Transition Metals Aqueous Chemistry - Formation of Metal-Aqua Ions and Acidity Differences

TRANSITION METALS - AQUEOUS CHEMISTRY Learning Objectives: 1. Describe how metal-aqua ions are formed in aqueous solution and explain the differences in acidity between M2+ and M3+ complexes. 2. Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases. 3. Evaluate experimental observations to identify products and reactants and give equations for the reaction of M2+ and M3+ complexes with bases. Specification Reference: 3.2.6

THE AQUEOUS CHEMISTRY OF IONS - HYDROLYSIS when salts dissolve in water the ions are stabilised this is because water molecules are polar hydrolysis can occur and the resulting solution can become acidic the acidity of the resulting solution depends on the cation present the greater the charge density of the cation, the more acidic the solution cation charge 1+ 2+ 3+ ionic radius 0.095 nm 0.065 nm 0.050 nm reaction with water / pH of chloride Na Mg Al the greater charge density of the cation... the greater the polarising power and the more acidic the solution LO1: Describe how metal-aqua ions are formed in aqueous solution and explain the differences in acidity between M2+ and M3+ complexes..

THE AQUEOUS CHEMISTRY OF IONS Theory aqueous metal ions attract water molecules many have six water molecules surrounding them these are known as hexaaqua ions they are octahedral in shape water acts as a Lewis Base a lone pair donor water forms a co-ordinate bond to the metal ion metal ions accept the lone pair - Lewis Acids Acidity as charge density increases, the cation has a greater attraction for water the attraction extends to the shared pair of electrons in water s O-H bonds the electron pair is pulled towards the O, making the bond more polar this makes the H more acidic (more +) it can then be removed by solvent water molecules to form H3O+(aq). LO1: Describe how metal-aqua ions are formed in aqueous solution and explain the differences in acidity between M2+ and M3+ complexes..

HYDROLYSIS - EQUATIONS M2+ ions [M(H2O)6]2+(aq) + H2O(l) [M(H2O)5(OH)]+(aq) + H3O+(aq) the resulting solution will now be acidic as there are more protons in the water this reaction is known as hydrolysis - the water causes the substance to split up Stronger bases (e.g. CO32- , NH3 and OH ) can remove further protons... LO1: Describe how metal-aqua ions are formed in aqueous solution and explain the differences in acidity between M2+ and M3+ complexes..

HYDROLYSIS - EQUATIONS M3+ ions [M(H2O)6]3+(aq) + H2O(l) [M(H2O)5(OH)]2+(aq) + H3O+(aq) the resulting solution will also be acidic as there are more protons in the water this SOLUTION IS MORE ACIDIC due to the greater charge density of 3+ ions Stronger bases (e.g. CO32- , NH3 and OH ) can remove further protons... LO1: Describe how metal-aqua ions are formed in aqueous solution and explain the differences in acidity between M2+ and M3+ complexes..

