Transition Metals Flashcards
What is a transition metal?
D-block element that forms at least 1 stable ion with a partially filled d sub shell
Why are scandium and zinc not transition metals?
Neither scandium or zinc produce ions which have a partially filled d sub shell. (Sc d^0) (Zn d^10)
How does the d orbital fill up with electrons?
4s is the first to gain electrons due to a lower energy level. The the 3 d orbital starts to fill up with electrons. Once electrons are in the 3 d orbital, this orbital has a lower energy so the 4s electrons are also lost first.
What are some physical properties of transition metals?
All metals are shiny with a high density, melting and boiling point.
When they are solid they form giant metal lattices with delocalised electrons and therefore can conduct electricity.
What are some chemical properties of transition metals?
All transition metals have more than one oxidation state.
They all form 2+ ions from losing their 4s orbital.
Highest oxidation states will readily accept electrons and become reduced making them powerful oxidising agents.
What are the main characteristic properties of transition metals?
Form complex ions
Form coloured ions
Have variable oxidation states
Catalytic activity
What is disproportionation?
A process where a species is both oxidised and reduced in the same reaction
Eg. Hit dilute sulphuric acid and copper oxide forms a blue copper sulphate solution and brown copper precipitate as the copper is oxidised and reduced.
How do transition metals form coloured compounds?
When light passes through a solution of transition metals, the ions will absorb some wavelengths of light. Due to the splitting in the d orbital in an ion complex an electron may get promoted up to the higher energy level as the ions absorb light producing a colour.
How do transition metals act as catalysts?
They provide a surface for the reactions to take place on.
Many oxidation numbers and therefore can bind to reactants and form intermediates.
What transition metal catalyst are used in industrial processes?
Harder process- Iron (II) catalyst
Contact process- (vanadium catalyst)
Decomposition of hydrogen peroxide- manganese catalyst
Hydrogen production-copper sulphate catalyst
What are complex ions?
Formed when a central metal is surrounded by ligands or datively coordinate bonding to the metal ion.
Formed when a transition metal is present in a solution
What is a ligand?
A molecule or ion which can donate a pair of electrons to form a dative covalent ( coordinate) bond
What is the coordination number?
The number of coordinate bonds to the metal ion in a complex.
Give examples of ligands and state their formula and charge.
Water (:OH2) - neutral
Ammonia (:NH3) - neutral
Thiocyanate (:SCN^-) - -1
Cyanide (:CN^-) - -1
Chloride (:Cl^-) - -1
Hydroxide (:OH^-) - -1
What is a monodentate ligand?
Give examples
A ligand which is able to form 1 lone pair of electrons and from 1 coordinate bond.
Eg H2O NH3 Cl^-
Why is a chloride ligand only able to get 4 chloride molecules around the central atom?
Chloride ligand is larger than water and ammonia and hence will only be able to fit 4 chloride ions around the central metal ion.
What are bidentate ligands?
Give examples
Can donate 2 lone pairs of electrons from different atoms within the ligand to form 2 coordinate bonds
Eg ethane-1,2-diamine (en)
C2O4^2-
What are multidentate ligands?
Give examples
Ligands which can donate multiple lone pairs of electrons
Eg EDTA ——> EDTA^4- has 6 lone pairs
Hexadentate ligand
What is the main use of EDTA?
Used to bind to metal ions to decrease their concentrations
What shape is formed by a complex with 2 coordinate bonds?
Linear shape
[Ag(NH3)2]^2+
What shape is formed by a complex with 4 coordinate bonds?
Tetrahedral shape
Chloride ions
What shape is formed by a complex with 6 coordinate bonds?
Octahedral shape
Water and ammonia
(Most common shape)
What is special about ligand exchange between NH3 and H2O?
This exchange occurs without s change of s coordination number
What happens when a H2O ligand is exchanged by Cl-?
Involves a Change in the coordination number since chloride ions are a lot larger
What is the chelate effect?
When bidentate and multidentate ligands replace monodentate ligands from complexes
Eg.
[Cu(H2O)6]^2+ + EDTA^4- ———> [Cu(EDTA)]^2- + 6H2O
What complex is formed in the body to live?
