Unit 1)9) Colour , Loss Of Degeneracy , Catalysts Flashcards
Several transition metal ________ are coloured
Several transition metal complexes are coloured
- Why are they coloured
- what colour is seen
- they absorb light in certain parts of the visible spectrum
- the colour seen is the complementary colour to that absorbed
- look in flashcards ahead this is not the actual full explanation as to why light is absorbed / colour shown
What is white light a combination of
White light is a combination of 3 colours : red , blue and green
What is seen when
- red light is absorbed
- blue light is absorbed
- if green light is absorbed
- if all colours are absorbed
- if red light is absorbed , green and blue light are transmitted and we see this as a blue / green colour or cyan
- if blue light is absorbed , the colours transmitted are red and green , seen as yellow
- if green light is absorbed , a combination of red and blue light is transmitted and we see this as purple or magenta
Explain how transition metals absorb light
- they can absorb light due to the 5 degenerate d orbitals splitting in terms of energy
- in a free transition metal ion (without ligands so not a complex ) the 5 d orbitals are degenerate (equal in energy)
- on the formation of a complex when ligands approach the x , y ,z axis , the lone pair of the ligand repels the d orbitals
- as a result the d sub shell is split into two distinct sets of orbitals which now have different energy and are not degenerate
What affects the size of the energy gap between the two sets of d orbitals
- the transition metal ion
- the oxidation state of the transition metal
- type of ligand
Why do transition metals absorb light
- transition metal complexes absorb light due to the split in d orbitals
-electrons in lower energy orbitals can absorb energy and move to the higher energy orbitals - if the energy absorbed in these d - d transitions is in the visible part of the electromagnetic spectrum , the colour of the metal is absorbed and the complementary colour is observed
orbitals that lie on the axis , repel more strongly they get promoted to the higher d orbitals
Example of a complex absorbing light and showing colour
- on the formation of a complex eg (Ni(H2O6)2+) , six water ligands surround the central nickel ion forming an octahedral shaped complex
- the ligands approach the nickel ion along the x y z axes. The electrons in d orbitals that lie on this axis (nickels d orbitals )will be repelled by electrons in the water ligand molecules
- these orbitals now have a higher energy than the 3 d orbitals that lie between the axes , therefore the 5 d orbitals are no longer degenerate
- this is called splitting of d orbitals and is different in octahedral complexes compared to tetrahedral and other shapes of complexes
What specifically determines the colour produced by the complex
It is the size of the split which determines the colour produced by the compound
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What does the energy difference (size of split) between the different subset of d orbitals depend on
The energy difference between the different subsets of d orbitals depends on the position of the Ligand in the spectrochemical series
What is the spectrochemical series
- this is a series that puts in order the ability of the ligand to to split the d orbitals
- those ligands that cause a large difference in energy in the d orbitals are said to be strong field “ligands” in contrast with “weak field” ligands where the energy difference is small
Give a short form of the spectrochemical series
CN- > NH3 > H20 > OH- > F- > Cl- > Br- > I-
How can the effect of D-D transitions be studied
By using spectroscopy
- When is visible spectroscopy used
- when is ultraviolet spectroscopy used
- if the absorbed energy is in the visible part of the electromagnetic spectrum (400-700nm) giving a coloured compound, visible spectroscopy will be used
- if the absorbed energy is in the ultraviolet part of the electromagnetic spectrum (200-400nm) the transition metal complex will be colourless and uv spectroscopy will be used
Which complexes are more likely to absorb energy in visible light region and which are more likely to absorb energy in UV region
- If the ligands are strong field ligands (those that cause greatest splitting of the d orbitals ) the d transitions are more likely to occur in UV region
- if the ligands are weak filed ligands (those that split the d orbitals the least ) the energy absorbed is more likely to occur in the visible region
How can the absorbance of coloured solutions be measured
- a colorimeter fitted with coloured filters corresponding to certain wavelengths in the visible region can be used to measure the absorbance of coloured solutions
- a filter of the complementary colour should be used
How can absorbance if colourless solutions be measured
- an ultraviolet spectrometer is a bit more complicated than a colorimeter. Different wavelengths of ultraviolet light from 200 to 400nm are passed through the sample and the amount of ultraviolet light absorbed at different wavelengths is recorded
- the results are plotted automatically as an ultraviolet spectrum
- as with a colorimeter the absorbance is directly proportional to the concentration of the absorbing species
Diagrams
Look at bright red
Why do transition metals act as catalysts
They can form a variable number of bonds due to occupied and partially occupied d orbitals
Transition metals and their compounds are important catalysts in many industrial chemical reactions
- name a few and the process
- haber process : iron
- contact process : vanadium (V) oxide
- Ostwald process : platinum
- catalytic converter in cars : platinum , palladium , rhodium
- preparation of methanol : copper
- preparation of margarine : nickel
- polymerisation of Alkenes : titanium compounds
How do catalysts speed up chemical reactions
Catalysts speed up chemical reactions by providing an alternative reaction pathway of lower activation energy
- What is a homogeneous catalyst
- how can transition metals act as homogenous catalysts
- a homogeneous catalyst is in the state state as reactants
- the ability of the transition metal to have variable oxidation states enables them to provide alternative reaction pathways with lower activation energies so speeding up the reaction
- What is a heterogenous catalyst
- examples
- how can transition metals act as a heterogenous catalyst
- a heterogeneous catalyst is a catalyst in a different state to the reactants. Most industrial processes are examples of heterogeneous catalysis
- iron in the haber process
- platinum in Ostwald process
- platinum , rhodium and palladium in catalytic converters
- the fact : transition metal atom /ion on the surface of active site of catalyst can form weak bonds with reactants . - the presence of unpaired d electrons or unfilled d orbitals allows intermediate activated complexes to form - this weakens the covalent bonds inside the reactant molecule and are in a more favourable position and susceptible to attack by molecules of the other reactant - overall effect is that an alternative reaction pathway with a lower activation energy is provided so rate of reaction is increased