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. OH OH [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) H+ H+ [M(OH)2(H2O)4](s) OH OH [M(OH)2(H2O)4](s) [M(OH)3(H2O)3] (aq) H+ [M(OH)4(H2O)2]2-(aq) H+ OH OH [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) H+ [M(OH)6]4-(aq) H+ When sufficient protons have been removed the complex becomes neutral and precipitation of a hydroxide or carbonate occurs. e.g. M2+ ions M3+ ions [M(H2O)4(OH)2](s) or [M(H2O)3(OH)3](s) or M(OH)2 M(OH)3 LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. OH OH [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) H+ H+ [M(OH)2(H2O)4](s) OH OH [M(OH)2(H2O)4](s) [M(OH)3(H2O)3] (aq) H+ [M(OH)4(H2O)2]2-(aq) H+ OH OH [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) H+ [M(OH)6]4-(aq) H+ When sufficient protons have been removed the complex becomes neutral and precipitation of a hydroxide or carbonate occurs. e.g. M2+ ions M3+ ions [M(H2O)4(OH)2](s) or [M(H2O)3(OH)3](s) or M(OH)2 M(OH)3 LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. OH OH [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) H+ H+ [M(OH)2(H2O)4](s) Precipitated OH OH [M(OH)2(H2O)4](s) [M(OH)3(H2O)3] (aq) H+ [M(OH)4(H2O)2]2-(aq) H+ OH OH [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) H+ [M(OH)6]4-(aq) H+ In some cases, if the base is strong, further protons are removed and the precipitate dissolves as soluble anionic complexes such as [M(OH)6]3- are formed. Very weak bases H2O Weak bases Strong bases remove few protons remove protons until precipitation can remove all the protons NH3, CO32- OH LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. OH OH [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) H+ H+ [M(OH)2(H2O)4](s) Precipitated OH OH [M(OH)2(H2O)4](s) [M(OH)3(H2O)3] (aq) H+ [M(OH)4(H2O)2]2-(aq) H+ OH OH [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) H+ [M(OH)6]4-(aq) H+ In some cases, if the base is strong, further protons are removed and the precipitate dissolves as soluble anionic complexes such as [M(OH)6]3- are formed. Very weak bases H2O Weak bases Strong bases remove few protons remove protons until precipitation can remove all the protons NH3, CO32- OH LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. OH OH [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) H+ H+ [M(OH)2(H2O)4](s) Precipitated OH OH [M(OH)2(H2O)4](s) [M(OH)3(H2O)3] (aq) H+ [M(OH)4(H2O)2]2-(aq) H+ OH OH [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) H+ [M(OH)6]4-(aq) H+ AMPHOTERIC CHARACTER Metal ions of 3+ charge have a high charge density and their hydroxides can dissolve in both acid and alkali. H+ OH [M(H2O)6]3+(aq) Soluble [M(OH)3(H2O)3](s) [M(OH)6]3-(aq) Soluble Insoluble LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTION TYPES The examples aim to show typical properties of transition metals and their compounds. One typical properties of transition elements is their ability to form complex ions. Complex ions consist of a central metal ion surrounded by co-ordinated ions or molecules known as ligands. This can lead to changes in ... colour shape co-ordination number stability to oxidation or reduction Reaction types ACID-BASE A-B LIGAND SUBSTITUTION LS PRECIPITATION Ppt REDOX RED OX REDOX LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTION TYPES The examples aim to show typical properties of transition metals and their compounds. LOOKFOR... substitution reactions of complex ions variation in oxidation state of transition metals the effect of ligands on co-ordination number and shape increased acidity of M3+ over M2+ due to the increased charge density differences in reactivity of M3+ and M2+ ions with OH and NH3 the reason why M3+ions don t form carbonates amphoteric character in some metal hydroxides (Al3+ and Cr3+) the effect a ligand has on the stability of a particular oxidation state LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTIONS OF COPPER(II) Aqueous solutions of copper(II) contain the blue, octahedral hexaaquacopper(II) ion Most substitution reactions are similar to cobalt(II). OH [Cu(H2O)6]2+(aq) + 2OH (aq) > [Cu(OH)2(H2O)4](s) + 2H2O(l) blue, octahedral Blue ppt. insoluble in XS NaOH A-B A-B NH3 [Cu(H2O)6]2+(aq) + 2NH3(aq) > [Cu(OH)2(H2O)4](s) + 2NH4+(aq) Blue ppt. soluble in excess NH3 then [Cu(OH)2(H2O)4](s) + 4NH3(aq) > [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH (aq) NOTE THE FORMULA Deep blue LS CO32- [Cu(H2O)6]2+(aq) + CO32-(aq) > CuCO3(s) + 6H2O(l) Green-blue ppt. Ppt LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTIONS OF COPPER(II) Cl [Cu(H2O)6]2+(aq) + 4Cl (aq) > [CuCl4]2-(aq) + 6H2O(l) yellow, tetrahedral LS Cl ligands are larger than H2O and are charged the complex is more stable if the shape changes to tetrahedral adding excess water reverses the reaction I 2Cu2+(aq) + 4I (aq) > 2CuI(s) + I2(aq) off - white ppt. REDOX a redox reaction used in the volumetric analysis of copper using sodium thiosulphate LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTIONS OF IRON(II) When iron reacts with acids it gives rise to iron(II) (ferrous) salts. Aqueous solutions of such salts contain the pale green, octahedral hexaaquairon(II) ion OH [Fe(H2O)6]2+(aq) + 2OH (aq) > [Fe(OH)2(H2O)4](s) + 2H2O(l) Green A-B Green ppt. it only re-dissolves in very conc. OH but... it slowly turns a rusty brown colour due to oxidation by air to iron(III) increasing the pH renders iron(II) unstable. Fe(OH)2(s) + OH (aq) > Fe(OH)3(s) + e Green OX rusty brown A-B NH3 Iron(II) hydroxide precipitated, insoluble in excess ammonia CO32- Green iron(II) carbonate, FeCO3, precipitated Ppt LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTIONS OF IRON(III) Aqueous solutions contain the yellow-green, octahedral hexaaquairon(III) ion OH [Fe(H2O)6]3+(aq) + 3OH (aq) > [Fe(OH)3(H2O)3](s) + 3H2O(l) yellow A-B Brown ppt. insoluble in XS CO32- 2[Fe(H2O)6]3+(aq) + 3CO32-(aq) > 2[Fe(OH)3(H2O)3](s) + 3H2O(l) + 3CO2(g) Brown ppt. A-B The carbonate is not precipitated but the hydroxide is; the high charge density of M3+ makes the solution too acidic to form a carbonate CARBON DIOXIDE EVOLVED. A-B NH3 [Fe(H2O)6]3+(aq) + 3NH3(aq) > [Fe(OH)3(H2O)3](s) + 3NH4+(aq) Brown ppt. insoluble in XS SCN [Fe(H2O)6]3+(aq) + SCN (aq) > [Fe(SCN)(H2O)5]2+(aq) + H2O(l) Very sensitive; BLOOD RED COLOUR confirms Fe(III). No reaction with Fe(II) LS blood-red colour LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

