Transition metals Flashcards
What is a transition element?
a partially filled d-subshell
What are the characteristics of transition metals
- complex formations
- variable oxidation states
- form coured ions
- catalytic activity
What are complex formations?
when a transition metal ion is surrounded by ligands joined by co-ordinate bonds
What is a ligand?
a molecule/ion that forms coordinate bond with a transition metal atom/ion
- electron pair donor
What is a complex?
a central metal atom/ion surrounded by ligands joined by coordinate bonds
What is a co-ordinate number?
the number of coordinate bonds to the central metal atom/ion surrounded
What is a coordinate bond
covalent bond where both electrons come from one atom
How many bonds are usually bonded to transition metals
2,4 or 6
When will the shape of the coordinate bond and transition metal be linear?
When the complex is Ag (silver)
- coordinate number is 2
When will the shape of the coordinate bond and transition metal be Square planar?
When the complex is Pt2+ (platinum) or Ni2+ (Nickle)
- coordinate number is 4
When will the shape of the coordinate bond and transition metal be Tetrahedral?
when ligands are too big to fit 6 e.g Cl-
- coordinate number is 4
When will the shape of the coordinate bond and transition metal be Octahedral?
Most of the time as this is the most common
What is a monodentate ligand?
forms one coordinate bond to the TM
Why does the complex ion of chloride ligands have a different shape than H20?
Cl- is too big to fit more than 4 around one ion
How do you draw a complex?
- draw and name the shape
- deduce the bind angle and oxidation state
- give the coordination number
When do cis-trans isomerisms occur?
in octahedral and square planar complexes where theres two ligands of a differente
What is a Cis isomer?
atoms are on the same side
90° apart
What is a Trans isomer?
atoms are on opposite sides
180° apart
What is a bidentate ligand?
forms two coordinate bonds to a transition metal ion thru two different atoms on the same ligand
- only show octahedral shape
how do bidentate ligands bond to the TM?
two lone pairs must come from separate atoms
What is the formula for ethanedioate ions?
C2O4 ^2-
What are the two examples of bidentate ligands
- 1,2-diaminoethane
- ethanedioate ions
What is the formula for 1,2-diaminoethane?
NH2CH2CH2NH2
which complexes will show optical isomerisms
octahedral complexes formed from bidentate ligands
What is a multi dentate ligands?
ligand that can form two or more coordinate bonds to a TM
- max number of bonds to 1 ligand is 6
- ligand must have 6 lone pairs on different atoms
What is the general formula for a multidentate complex
M2+
[M(EDTA)]2-
What is the main ligand that forms multidentate ligands
EDTA ^4-
Why is EDTA most effective in alkaline conditions
- OH- will react with hydrogen
- equilibrium will move to the right to restore H+
What are the uses of EDTA?
medicine
complexometric titrations
How is EDTA used in medecine?
chelation therapy
used to treat patients suffering from lead poisoning
EDTA makes the toxic ions present in the body harmless
How is EDTA used in complexometric titrations?
- used to calculate the conc of metal +ve ions in aw solutions
- all metal EDTA complexed have a 1:1 ratio
- indicator (e.g organic dyes) is added to determine the end point
-indicator shows one colour as a ligand in a complex with the TM and another colour when not in a complex
Outline an EDTA complexometric titration
- EDTA from a burette is added to the metal ion aqeous solution until the indicator changes colour
- at endpoint EDTA will have displaced all the indicator ligands due to the chelate effect and the colour of the free indicator is observer
- all metal EDTA complexes have a 1:1 ratio so the moles are = to the metal ion
What is haemoglobin
an iron complex which is used to transport oxygen around the body
What is contained in the structure of Haemoglobin
Fe2+ as the central metal ion
Porphyrin ring - a multidentate ligand (4 coordinate bond leaving 1 space to “carry” a ligand)
What is globin
a protein that acts as a monodentate ligand with a lone pair from a N atom
How does O2 act as a multidentate ligand
when oxygenated O2 acts as a monodentate ligand
O2 is easy to pick up in O2 rich lungs
What effect does Carbon monoxide have on Hb
When CO is inhaled its forms a coordinate bond with the Fe2+ instead of O2
(It forms a much stronger bond w/ Hb)
This prevents the transportation of oxygen around the body
What is a ligand substitution reaction?
