Unit 2: Section 4 - Transition metals CDS * Flashcards
transition metals - the basics complex ions more on complex ions formation of coloured ions substitution reactions variable oxidation states titrations with transition metals catalysts metal-aqua ions more on metal-aqua ions
what is a transition metal?
A transition metal is a metal that can form one or more stable ions with a partially filled d sub-shell
where can transition metals be found in the periodic table?
in the d - block
which elements in the period 4 d-block are transition metals?
all of them except scandium and zinc
remember chromium and copper only have 1 electron in their 4s orbital and all the rest have 2
what causes the special chemical properties of transition metals?
the incomplete d sub-shell
why isn’t scandium a transition metal?
scandium only forms 1 ion, Sc 3+, which has an empty d sub-shell
electron configuration of Sc 3+ = [Ar]
why isn’t zinc a transition metal?
it only forms 1 ion, Zn 2+, which has a full d sub-shell
electron configuration of Zn 2+ = [Ar] 3d^10
what happens when transition metals form ions?
the s electron are removed first, then the d electrons to form positive ions
what physical properties do transition metals have?
high density
high melting and boiling points
ionic radii are very similar
what chemical properties do transition metals have?
can form complex ions
form coloured ions
good catalysts
exist in variable oxidation states
what colours can vanadium ions be?
VO2 = yellow
VO 2+ = blue
V 3+ = green
V 2+= violet
what colour can chromium ions be?
Cr2O7 2- = orange
Cr 3+ = green (violet with water ligands)
what colour can manganese ions be?
MnO4 - = purple
Mn 2+ = pale pink
what colour can iron ions be?
Fe 3+ = yellow (purple with water ligands)
Fe 2+ = pale green
what colour can cobalt, nickel and copper ions be?
Co 2+ = pink
Ni 2+ = green
Cu 2+ = blue
why do transition metals have variable oxidation states?
the energy levels of the 4s and 3d sub - levels are very close to one another. so different numbers of electrons can be gained or lost using fairly similar amounts of energy
what is a complex?
a complex is a central atom or ion surrounded by coordinately bonded ligands
what is a coordinate (dative covalent) bond?
a covalent bond in which both electrons in the shared pair come from the same atom.
in a complex they come from ligands
what is a ligand?
an atom, ion or molecule that donates a lone pair of electrons to a central transition metal ion to form a co-ordinate bond
what is the coordination number?
the number of coordinate bonds that are formed with the central metal ion
when is the co-ordination number 6?`
when the ligands are small: H2O, NH3
shape: octahedral
bond angles: 90*
the O or the N donates the electrons
when is the co-ordination number 4?
if the ligands are larger: Cl-
shape: tetrahedral
bond angle: 109.5*
when is a complex square planar?
platinum or nickel centre
e.g. platinum centre with 2 NH3 ligands and 2 Cl - ligands
bond angle: 90*
when is the co-ordination number 2?
when the metal ion centre is silver
shape: linear
bond angle: 180*
what is the overall charge of a complex ion?
a.k.a. total oxidation state
the charge outside the square brackets of the complex
how to work out the oxidation state of the metal ion in a coplex?
total oxidation state - sum of the oxidation states of the ligands
what are monodentate ligands?
ligands that can only form 1 coordinate bond e.g. H2O, NH3, Cl-
what are bidentate ligands?
ligands that can form 2 coordinate bonds
they each have 2 lone pairs, so can form 2 co-ordinate bonds
e.g. NH2CH2CH2NH2
OOCCOO 2-
what are multidentate ligands?
ligands that can form more than 1 coordinate bond
e.g. EDTA 4- has 6 lone pairs to form 6 coordinate bonds
what is haemoglobin?
a Fe2+ ion bonded to a multidentate ligand (haem) hat forms 4 co-ordinate bonds from nitrogen
a globin protein acts as another ligand
the final ligand is either oxygen or water
how can carbon monoxide disrupt haemoglobin|?
the water ligand is swapped for a carbon monoxide ligand, which is very strong so doesn’t swap back for an oxygen
how can complex ions show optical isomerism?
it happens to octahedral complexes with 3 bidentate ligands
the ion can exist as 2 non-superimposable mirror images
what is cis-trans isomerism?
a type of stereoisomerism which is a special case of E/Z isomerism
how can cis-trans isomerism occur in octahedral complexes?
