23. Transition metals Flashcards
Define a transition metal
An element that forms at least one stable ion with a part full d-shell of electrons
Why is Scandium (21) not a transition metal
Only forms Sc3+ (3d0) which does not have a partially filled d orbital so is not a transition metal
Why is Zinc (30) not a transition metal
Only forms Zn2+ (3d10) which does not have a partially filled d orbital so is not a transition metal
Name the 4 main features which are common to transition metals
- variable oxidation states
- coloured
- catalysts
- complex formation
What does complex formation mean
Transition elements form complex ions
What is a complex ion
A complex ion is formed when a transition metal ion is surrounded by other ions or molecules (ligands) which are bonded by coordinate bonds
What is a ligand
An ion or molecule with a lone pair of electrons that forms a co-ordinate bond with a transition metal
What is the co-ordination number
The number of co-ordination bonds to ligands that surround the transition metal ion
Ions with co-ordination number six are usually…
Octahedral
Ions with co-ordination number four are usually…
Tetrahedral
Some ions with co-ordination number four are usually…
Square planar
What happens when you dissolve the salt of a transition metal ion water
The positively charged ion becomes surrounded by water molecules that act as ligands. These are called aqua ions.
What is an aqua ion
- when you dissolve the salt of a transition metal ion water
- the positively charged ion becomes surrounded by water molecules that act as ligands
What is a multidentate ligand
A molecule or ion that have more than one atom with a lone pair of electrons which can bond to a transition metal ion
Give 3 examples of bidentate ligands
- ethane- 1,2-diamine
- ethanedioate ion
- benzene- 1,2-diol
Name an important multidentate ligand that can act as a hexadentate ligand
EDTA 4-
What are chelates
Complex ions with polydentate ligands
What can chelates be used for
Remove d-block metal ions from solution
What will happen when you add a hexadentate ligand into a solution of transition metal salt
It will replace all six water ligands
Why is chelate complexes with polydentate ligands favoured over complexes with monodentate ligands
When polydentate ligands replace the monodentate ligands there is an increase in the number of particles and therefore entropy so it is the favourable reaction
What is the chelate effect
When chelate complexes with polydentate ligands favoured over complexes with monodentate ligands due to entropy
What shape is [Co(NH3)6]3+
Octahedral as it has 6 ligands
What shape is [CoCl4]2-
Tetrahedral as it has 4 ligands
What type of isomerism occurs in octahedral and square planar complexes
E-Z (geometrical)
If two Cl- ligands in an octahedral complex ion are next to each other which type of EZ isomerism is it
Cis or Z form
If two Cl- ligands in an octahedral complex ion are on opposite sides to each other which type of EZ isomerism is it
Trans or E form
When does optical isomerism occur in transition metals
When there are two or more bidentate ligands in a complex
Why are transition metal complexes coloured
- because they have partially filled d-orbitals
- it is therefore possible for electrons to move from one d orbital to another
- in a compound d-orbitals have slightly different energies (lower = ground excited = higher)
- when d electrons move from the ground state to the excited state they absorb light energy
- equal to the difference in energy between levels
- this colour is therefore missing from the spectrum
- you see a combination of colours not absorbed
What does ΔE =
ΔE= hv
v is the frequency
h constant called Planks constant
What effects the colour of a metal complex and why
- oxidation state
- ligands
- coordination number
- as the energy gap ΔE changes with each of these things
Why do transition metals have a variety of oxidation states
They can use 3d and 4s electrons in bonding
What kind of isomerism occurs with monodentate ligands
E-Z
What kind of isomerism occurs with bidentate ligands
Optical
Describe cisplatin
- platinum transition metal
- 2 chlorine ion ligands
- 2 ammonia ligands
- cis form (z form)
What is cisplatin used for
Chemotherapy drug
Give an example of a linear complex
[Ag(NH3)2]+
What is [Ag(NH3)2]+
- tollens reagent
- used in organic chemistry to distinguish aldehydes from ketones
- aldehydes reduce the [Ag(NH3)2]+ to Ag while ketones do not
Give the equation for the difference in energy between the ground and excited state
ΔE= hv v is the frequency h constant called Planks constant or ΔE= hc/ λ c is velocity of light λ is wavelength
Name all the oxidation states of Vanadium and how they are produced
- Vanadium V IV III II
- formed by the reduction of Vanadate (V) ions by zinc in an acidic solution
When does oxidation of lower oxidation states tend to occur
In alkaline solutions
Why does oxidation of lower oxidation states tend to occur in alkaline solutions
- in alkaline there is a tendency to form negative ions
- since oxidation is electron loss
- it is easier from negatively charged species
When does reduction of a high oxidation state occur
In acidic solution
Why does reduction of higher oxidation states tend to occur in acidic solutions
- in an acidic there is a tendency to form positive ions
- since reduction is electron gain
- it is easier from positively charged species
How does M(H2O)4(OH)2 (typical transition metal complex) change with pH
acid solution: M(H2O)6 2+
alkaline solution: M(H2O)2(OH)4 2-
How are low oxidation state transition metals such as Fe2+ stabilised against oxidation
- keeping them in an acidic solution.