ligands on a complex can be substituted for other ligands
What are examples of ligand substitution reactions
- NH3 ligands replace H2O
either **all* ligands are replaced or just some - 4 Cl- ligands replace H2O leaving 6H2O
- Bidentate ligands can replace monodentate ligands
What is the Chelate effect
process where bidentate or multidentate ligands substitute monodentate ligands to create a more stable complex
- forms a stable multidentate ligand compelx by a ligand substitution reaction
Why is the product formed from the Chelate effect more stable?
there’s been +ve ∆S
(more moles of the product than reactants)
What are Chelating agents?
Multidentate ligands are often used as Chelating agents
- to remove harmful metal ions and render them useless
What oxidation states does Vanadium have in its compounds?
+5, +4, +3 and +2
How is Vanadium (V) reduced to Vanadium (II) using Zinc
- Ammonium metavanadate (NH4VO3) is dissolved in Sodium hydroxide solution
- Acid is added to ensure that Dioxovanadium (V) ion - VO2- is the main ion present
- transfer to a small conical flask and ass small amount of zinc powder and HCl
- use cotton wool stopper in neck of flask to allow H2 gas to escape and it also minimises the reoxidation from O2 in air
- swirl slowly to mix
- colour will form
What colours may form from the reduction of Vanadium (V)
Yellow - VO2+
(oxidation state = +5, V+5)
Blue - VO^2+
(oxidation state = +4, V+4)
Green - [V(H2O)6]3+
(oxidation state = +3, V+3)
Violet - [V(H2O)6]2+ (oxidation state = +2, V+2)
How is Vanadium(II) oxidised
remove the cotton wool from the flask and pour some solution into the test tube
it will turn green because of its contact with O2 in the air
- oxidised back to Vanadium(III)
What colour and ions does Vanadium(V) form
Yellow
VO2+
Oxidation state = +5
What colour and ions does Vanadium(IV) form
Blue
VO^2+
Oxidation state = +4
What colour and ions does Vanadium(III) form
Green
[V(H2O)6]3+ / (V3+)
Oxidation number = +3
What colour and ions does Vanadium(II) form
Violet
[V(H2O)6]2+ / (V2+)
Oxidation number = +2
What happens if Vanadium(III) is allowed to stand for a long time
if allowed to stand for a long time solution eventually turns blue as air oxidises it back to Vanadium(IV) state (VO^2+) ions
What happens if nitric acid is added to Vanadium(III)
nitric acid is a powerful oxidising agent
adding it to Vanadium(II) produces VO2+ ions
The vanadium(II) is again oxidised back into Vanadium(IV)
Why is an indicator not needed for a redox titration with MnO4-
MnO4- is self indicating and causes a distinctive colour change
Outline a redox titration with MnO4-
- purple MnO4- ions are added to the conical flask and will oxidise the flasks content
- as MnO4- ions are being reduced to Mn2+ the purple colour disappears as Mn2+ is formed so the solution is colourless
- when all the content of the flask has been oxidised the flake content turns pale pink on the next addition of MnO4- as it is now in excess
What is the function of H2SO4 in a redox titration with MnO4-
to provide H+ ions
if HCl is used the MnO4- will oxidise the Cl- ions
What is the aim of the redox titration of Iron(II) ions and Potassium magnate (VII)
to determine the percentage by mass of iron in a sample of fertiliser
Outline a redox titration of Iron(II) ions and Potassium magnate (VII)
- potassium magnate (VII) is added from a burette into the acidified iron salt solution and is immediately decolourised
- when all the iron (II) is used up the next drop of potassium magnate isn’t decolorised and the solution in the conical flask becomes pale pink
- the end point is the first appearance of the pale pink colour
- neutral solutions of iron (II) salts are oxidised in air to prevent this, the solution is acidified
What causes colour changes in transition metals?