Haeme complex with a multidentate ligand.
Explain when the haeme complex is important
Oxygen can form a coordinate bond with the Fe^2+ ions in haemoglobin.
This enables oxygen to be transported around via the blood
Carbon monoxide is toxic as it replaces oxygen coordinately bonded to the haeme group.
The binding of carbon monoxide prevents oxygen binding and so people die.
Explain why the chelate effect is a feasible reaction?
Eg. [Cu(H2O)6]^2+ + EDTA^4- ———> [Cu(EDTA)]^2- + 6H2O
6 coordinate bonds to the copper ions are broken but EDTA can also form 6 coordinate bonds and so that many are reformed. This means that the enthalpy of this reaction is very small.
Therefore Gibbs energy = -T (entropy)
In addition to this the reaction has gone from 2 molecules to 7 molecules and hence has a large increase in entropy meaning that negative Gibbs energy ensures the reaction is feasible
This is the chelate effect
How can transition metals be identified?
By the colours of their ions
How do the coloured ions arise in transition metals?
Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths reflected.
The d orbital energy splits when a ligand attaches to a dissolved compound and electrons are promoted from their ground state to the excited state when light energy is absorbed.
The absorbance of the light energy and promotion of the electrons leaves the solution coloured.
What equation is used to calculate the energy present in light?
Energy = planks constant X frequency
E = hf
What are zinc ions colourless?
Zinc has a full d orbital and therefore when in a complex and the orbital has split energy levels, no electrons can be promoted absorbing light and hence no colours are absorbed or refracted producing no coloured ions
How can a colour change of a transition metal occur?
Changes in oxidation state
Change Coordination number
Change of ligand
These all alter the E and hence leads to a change in colour.
How can we determine the concentration of coloured ions in solution?
Using colourimetry to calculate the concentrations
What type of isomerism is shown by octahedral isomers?
Cis trans isomers are shown in octahedral complexes by monodentate and bidentate ligands
What kind of isomerism is shown by octahedral complexes with bidentate ligands?
Forms optical isomers with bidentate ligands
A complex with 3 bidentate ligands
A complex with 2 bidentate and 2 monodentate
A complex with a hexadentate ligand
All form optical isomers
What is a cis and trans isomer with monodentate ligands?
When the 2 ligands are next to each other’s it is cis
When 2 of the same ligand are opposite each other (180 degrees) it is trans isomerism
What happens when aqueous copper ions react with ammonia?
Overall reaction:
[Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4(H2O)3]2+ + 4H2O
Adding small amounts of ammonia (acts as a base)
[Cu(H2O)6]2+ + 2NH3 —> [Cu(OH)2(H2O)4]2+ + 2NH4+
Forms a blue precipitate
Adding excess ammonia causes the precipitate to dissolve and ammonia replaces the water as a ligand
[Cu(OH)2(H2O)4]2+ + 4NH3 —> [Cu(NH3)4(H2O)]2+ + 2H2O + 2OH-
Give an example of a square planar complex
Cisplatin
Square planar complexes also show cis trans isomerism
What is a ligand substitution reaction?
Where a ligand in a complex ion is replaced by a different ligand
Often water is being replaced
what happens when aqueous copper ions react with HCl?
[Cu(H2O)6] + 4Cl- [Cu(Cl)4]2- + 6H2O
This forms yellow copper chloride as the coordination number has decreased.
since this is a reversible reaction, adding water will return the complex back to [Cu(H2O)6]2+
Give an example of ligand substitution reactions in the body?
Oxygen and haemoglobin in our blood
why do we use Manganate VII ions as an oxidising agent?
Manganate VII ions can readily be reduced to form Mn2+ ions in acidic conditions
used as an oxidising agent as the MnO4- ions are a different colour to the Mn2+ ions.
this colour change can be used to show the end point of a titration
Why do we carry out a redox titration between Fe2+ and Mn04-?
We can use this titration to work out the percentage composition of iron in a sample as we assume that all the iron has formed 2+ ions in solution
What is the redox equation between Fe2+ and MnO4- and how do we know that titration has reached the end point?