REACTIONS OF ALUMINIUM aluminium is not a transition metal as it doesn t make use of d orbitals BUT, due to a high charge density, aluminium ions do behave as typical M3+ ions aqueous solutions contain the colourless, octahedral hexaaquaaluminium(III) ion OH [Al(H2O)6]3+(aq) + 3OH (aq) > [Al(OH)3(H2O3](s) + 3H2O(l) colourless, octahedral white ppt. soluble in XS NaOH As with all hydroxides the precipitate reacts with acid A-B [Al(OH)3(H2O)3](s) + 3H+ (aq) > [Al(H2O)6]3+(aq) being a 3+ hydroxide it is AMPHOTERIC and dissolves in excess alkali A-B A-B [Al(OH)3(H2O)3](s) + OH (aq) > [Al(OH)4(H2O)2]-(aq) + 3H2O(l) colourless, octahedral CO32- 2 [Al(H2O)6]3+(aq) + 3CO32-(aq) > 2[Al(OH)3(H2O)3](s) + 3H2O(l) + 3CO2(g) white ppt. A-B As with 3+ ions, the carbonate is not precipitated but the hydroxide is. [Al(H2O)6]3+(aq) + 3NH3(aq) > [Al(OH)3(H2O)3](s) + 3NH4+(aq) white ppt. insoluble in XS NH3 NH3 A-B LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

SUMMARY OF TRANSITION METAL COMPLEX REACTIONS LO2: Describe and explain the simple test tube reactions of M2+ and M3+ complexes with bases.

PRACTICE QUESTION 12 MINUTES LO3: Evaluate experimental observations to identify products and reactants and give equations for the reaction of M2+ and M3+ complexes with bases.