oxidation state
coordination number
ligand
Why do colour changes occur in translation metals
electron promotion
due to the partially filled d sub levels
electrons sitting in this sub level can shift up and down between the un occupied orbitals
Outline electron promotion
- d orbitals split energy levels
- d electrons are promoted to higher energy d orbitals by absorbing energy in the form of light
- the colour transmitted is the light not absorbed
What is an energy gap?
d orbitals are at specific energy values therefore the gap between them has a specific energy gap value ΔE
an electron must be given at least the ΔE value to be promoted from the ground state to the excited state
How are electrons in a transition metal promoted
The TM must absorb a specific frequency of light
What are the 2 equations for ΔE
ΔE = hv
ΔE = hc/λ
What is ‘v’ and its units
the frequency of the wave of light absorbed
s-1
What is ‘c’ and its units
the speed of light
3.00 x 10-8 ms-1
What is λ and its units
wavelength of light
m
What are the units for ΔE
Joules
How do you change wavelength from nanometres (nm) to meters (m)
times by 10^-9
Why are aqueous cobalt ions always pink
d orbitals split in energy levels
d electrons ate promoted to higher energy d orbitals by absorbing energy in the form of light
pink light is not absorbed it is transmitted and observed
why do complexes of magnesium aluminium and zinc all per colourless in solution?
They don’t have partially filled d sub levels
What changes colour
Changing colour arise due to the size of ΔE between the d orbital’s changing
If ΔE changes different frequencies of light will be absorbed and different frequencies of light will be transmitted (then observed) through the sample
What can cause a change to ΔE thus change the colour of the complex
- Ligands bonded to the TM on
- Coordinate number
- Oxidation state
What is a colorimeter for
used to measure the absorbance of light in the visible and uv region
- contains a coloured filter. The colour of the filter is chosen to allow wavelengths of light that would be most strongly absorbed by the coloured solution
What is the relationship with light absorbed and the complex ion
The amount of light absorbed is proportional to the concentration of the complex ion
How could the colour of a complex be intensified?
Add a suitable ligand to intensify the colour
how can you determine the concentration of a transition metal ion?
- add an appropriate ligand to intensify the colour
- set the colorimeter wavelength to λmax
- make a series of standard solutions of known concentrations of the metal ion
- Measure the absorbance of the standard solutions
- Plot a graph of absorbance v. concentration (calibration graph)
- Measured the absorbance of the unknown solution and determine its concentration from the calibration graph
what is a catalyst?
A substance that increases the rate of reaction but isn’t used up
How do catalysts work?
Provide an alternative route with a lower activation energy
How does a catalyst affect a reaction at equilibrium?
catalyst has no effect on the position of equilibrium as it increases the rate of the reaction forwards AND backwards equally
It only decreases the time taken to reach equilibrium
What are the properties of a catalytic converter
Platinum Pallidium or Rhodium are sprayed over a ceramic honeycomb structure to limit costs and maximise surface area
What is a heterogeneous catalyst?
catalyst which are in a different state to the react
The catalyst is usually a solid and the reaction takes place on the surface
What are examples of heterogeneous catalytic processes?
Hebrew process – industrial production of ammonia
Contact process – for making sulphuric acid (most common) do you think about the day that we sat down smoking? I don’t drinking hazel the other way is it the other way?
Catalytic converter
how do heterogeneous catalysts work?
- reactants are adsorbed onto active site on the catalyst surface.
- reaction occurs with a low activation energy as bonds of weed or new buns are made between reactants held close together.
- The products are desorbed (the surface).
how can adsorption resort in an increased reaction?
