Transition metals Flashcards
What is a transition metal?
A transition metal is a metal that can form one or more stable ions with an incomplete d sub-shell
What are the physical properties of transition metals?
- high density
- high melting and boiling points
- ionic radii is more or less the same
What are the chemical properties of transition metals?
- complex formation
- formation of coloured ions
- variable oxidation state
- catalytic activity
What is a ligand?
A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons
Why are zinc and scandium not transition metals?
Their stable ions do not have incomplete d sub-shells
Scandium has an empty d shell
Zinc has a full d shell
What is a complex?
A complex is a central atom or ion surrounded by ligands
What is the co-ordinate number?
It is the number of coordinate bonds to the central metal atom or ion
Give three examples of monodentate ligands
NH3
H2O
Cl-
Why is there not a change in the coordination number when NH3 and H2O are substituted?
They are similar in size and are uncharged
Give two examples of bidentate ligands
Ethane-1,2-diamine(en)
Ethanedioate
Give an example of a multidentate ligand
EDTA(4-)
What is haem?
It is an iron(II) complex with a multidentate ligand
How is oxygen transported in the blood
Oxygen forms a coordinate bond with Fe(II) in haemoglobin
Why is CO toxic to humans?
It forms a stronger coordinate bond with Fe(II) compared to oxygen. Therefore, it replaces the oxygen attaching to the haemoglobin
What is the chelate effect?
The substitution of a monodentate ligand with a bidentate or multidentate ligand which leads to a more stable complex
Explain the chelate effect in terms of enthalpy and entropy change
The enthalpy change is small a there is a similar number of bonds being made and broken
The entropy change is positive
Therefore, the free energy is negative
Which shapes can be formed by complex ions?
Octahedral
Tetrahedral
Square planar
Linear
Which complexes show cis-trans isomerism?
Square planar
Octahedral
Which complexes show optical isomerism?
Octahedral
Which factors lead to the colour change in transition metals?
Oxidation state
Coordination number
Ligands
How does colour arise in coloured ions?
Colour arises from electronic transitions from the ground state to excited states.
A portion of visible light is absorbed and the light that is transmitted gives the coloured ion its colour.
Give the equation that links the energy difference between the split d orbitals to the frequency of light absorbed
E=hv
E is the energy split
h is Planck’s constant
v is the frequency of the light absorbed
Why does Sc3+ have no colour?
It has no electrons in the d shell. Therefore, there is no energy transfer.
Why do Zn+2 and Cu+ ions have no colour?
The ions have full d shells. There is no space for electrons to transfer, therefore, there is no energy transfer.
How can a simple colourimeter be used to determine the concentration of coloured ions in a solution?
When visible light is passed through a sample of a coloured complex ion, some of the light is absorbed.
The amount of light absorbed is proportional to the concentration of the absorbing species.
Which factors influence the redox potential for a transition metal ion changing from a higher to a lower oxidation state?
pH
Ligands
What is the oxidation state of vanadium in VO2(+) and the colour of the solution?
+5
yellow
What is the oxidation state of vanadium in VO(2+) and the colour of the solution?
+4
blue
What is the oxidation state of vanadium in V(3+) and the colour of the solution?
+3
green
What is the oxidation state of vanadium in V(2+) and the colour of the solution?
+2
violet
What is the observation when [Ag(NH3)2]+ is used to distinguish aldehydes and ketones?
Aldehydes- silver mirror
Ketones- no visible change
Why is an indicator unnecessary for the MnO4- and Fe2+ redox titration?
The titration is self indicating because of the significant colour change
What is the colour change for the MnO4- and Fe2+ redox titration?
Purple to colourless
Give the equation for the MnO4- and Fe2+ redox titration
MnO4-(aq) + 8H+(aq) + 5Fe(2+)(aq) → Mn2+(aq) + 4H2O(l) + 5Fe(3+)(aq)
Which acid can only be used for manganate titrations?
dilute sulfuric acid
What will happen when insufficient volumes of sulfuric acid are used?
the solution is not acidic enough
MnO2 will be produced instead of Mn2+
the brown MnO2 will mask the colour change leading to a greater inaccurate volume of manganate being used
Why can conc. HCl not be used in manganate titrations?
Cl- ions will be oxidised to Cl2
a greater volume of manganate used
poisonous Cl2 gas produced
Why can nitric acid not be used?
It is an oxidising agent
It oxidised Fe2+ to Fe3+
a smaller volume of manganate used
Give the overall equation for the manganate titration with hydrogen peroxide
2MnO4(-)(aq) + 6H+(aq) + 5H202 → 5O2 + 2Mn(2+)(aq) + 8H2O(l)
Give the overall equation for the manganate titration with ethanedioate
2MnO4(-)(aq) + 16H+(aq) + 5C2O4(2-) → 10CO2 + 2Mn(2+)(aq) + 8H2O(l)
What is a heterogeneous catalyst?
It is in a different phase from the reactants
What is a homogeneous catalyst?
It is in the same phase as the reactants
What are the steps in heterogeneous catalysis?
Reactants are adsorbed at active sites on the surface of the catalyst
The bonds in the reactants weaken and break
New bonds are formed between the reactants
Then the product desorbs from the catalyst surface
How is the surface are of a solid catalyst maximised?
A support medium is often used to maximise the surface area and minimise cost
Give two reactions which show how V2O5 is used as a catalyst in the Contact Process
SO2 + V2O5 → SO3 + V2O4
V2O4 + 0.5O2 → V2O5
How can the efficiency of heterogeneous catalysts be reduced?
Impurities block the active sites
Why can transition metals act as homogeneous catalysts?
they can form various oxidation states
they are able to oxidise and reduce because of their partially filled d-shells
Explain, with the aid of equations, how Fe2+ ions catalyse the reaction between I- and S2O8(2-).
S2O8(2-) + 2Fe(2+) → 2SO4(2-) + 2Fe(3+)
2I- + 2Fe(3+) → 2Fe(2+) + I2
Explain, with the aid of equations, how Mn2+ ions catalyse the reaction between MnO4(-) and C2O4(2-).
4Mn(2+) + MnO4(-) + 8H+ → 5Mn(3+) + 4H2O
2Mn(3+) + C2O4(2-) → 2Mn(2+) + 2CO2
Why is the uncatalysed reaction slow?
The reaction involves a collision between two negative ions which repel each other leading to a high activation energy
