬是很难白色金属，具有极强的抗化学侵蚀，在室温的温度 这是用在'生产非常困难，例如镀钢合金球轴承和钢铁制品的防腐涂料（'铬），铬金属，是在'的重要组成部分不锈钢 铬的提取 铬矿石和铬处理，净化成（三）氧化物。这是反应非常放热，在铝热反 应的风格，与铝（见铝的反应 ），以自由的铬金属。 Cr2O3 + 2Al ==> Al2O3 + 2Cr 铬（Ⅲ）氧化物减少损失Ø铬，铝被氧化为三氧化二铝由O增益，铝是还原剂的O，即去除 这些例子是钛金属或位移反应如反应少铬的铝流离失所更反应钠，镁或。 铬（三）化学 铬（六）化学 *
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