- tendency to form more positive ions
- harder to lose electrons (oxidise)
How are transition metals with high oxidation state oxidised
- alkali solution is added
- in alkaline there is a tendency to form more negative ions
- since oxidation is electron loss
- it is easier from more negatively charged species
- then oxidising agent is added
Write the redox equation for the oxidising of Fe2+ to Fe3+ by potassium manganate (VII). During the reaction the oxidation number of manganese falls from 7+ to 2+. First construct the half equations
Reduction :
MnO4- (aq) + 8H+ (aq) + 5e- —-> Mn2+ (aq) +4H2O (l)
Oxidation: Fe2+ (aq) —> Fe3+ (aq) + e-
Overall:
5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) —-> 5Fe 3+ (aq)
+ Mn2+ (aq) + 4H2O (l)
Write the redox equation for the oxidising of Fe2+ to Fe3+ by acidified dichromate (VI). During the reaction the oxidation number of chromium falls from 6+ to 3+. First construct the half equations
Reduction:
Cr2O7 2- (aq) + 14H+ (aq) + 6e- —> 2Cr3+ (aq) + 7H2O (l)
Oxidation: Fe2+ (aq) —> Fe3+ (aq) + e-
Overall:
6Fe2+ (aq) + Cr2O7 2- (aq) + 14H+ (aq) —>
6Fe3+ (aq) 2Cr3+ (aq) + 7H2O (l)
Many M2+ ions will be oxidised to M3+ in what kind of solution
Alkaline
Describe the process of how Co2+ can be oxidised by air to Co3+
By adding excess ammonia solution to solution containing Co2+ ions
1. First precipitate is formed with OH- ions from the ammonia
[Co(H2O)6]2+ + 2OH- —> Co(H2O)4(OH)2 (s) + 2H2O (l)
2.Then the precipitate dissolves in excess ammonia
Co(H2O)(OH) (s) + 6NH3 (aq) —-> [Co(NH3)6]2+ +2OH- (aq) + 4H2O (l)
3. The resulting complex ion is oxidised by air to [Co(NH3)6]3+
How can Co2+ be oxidised to Co3+ by H2O2
2[Co(OH)6]4- (aq) + H2O2 (aq) —-> 2[Co(OH)6]3- (aq) + 2OH- (aq)
What are the two groups catalysts can be divided into
- heterogeneous
- homogeneous
What are heterogeneous catalyst
- catalysts that are present in a reaction in a different phase
- usually present as solids whilst the reactants may be gases or liquid
- reactants pass over the solid catalyst are react on its surface
How are heterogenous catalysts made more efficient
- increase their surface area
- spread catalyst on inert support medium. Increases surface to mass ratio (used by catalytic converters)
- do not use reactants with impurities to stop poisoning (decreases life of the catalyst)
Why do heterogeneous catalysts not last forever
- overtime the surfaces become covered with unwanted impurities (poising).
- finely divided catalyst may gradually be lost from the support medium
Name two processes that use heterogeneous catalysts
- the Haber process
- the Contact process
What is the Haber Process
- for the production of ammonia
- N2 (g) + 3H2 (g) 2NH3 (g)
- iron catalyst
- iron catalyst becomes poised by impurities in the gas stream such as sulfur
What is the Contact Process
- for the production of sulfuric acid
- key step : 2SO2 + O2 —-> 2SO3
- this is catalysed by vanadium (V) oxide V2O5 in two steps
Write the two steps in which vanadium oxide catalyses the key step in the Contact process
- catalyst oxidises sulfur dioxide to sulfur trioxide and itself is reduced
SO2 + V2O5 —> SO3 + V2O4 - vanadium (IV) oxide is then oxidised back to vanadium (V) oxide
2V2O4 + O2 —-> 2V2O5
What is a homogenous catalyst
- catalysts that are present in a reaction in the same phase as the reactant
- an intermediate species is formed
Name an example of homogenous catalysis
- Fe2+ in the
- oxidation of iodide ions to iodine by S2O8 2-
What is the overall reaction for of the oxidation of iodide ions to iodine by S2O8 2-
S2O8 2- (aq) + 2I- (aq) —> 2SO4 2- (aq) + I2 (aq)
Write the two steps in which Fe 2+ catalyses the reaction for iodide ions to iodine
- first the peroxodiosulfate ions oxidise Fe2+ to Fe3+
S2O8 2- (aq) + 2Fe 2+ (aq) —>2SO4 2- (aq) + 2Fe3+ (aq) - Fe3+ then oxidises the I- to I2 regenerating the Fe2+
2Fe3+ (aq) + 2I- (aq) —> 2Fe2+ (aq) + I2 (aq)
What is autocatalysis
- When a product of a reaction is a catalyst for the same reaction
- As concentration of the product builds up the speed of the reaction increases
Name an example of autocatalysis
The oxidation of enthandioic acid by manganate (VII) ions ( acidificed potassium maganagte (VII) )
Write the overall equation for the oxidation of enthandioic acid by manganate (VII) ions
2MnO4 - (aq) + 16H+ (aq) + 5C2O4 2- (aq) —->
2Mn2+ (aq) + 8H2O (l) + 10CO2 (g)
How does the products of the reaction of the oxidation of enthandioic acid by manganate (VII) ions catalyse the reaction
- Mn2+ ions react with the MnO4 - to form Mn3+ (the intermediate species)
- Mn3+ then reacts with C2O4 2- to reform Mn2+
Write the two steps where Mn2+ catalyses the reaction for the oxidation of enthandioic acid by manganate (VII) ions
- 4Mn2+ (aq) + MnO4 - (aq) + 8H+ (aq) —-> 5Mn3+ (aq) 4H2O (l)
2.
2Mn3+ (aq) + C2O4 2- (aq) —> 2Mn2+ (aq) + 2CO2 (g)
Give an example of a complex ion in the body
Haemoglobin
What is haemoglobin
- Fe2+ ion with a coordination number of 6
- 4 coordination sites are taken up by porphyrin (a ring system) which acts as a tetradentate ligand. This is the haem
- 5th site taken up by nitrogen from a complex protein. This is the globin
- 6th site can accept a oxygen molecule as a ligand
Acidic conditions favour….
positive ions
Alkaline conditions favour…
negative
