Inorganic Unit 2.5: Transition Metals Flashcards
define transition metal
an element that has an incomplete d sub-shell in either its atoms or its common ions
Ti-Cu
what are the properties of transition metals?
formation of complexes
formation of coloured ions/compounds
variable oxidation states
catalytic activity
define ligand
a molecule or ion that forms a co-ordinate bond with a transition metal by donating a lone pair of e-s
define complex
central metal atom/ion surrounded by ligands co-ordinately bonded
define co-ordination number
the number of co-ordinate bonds from ligands to the central metal atom/ion
define Lewis base
lone pair donor
define Lewis acid
lone pair acceptor
describe the formation of complexes
ligands form co-ordinate bonds with transition metal ion by donating their lone pair of e-s
describe the formation of complexes in terms of Lewis acid & Lewis base
ligands act as Lewis bases when hey bond to transition metals as they donate a lone pair of e-s to form a co-ordinate bond
the transition metal ion acts as a Lewis acid as is accepts this lon pai of e-s
what are examples of common ligands?
H2O (small)
NH3 (small)
Cl- (big)
table of common shapes of complexes
co-ordination number
shape
occurrence
example
see booklet
tetrahedral with large ligands (Cl-)
octahedral with small ligands (H2O & NH3)
what are the different types of ligands & define them
unidentate/monodentate - ligands which form 1 co-or. bond to a metal ion
bidentate - ligands which form 2 co-ord. bonds to a metal ion (form cycles)
multidentate - ligands which form more than 2 co-of. bonds to a metal ion
list unidentate ligands
:Cl-
:OH-
:CN-
H2O:
:NH3
give 2 bidentate ligands
1,2-diaminoethane (en) H2NCH2CH2NH2
ethanediote ion (C2O4^2-)
what makes a cycle (formed from bidentate ligands) less stable?
the further the bond nile is from 109.5, the less stable the cycle
give 2 multidentate ligands
EDTA4- forms 6 bonds
porphyrin forms 4 bonds e.g. in haemoglobin
describe the structure of haemoglobin & how does oxygen bind?
haem is an iron(II) complex with a multidentate ligand
haemoglobin has 4 haem groups (each with a porphyrin ligand taking up 4/6 co-ord. sites)
oxygen forms a co-ord. bond to Fe2+ in haemoglobin, enabling oxygen to be transported in the blood
why is CO toxic?
it replaces oxygen by forming a co-ord. bond to Fe2+ in haemoglobin
CO & CN- are better ligands than O2 so bond to haemoglobin in preference to oxygen
so they prevent O2 transport around the body
when does each type of stereoisomersim occur in complexes?
cis-trans isomerism can be displayed in:
octahedral complexes with monodentate ligands
square planar complexes
when there are 2 of the same ligands that are different to the other ligands
optical isomerism can be displayed in:
octahedral complexes with at least 2 bidentate ligands (often 3)
draw a pair of isomers for each type of isomerism
see booklet
what linear complex is used in Tollen’s reagent?
Ag+ forms [Ag(NH3)2 ]+
define ligand substitution/exchange
reaction where one ligand is replaced by another ligand
what happens in substitution of similar sized ligands & give example equations with Co2+ & Cu2+
H2O & NH3 are similar in size
no change in co-ordination number
see booklet for equations
what happens in ligand substitution reactions that are incomplete?
only some of the ligands are replaced
what happens in substitution of larger or smaller ligands & give example equations with Co2+, Cu2+, Fe3+
co-ordination number can change
e.g. Cl- ligands are larger than NH3 & H2O
only 4 Cl- ions can bond to most transition metals so co-ordination number goes from 6 to 4
what is the chelate effect?
substitution of ligands that form more co-ordinate bonds
bidentate & multidentate ligands replace monodentate ligands
describe the chelate effect
ΔH is negligible bc same number of same type of similar bonds are being broken & formed
when ligands are replaced by those that form more co-ord. bonds, there is a large increase in entropy (Δ S)
ΔG is -ve so ligand substitution where ligands are replaced by those that form more co-ord. bonds are feasible
what are chelating agents?
ligands that form more than one co-ordinate bond (often form many)
they bond easily to metal ion & are v difficult to remove —> so metal ion cannot bond to anything else so is harmless
what ligand is a good chelating agent & why?
EDTA4- as it forms 6 co-ordinate bonds
why are complexes coloured?
presence of ligands around central metal ion
causes split in energy b/w d orbitals
e-s absorb UV/visible light
so e-s are promoted from lower energy orbital to higher energy orbital
the observed colour is the frequency of light transmitted/reflected
when does colour arise?
when some wavelengths of light are absorbed & the remaining wavelengths are transmitted or reflected
what happens to e-s when light is absorbed?
d e-s move from the ground state to an excited state
what is the formula for the energy difference b/w ground state & excited state of the d -s
ΔE = hv = hc/λ
ΔE - energy gap
h - Planck’s constant
v - frequency of light
c - velocity of light
λ - wavelength of light
what factors affect the colour of a complex?
identity of metal
oxidation state of metal
identity of ligands
co-ordination number –> which influences the shape of the compound
what metal ions are not coloured & why?
