23.3 coloured ions

. The compounds absorb energy that corresponds to light in the visible region of the spectrum . Eg a purple solution absorbs all light from a beam of white light, but transmits red and blue waves so it appears purple

. They have part filled d orbitals . So it is possible for electrons to move from one d orbital to another Eg in an isolated transition metal atom, all the d orbitals are of exactly the same energy, but in a compound, the presence of other atoms nearby makes the d orbitals have slightly different energies . When electrons move from one d orbital to a higher one (called an excited state), they often absorb energy in the visible region of the spectrum This energy is equal to the difference in energy between levels .This colour is therefore missing from the spectrum so you see a combination of colours that aren’t absorbed

. Oxidation number 5: VO2^+ Is yellow . Oxidation number 4: VO^2+ is blue . Oxidation number 3: V^3+ Is green . Oxidation numer 2: V^2+ Is violet

v = fλ wave speed = frequency X wavelength

Energy change= (planks constant) X frequency E=HF use mnemonic Eat Hetty Feather So find frequency by doing v = fλ so velocity divided by wavelength

.. Energy gap between the levels of orbitals with different amounts of energy So this relies on: . the oxidation state of the metal, . the types of ligands . the transition metal, . the coordination number, therefore the shape of the complex ion So different compounds of the same metal will have different colours

[Fe(H2O)6]2+ is green [Fe(H2O)6]3+ is pale brown

. [Cr(H2O)6]2+ is blue . [Cr(H2O)6]3+ is red-violet

[Co(NH3)6]2+ is brown [Co(NH3)6]3+ is yellow

Uses a light source and a detector to measure the amount of light of a particular wavelength that passes through a coloured solution The more concentrated the solution, the less light is transmitted through

. It can find ratio of metal ions to ligands in a complex . Two solutions are mixed together, and one contains the ligand and one contains the metal ion in different proportions . When they are mixed in the same ratio that they are in in the complex, there is the maximum concentration of complex in solution So the solution will absorb the most light and not transmit much

They give a reading of absorbance, so how much light the solution absorbs

. First it is calibrated to zero by measuring absorbance of the solvent (Held in cuvette) with no transition metal complex dissolved in it. Eg water . Then, white light is shone through a filter which makes it into a narrow range of frequencies, this produces monochromatic light ( a single colour) But the choice of filter is important because the light produced must be absorbed by the transition metal ion solution . This monochromatic light is passed through the sample (which is held in a cuvette), and some light is absorbed. . The light that is transmitted (so isn’t absorbed) passes to a detector which measures the level of absorbance by comparing it to the absorbance of the blank sample . Then plot a calibration graph

. First plot calibration curve . Do this by using the colorimeter to measure the absorbance of 5 different samples of your solution, each with a different but known concentration of transition metal ion complex in it Eg 0, 20%,40%,60%,80% Plot these on a graph with the absorbance on y axis and these on x axis To make these samples you must use the same transition metal complex, and same solvent. Use a pipette for this to ensure that accurate volumes are used