8H+ + MnO4- + 5Fe2+ ——> Mn2+ + 4H2O + 5Fe3+
end point is reached when all the Fe2+ is used and there will be a build up of purple MnO4- ions
what is special about vanadium?
Has many oxidation states
How are the different oxidation states of vanadium formed?
By the reduction of vanadate(v) ions by zinc in acidic solutions
What are the 4 key oxidation states of vanadium and their colours?
V^2+ purple
V^3+ green
VO^2+ blue
VO2^+ yellow
how can you assess the thermodynamic feasibility of a reduction?
by using the electrode potentials for each reduction
how far will zinc reduce vanadium ions to and give equations to prove this?
Zinc will reduce VO2^+ to V^2+ but no further since it’s electrode potential is more negative than the first 4 oxidation states of vanadium.
reduction from 5+ to 4+:
2VO2^+ + 4H^+ +Zn —-> 2VO^2+ + Zn^2+ +2H2O
Reduction from 4+ to 3+:
2VO^2+ + 4H^+ + Zn —-> 2V^3+ + Zn^2+ + 2H2O
Reduction from 3+ to 2+:
2V^3+ + Zn —-> Zn^2+ + 2V^2+
The positive Ecell for these 3 reductions shows that these reactions are thermodynamically feasible
What influences the redox potential for a transition metal ion?
Changing from a higher to a lower oxidation state is influenced by pH and by the ligand
What is a heterogeneous catalyst?
A catalyst that is in a different state to the reactants and is the reaction occurs at active sites on the catalyst
What is a homogeneous catalyst?
A catalyst that is in the same state as the reactants
Explain how V2O5 acts as a heterogeneous catalyst in the contact process?
The contact process produces sulphuric acid
2SO2 + O2 ——> 2SO3
Catalyses by vanadium oxide:
Vanadium oxide oxidised the sulphur dioxide and itself is reduced
SO2 + V2O5 ——> SO3 + V2O4
The vanadium is then oxidised back to vanadium oxide by oxygen
2V2O4 + O2 ——> 2V2O5
Show how V2O5 acts as a catalyst in the contact process with the aid of equations.
V2O5 +SO2 ——> V2O4 + SO3
V2O4 + 1/2O2 ——> V2O5
How is iron used in the haber process?
Iron is used as a heterogeneous catalyst
3H2 + N2 ——> 2NH3
Iron is in small lumps to increase its surface area
How do heterogeneous catalysts become poisoned?
By impurities which block the active sites and consequently have reduced efficiency
How are heterogeneous catalysts made more efficient?
Increased their surface area (by powdered form)
Spread the catalyst onto an inert support medium as this increases the surface to mass ratio
What are catalytic converters?
Found in exhausts of cars
Limit amounts of carbon monoxide and nitrogen monoxide released
Contain platinum rhodium and palladium transition metal catalysts
Have honey comb structure to increase the surface area
Explain with the aid of equations how Fe2+ catalyses the reaction between I- and S2O82-
Overall reaction:
S2O8^2- + 2I^- ——> 2SO4^2- + I2
Peroxodisulphate ions oxidise Fe2+ to Fe3+
S2O8^2- + 2Fe^2+ ——> 2SO4^2- + 2Fe^3+
Then Fe3+ oxidised the I- to I2 regenerating the Fe2+ ions used up
2Fe^3+ + 2I^- ——> 2Fe^2+ + I2
What is auto catalysis?
Where a product of the reaction becomes a catalyst and speeds up the rate of reaction.
Explain with the aid of equations how Mn2+ ions autocatalyse the reaction between C2O4^2- and MnO4-
2MnO4^- +16H^+ +5C2O4^2- —-> 2Mn^2+ + 9H2O + 10CO2
The catalyst Mn^2+ is not present at the begging of the reaction.
a Mn3+ intermediate is formed which reacts with C2O4^2- to reform the Mn2+
4Mn^2+ +MnO4^- + 8H^+ —-> 5Mn^3+ +4H2O
2Mn^3+ + C2O4^2- —-> 2CO2 + 2Mn^2+