Zn2+ as d sub-shell is full
Cu+ as d sub-shell is empty
Sc3+ d sub-shell is empty
so e-s cannot be promoted from the lower to higher energy d orbitals
how is the concentration of coloured ions in a solution determined?
colorimetry
how is the absorption of visible light using in spectroscopy?
frequencies at which a complex absorbs UV/visible light can be measured w a UV/visible light spectrometer
UV/visible light is passed through the complex & frequencies passing through/transmitted are detected
frequencies that do not pass through are absorbed
the more concentrated the solution, the more light is absorbed & the less light is transmitted
how does colorimetry work?
the more concentrated a solution, the more it absorbs
for some ions, a ligand is added to intensify the colour
a colour of light is chosen that the compound absorbs
the strength of absorption of a range of solutions with known concentrations is measured & calibration curve produced
the conc. of a solution with unknown conc. is found by measuring the absorption & reading off the calibration curve
what are common oxidation states of iron, manganese & copper?
Fe: +2, +3, +6
Mn: +2, +4, +6, +7
Cu: +1, +2
what are the uses of variable oxidation states?
testing for aldehydes:
Tollen’s reagent - [Ag(NH3 )2 ]+ is reduced
Fehling’s solution - Cu2+ reduced to Cu+
testing for alcohols:
acidified potassium dichromate K2Cr2O7
Cr2O72- –> Cr3+
Cr (+6) is reduced to Cr (+3)
redox titrations:
acidifies KMnO4 used to analyse Fe2+, C2O4- & H2O2
catalysis:
reactions catalysed by TM often involves TM changing oxidation state
what does ‘redox potential’ mean?
how easily it is to change the oxidation state of a TM
what does redox potential for a TM depend on?
pH & the ligand
how does pH affect redox potential of TM?
in general it is easier to:
oxidise a TM in alkaline conditions = oxidising agent in alkaline conditions promotes the higher oxidation state
reduce a TM in acidic conditions = reducing agent in acidic conditions promotes the lower oxidation state
how are the effect of ligand, effect of pH & effect of ligand & pH shown with electrode potentials?
see booklet
how is vanadium in ammonium vanadate(V) reduced?
reduced from V(+5) to V(+4) to V(+3) to V(+2) in acidic conditions (HCl or H2SO4) using zinc as the reducing agent
cotton wool inserted into neck of flask to keep air out to prevent reoxidation
what are the species present & the colour for each oxidation state of vanadium?
V(+5) - VO2 + - yellow
V(+4) - VO 2+ - blue
V(+3) - V 3+ - green
V(+2) - V 2+ - violet
in what conditions prevents V(+5) from being reduced to V(+2)?
alkaline conditions
redox titration calculations
see booklet & practice
what are the conditions for using potassium manganate as an oxidising agent & why can’t specific acids be used?
dilute H2SO4 is used (as S cannot be oxidised bc it is +6)
HCL cannot be used as MnO4- would also oxidise Cl- to Cl2 toxic gas which increases the volume of KMnO4 required in the titration
conc. H2SO4 & conc. HNO3 cannot be used as they are oxidising agents so decrease the volume of KMnO4 required in the reactions
ethanoic acid cannot be used as it is a weak acid so does not provide enough H+ ions
describe the use of acidifies KMnO4 in redox titrations
purple KMnO4/potassium manganate (VII) is in the burette & sample being analysed is in the flask with XS dilute H2SO4
as KMnO4 is added it reacts to form colourless Mn2+
at the end point, purple MnO4- do not react so end point is pink
titration is slef-indicating
what is the half equation for reduction of MnO4-?
MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O
dilute H2SO4 can be used with any oxidising agent to create acidic conditions
describe the analysis of Fe2+ in redox titrations & give half equation & overall equation
analysed in redox titrations with acidified KMnO4
half equation: Fe2+ –> Fe3+ + e-
overall equation: 5Fe2+ + MnO4- + 8H+ –> 5Fe3+ + Mn2+ + 4H2O
reacting ratio of Fe2+ to MnO4- is 5:1
what happens if iron is not in +2 oxidation state?
if as element Fe (0), it reacts with H2SO4 –> Fe2+ ready to analyse
if is it Fe3+ then it is reacted with zinc to reduce it to Fe2+ ready for analysis
describe the analysis of ethanedioate/C2O4- & give half equation & overall equation
analysed in redox titrations with acidified KMnO4
half equation: C2O42- –> 2CO2 + 2e-
overall equation: 5C2O42- + 2MnO4- + 16H+ –> 10CO2 + 2Mn2+ + 8H2O
the reacting ratio of C2O42- to MnO4- is 2.5:1 or 5:2
why is the reaction of C2O42- & MnO4- slow?
C2O42- & MnO4- are both negative so they repel each other
so reaction needs warming at the start of the titration
so takes some time for purple manganate (VII)
how does Mn2+ act as a catalyst (give equations)?
