Chromium Oxidation States

Chromium Oxidation States ! Oxidation states range from +2 to +6. ! We will see +6, +3, and +2 Oxidation state +6 +3 +2 Ions CrO42–/Cr2O72– Cr3+(aq) Cr2+(aq) Color orange/red-orange green blue-violet 3d0 3d3 3d4 Configuration Magnetism diamagnetic paramagnetic paramagnetic Chromium(II) Acetate Dihydrate Cr2(O2CCH3)4(H2O)2 ! Discovered in 1844 ! Unusual for Cr(II) • Brick red • Diamagnetic ! One of a family of compounds Cr2(O2CR)4L2 ! All have short Cr–Cr distances (228 to ~254 pm) due to quadruple bonds. ! Cr–Cr quadruple bonding accounts for diamagnetism and atypical color. Chromate and Dichromate Ions U Potassium dichromate, K2Cr2O7, is the starting material for this synthesis. ! In alkaline or basic solution, chromium(VI) exists as the orange-colored tetrahedral CrO42– ion ! In acid solution, this can become HCrO4–, but dimerization to red-orange Cr2O72– predominates: H2CrO4 + H2O º HCrO4– + H3O+ HCrO4– + H2O º CrO42– + H3O+ 2 CrO42– + 2 H+ º Cr2O72– + H2O ! K1 = 4.1 K2 = 1.3 × 10–6 K = 6.3 × 10–3 Dichromate ion has two tetrahedrally coordinated Cr atoms linked by an oxygen bridge: 177 pm O O 110o O Cr Cr O O 123o O O 162 pm ! Intense colors of CrO42– and Cr2O72– are the result of ligand-to-metal (L÷M) charge transfer bands, Cr6+–O2– ÷ Cr5+–O– Synthesis Redox U In this synthesis we will reduce Cr(VI) to Cr(III) and then to Cr(II). ! Dichromate is a strong oxidizing agent: Cr2O72– + 14 H+ + 6 e– º 2 Cr3+ + 7 H2O ! Cr(III) is a weak oxidizing agent, which requires a strong reducing agent to produce Cr(II): Cr3+ + e– º Cr2+ ! Eo = +1.33 V Eo = –0.41 V In this synthesis, zinc metal serves as the strong reducing agent: Zn º Zn2+ + 2 e– –Eo = +0.763 V Overall Redox Equation U Actual overall reaction in this synthesis is not certain. ! From the electrode potentials, it might be assumed that the reduction of Cr(VI) to Cr(II) is accomplished by Zn as the reducing agent, in which case the balanced equation would be Cr2O72– + 14 H+ + 4 Zn ÷ 2 Cr2+ + 4 Zn2+ + 7 H2O ! But in this preparation, Zn and H+ from the acid are reacting to produce H2(g): 2 H+ + Zn ÷ H2 + Zn2+ ! Hydrogen gas might be the reducing agent for converting Cr(VI) to Cr(III). If so, the reduction of Cr(III) to Cr(II) would then be effected by Zn as the reducing agent in a second step. Cr2O72– + 3 H2 + 8 H+ ÷ 2 Cr3+ + 7 H2O 2 Cr3+ + Zn ÷ 2 Cr2+ + Zn2+ –––––––––––––––––––––––––––––––––––––––––––––– Cr2O72– + 3 H2 + 8 H+ + Zn ÷ 2 Cr2+ + Zn2+ + 7 H2O ! More likely, the first step is a mixture of reduction by H2 and reduction by Zn. L Regardless of the exact stoichiometry, the limiting reagent is Cr2O72–, from which twice as many moles of Cr2+ is produced. Cr2+(aq) to Cr2(O2CCH3)4(H2O)2 ! Color of the original dichromate solution changes from redorange to green and then to blue. ! When the solution is blue, conversion to Cr2+ is complete. ! At this point, using the pressure from the evolving H2(g), the solution is pumped into a solution of sodium acetate, where the complex forms and is precipitated as a brick-red solid: 2 Cr2+ + 4 CH3CO2– + 2 H2O ÷ Cr2(O2CCH3)4(H2O)2 L When wet, the complex is subject to oxidation to Cr(III) from oxygen: Cr2+ ÷ O2 + 4 H+ + 4 e– ÷ Cr3+ + e– 2 H2O –Eo = +0.41 V Eo = +1.229 V K Avoid sucking air through the wet product during the coldwater and alcohol washes. K After the final ether wash, allow air to pass through the product on the filter for about 30 sec., then collect it on a large piece of dry filter paper or a porcelain plate to allow the residual ether to evaporate.