An aqueous solution containing hexaaquachromium(III) ions, [Cr(H2O)6]3+\text{[Cr(H}_2\text{O)}_6\text{]}^{3+}[Cr(H2O)6]3+, is treated with aqueous sodium hydroxide, first added dropwise and then until the reagent is in excess.
Which of the following options correctly predicts the observation for the intermediate reaction and the final state of the mixture?
An intermediate green precipitate of Cr(H2O)3(OH)3(s)\text{Cr(H}_2\text{O)}_3\text{(OH)}_3\text{(s)}Cr(H2O)3(OH)3(s) forms, which dissolves in excess sodium hydroxide to give a deep green solution containing [Cr(OH)6]3−(aq)\text{[Cr(OH)}_6\text{]}^{3-}\text{(aq)}[Cr(OH)6]3−(aq).
An intermediate green precipitate of Cr(H2O)3(OH)3(s)\text{Cr(H}_2\text{O)}_3\text{(OH)}_3\text{(s)}Cr(H2O)3(OH)3(s) forms, which is insoluble in excess sodium hydroxide.
An intermediate green precipitate of Cr(H2O)3(OH)3(s)\text{Cr(H}_2\text{O)}_3\text{(OH)}_3\text{(s)}Cr(H2O)3(OH)3(s) forms, which reacts with excess sodium hydroxide to yield a bright yellow solution containing CrO42−(aq)\text{CrO}_4^{2-}\text{(aq)}CrO42−(aq).
An intermediate pale blue precipitate of Cr(H2O)4(OH)2(s)\text{Cr(H}_2\text{O)}_4\text{(OH)}_2\text{(s)}Cr(H2O)4(OH)2(s) forms, which dissolves in excess sodium hydroxide to give a royal blue solution containing [Cr(NH3)4(H2O)2]2+(aq)\text{[Cr(NH}_3\text{)}_4\text{(H}_2\text{O)}_2\text{]}^{2+}\text{(aq)}[Cr(NH3)4(H2O)2]2+(aq).
41 exam-style questions on OCR GCSE Chemistry Predicting chemical reactions. Each one has a worked solution and a mark scheme showing where the marks go.