4Mn2+ + MnO4- + 8H+ –> 5Mn3+ + 4H2O
+ve & -ve attract so fast
2Mn3+ + C2O42- –> 2CO2 + 2Mn2+
+ve & -ve attract so fast
define autocatalysis
a reaction where one of the products is a catalyst
what is a catalyst & how do they work?
substance that increases the rate of a chemical reaction but is not used up in the reaction
catalysts provide an alternative route of lower activation energy
catalysts do not change ΔG, ΔH or ΔS
define heterogeneous catalyst
a catalyst that is in a different phase to the reactants
usually a solid & the reaction takes place at active sites on the surface of the catalyst
in Q, define heterogeneous & then define catalyst
how do heterogeneous catalysts work?
at least one of the reactants is adsorbed onto the active site on the surface of the catalyst
molecules can move about the surface, bonding to different active sites
define adsorb
form bonds to the atoms in the solid surface
how does the adsorption of reactants onto the surface of a catalyst increase the rate of reaction?
it concentrates the reactants i.e. brings them closer together than in gas phase
it can weaken some of the bonds in the molecule, making the reaction easier
it can position the molecule in favourable orientation
–> so increases # successful collisions per second
give 2 examples of industrial processes that involve heterogeneous catalysts
Haber process:
Fe catalyst
2H2 + N2 <–> 2NH3
Contact process
V2O5 catalyst
2SO2 + O2 <–>2SO3
what happens when adsorption is too weak?
not many molecules will be adsorbed so the catalyst has little effect
what happens when adsorption is too strong?
molecules cannot move around active sites so are less likely to collide with particles of another reactant so less likely to react
product often stays adsorbed to the surface of the catalyst
graph of adsorption strength of transition metals
see booklet
what is the effect of the catalyst having a large surface area?
less quantity of catalyst needed to produce the same effect/to increase the rate of reaction by the same amount
how is surface area of a catalyst maximised?
using a v thin coating of catalyst on a support medium
honeycomb structure often used
this minimises the cost
describe catalyst poisoning
other substances/impurities adsorb strongly to the surface of the catalyst, which blocks the active sites
this reduces the efficiency of the catalyst
the poisons are difficult to remove so catalyst is ruined, which can be costly
examples of catalyst poisoning
lead poisoning of catalytic converters in cars
Rh & Pt poisoned by lead
sulphur poisoning in the Haber process
hydrogen is obtained from natural gas, which can be contaminated with S
S poisons Fe catalyst
describe the Contact process
manufacturing H2SO4
SO2 –> SO3 is a slow, reversible reaction
catalysed by vanadium(V) oxide V2O5:
V2O5 + SO2 –> V2O4 + SO3
V2O4 + 1/2O2 –> V2O5
overall: SO2 + 1/2O2 –> SO3
define homogeneous catalyst
a catalyst that is in the same phase as the reactants
often happen in solution
what happens when catalysts & reactants are in the same phase?
the reaction proceeds through an intermediate species, which reacts further to form the product faster than the original reactant & to regenerate the catalyst
how does an acid homogeneous catalyst work?
e.g. esterification
- the acid protonates one of the reactants: X + H+ –> HX+
- the species produced (HX+) then reacts with the other reactant, giving the products & reforming the catalyst (HX+ reacts faster than X): HX+ + Y –> products + H+
how does a transition metal homogeneous catalyst work?
TM compounds can vary oxidation states, which allows them to act as catalysts
describe the reaction of I- + S2O82- & give the equations
2I- + S2O82- –> I2 + 2SO42-
reaction is slow in absence of catalyst bc 2 -ve ions repel so collision is difficult
catalysed by Fe2+(aq) or Fe3+(aq) bc it can easily change b/w the oxidation states
- 2Fe2+ + S2O82- –> 2Fe3+ + 2SO42-
Fe2+ reduces S2O82- to SO42-
Fe2+ is oxidised to Fe3+ - 2Fe3+ + 2I- –> 2Fe2+ + I2
Fe3+ oxidises I- to I2, reforming Fe2+
NB these reactions can happen either way round
describe autocatalysis by Mn2+ in the reaction b/w MnO4- & C2O42-
2MnO4- + 16H+ + 5C2O42- –> 2Mn2+ + 8H2O + 10CO2
slow without catalyst bc -ve ions repel
catalysed by Mn2+(aq) which can act as a catalyst bc it easily changes b/w oxidation states Mn(+2) & Mn(+3)
the catalyst is a product of the reaction - the reaction is slow until some Mn2+ is formed, then it speeds up
define autocatalysis reactions
reactions which produce their own catalyst
what is the equation for the catalysed reaction bc Mn2+ reducing MnO4- to Mn3+?
4Mn2+ + MnO4- + 8H+ –> 5Mn3+ + 4H2O
what is the equation for Mn3+ oxidising C2O42- to CO2, reforming Mn2+?
2Mn3+ + C2O42- –>2CO2 + 2Mn2+
draw & label the graph of rate vs time
does not start at 0
rate increases as [Mn2+ catalyst] increases
rate then decreases as [reactants] decreases
see booklet
