23.3 coloured ions

Question 1

How can we see the colour of transition metal compexes

Answer 1

. 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

Question 2

Why are transition metal complexes coloured

Answer 2

. 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

Question 3

Describe the colours of the 4 vanadium species

Answer 3

. 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

Question 4

What is the equation linking frequency, wavelength and wave speed

Answer 4

v = fλ
wave speed = frequency X wavelength

Question 5

What equation would you use to find the difference in energy between d orbitals using planks constants

Answer 5

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

Question 6

What does the colour of a transition metal complex depend on

Answer 6

.. 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

Question 7

How does the aqua iron complex with oxidation state 2 appear in colour

What about oxidation state 3

Answer 7

[Fe(H2O)6]2+ is green

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

Question 8

How does the aqua chromium complex with oxidation state 2 appear in colour

What about oxidation state 3

Answer 8

. [Cr(H2O)6]2+ is blue

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

Question 9

How does the cobalt complex appear with oxidation state 2+ with its ligands being ammonia

and what about state 3+

Answer 9

[Co(NH3)6]2+ is brown

[Co(NH3)6]3+ is yellow

Question 10

What does a colorimeter do

Answer 10

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

Question 11

How can you find formula of a transition metal complex using colorimetry

Answer 11

. 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

Question 12

What reading do colorimeters give

Answer 12

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

Question 13

How do you use a colorimeter to measure the concentration of a transition metal ion in solution

Answer 13

. 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

Question 14

How do you use a calibration graph to measure concentration of transition metal complex in solution

Answer 14

. 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