- Adsorption onto the surface concentrates the reactants so increase in the likelihood of collision
- May weaken some of the bonds in a molecule making a reaction easier
- Positions the molecule in a favourable orientation for reaction
what happens if adsorption is too strong?
not many molecules will be adsorbed so the catalyst will have very little effect
what happens if adsorption is too weak?
Molecules won’t be able to move around in the active sites so will be less likely to meet another reactant therefore be less likely to react
what is adsorption?
going onto something
how can you maximise the efficiency of a surface catalyst?
- Maximising surface area has important cost savings
- Surface area is maximised by using a thin coating of the catalyst on a support medium
What is the equation for the Haber process?
N2 + 3H2 ⇌ 2NH3
Fe is a catalyst
what is the overall equation for the contact process?
SO2 + 1/2O2 > SO3
Catalyst - Vanadium(V) oxide / V2O5
what is the equation of the first step of the contact process?
V2O5(g) + SO2(s) > SO3(g) + V2O4(s)
What is the second step of the contact process?
regeneration of catalysts
V2O4 + 1/2O2 > V2O5
How does catalyst poisoning occur?
other substances adsorb strongly onto the surface blocking the active sites
- Lowers the efficiency of the catalyst/makes it ineffective depending on the extent of the poisoning
- poisons are very difficult to remove and the catalyst is ruined which can be costly
How does lead poisoning occur
The radium and platinum catalysts are poisoned by lead from leaded petrol this is very expensive to replace
how does sulphur poisoning occur?
In the Haber process the hydrogen is obtained from natural gas which is contaminated by sulphur if not removed this will poison the Fe catalyst
Sulphur is added to natural gas to give it an older so leaks can be smelt
What is a homogeneous catalyst?
catalyst which are in the same state as the reactants
Most reactions involving a homogeneous catalyst take place in solution.
The alternative reaction pathway involves formation of an intermediate – with a different oxidation state
Why can transition metals act as a catalyst?
Because of the variable oxidation states
outline the equation for the reaction between iodide ions and Persulfate ions
(homogeneous catalytic process)
S2O8 ^2-(aq) + 2I-(aq) > 2SO4 ^2-(aq) + I2(aq)
What is an example of a homogeneous catalytic process?
reaction between iodide ions and persulphate ions
has a high activation energy as both reactants are negative so they repel
why will the reaction with iodide ions and persulfate ions be faster with Fe2+ ions
Opposite charges on the ions attract and the activation energy is lowered
Why does the reaction between iodide ions and persulfate ions have a high activation energy
both reactants are negatively charged so will repel
In a reaction with iodide ions and persulfate ions what is the reducing agent
I-
In a reaction with iodide ions and persulfate ions what is the oxidation agent
S2O8^2-
What is the first step of the equation with iodide ions and persulfaste ions with Fe as a catalyst
S2O8^2- +2Fe^2+ > 2SO4^2- +2Fe^3+
What is the second step of the equation with iodide ions and persulfaste ions with Fe as a catalyst
2Fe^3+ + 2I- > 2Fe^2+ + I2
What is the overall equation between ethane dioate ions and magnate (VII) ions with Mn2+
2MnO4- + 5CrO4^2- +16H+ > 2Mn^2+ + 10CO2 + 8H2O
Why is the reaction between ethane dioate ions and Maganate (VII) ions very slow without a catalyst
Both reactants are negative and repel eachother
Why can Mn2+ act as a catalyst
it has variable oxidation states
- Mn2+ and Mn3+
How does Mn2+ work as a catalyst
its a product of the reaction
- the initial reaction is slow until Mn2+ is formed then its sped up
What does a normal concentration/time graph look like
a curve (downwards)
- reaction starts quickly and the conc of reactants is high
- reaction then slows down snd eventually stops as reactants are used up
What does an auto catalyst
occurs when one of the products from a reaction is a catalyst of the reaction
- the reaction starts out slowly and as the concentration of the catalyst product increases the rate also increases
What does an auto-catalysed concentration graph look like
a slope (s shape)
- reaction starts slowly then rate increases as the catalyst is produced
- then it slows down and stops