4 monodentate ligands of 1 type and 2 of another
cis = 2 odd ligands are next to each other
trans = 2 odd ligands are opposite each other
how can cis-trans isomerism occur in square planar complexes?
2 pairs of mondentate ligands
cis = the pairs are next to each other
trans = the pairs are opposite each other
what happens to the 3 d orbitals of transition metals when ligands bind?
some of the 3d orbitals gain energy and are split into different energy levels
electrons tend to occupy lower orbitals, to jump to higher energy levels they need energy equal to the energy gap from visible light
how can you work out the energy absorbed when electrons jump to a higher orbital in complex ions?
ΔE = hv = hc/λ v = frequency of light absorbed (Hz) h = Planck's constant (6.63 x 10 ^-34 Js) c = the speed of light (3 x 10 ^8 m/s) λ = wavelength of light absorbed (m)
what is the amount of energy needed for electrons to jump dependent on?
the central metal ion
its oxidation state
ligands
co-ordination number
what determines the colour of a complex ion?
the frequencies of light that are not absorbed to make electrons jump to higher orbitals are transmitted/reflected so you see them
when is a compound colourless?
if there are no 3d electrons or the shells are full, no electrons will jump, so no frequencies of light are absorbed
compound will look white or colourless
how can the colour of a complex be altered?
changes in oxidation state (Fe 2+ and Fe 3+)
changes in co-ordination number (octahedral to tetrahedral)
changes in ligand (H2O to NH3)
how can spectroscopy be used to find the concentration of a transition metal?
white light is shone through a filter
filter only lets light through that is absorbed by sample
light passes through sample to a colorimeter
the more concentrated the solution, the more light it absorbs
how to determine the concentration of a sample from its absorbance?
plot a calibration curve using the absorbance of known concentrations
use your absorbance to find the related concentration of the sample
what is a ligand substitution?
one ligand is swapped for another
what happens in ligand substitution if ligands are a similar size and same charge?
co-ordination number and shape don’t change
substitution may only be partial, so some of the old ligands are kep
what happens in ligand substitution if ligands are a different size?
co-ordination number and shape change
when can ligand substitutions not be reversed easily?
if the new ligands form stronger bonds than the old ligands with the central metal ion
if the new ligands are multidentate and the old ones are monodentate
what is the enthalpy change for a ligand exchange reaction?
co-ordinates bonds are broken and reformed, the strengths of bonds being broken is similar to the strengths of bonds being made, so enthalpy change is small
what is the chelate effect?
when monodentate ligands are substituted with bidentate or multidentate ligands, the number of particles in solution increases
the more particles the greater the entropy
increased entropy means reactions are more likely to occur, but the backwards reaction is not feasible
what are vanadium’s 4 oxidation states?
\+5 = VO2 + = yellow \+4 = VO 2+ = blue \+3 = V 3+ = green \+2 = V 2+ = violet
how are vanadium ions reduced?
by adding them to zinc metal in an acidic solution
e.g.
2VO2^+ + Zn + 4H+ –> 2VO^2+ + Zn^2+ + 2H2O
what is the redox potential of an atom or ion?
it tells you how easily it is reduced to a lower oxidation state
the same as electrode potentials
what does a large redox potential mean?
the larger the redox potential
the less stable the ion is
the more likely it is to be reduced
How does Tollens reagent work?
silver ions are easily reduced
[Ag(NH3)2]+ is colourless and is easily reduced with aldehydes to silver metal
how to carry out a titration with transition metals?
measure out quantity of reducing agent using pipette into conical flask
add dilute H2SO4 in excess
add oxidising agent using burette
when all reducing agent is used up, it will turn colour of oxidising agent
this is rough titration
do accurate titrations until concordant
what is the main oxidising agent used in titrations with transition metals?
aqueous potassium manganate (VII) which contain purple manganate (VII) ions
strong acidic conditions are needed for the manganate ions to be produced
how to balance titration questions for transition metals|?
potassium manganate (VII) = MnO4 -
add H2O to balance O’s
add H+ ions o balance H’s
then add electrons to balance charges
why do transition metals and their compounds make good catalysts?
they can change oxidation states by gaining or losing electrons within their d orbital.
this means they can transfer electrons to speed up reactions
what is the contact process?
vanadium oxide (V2O5) acts as a catalyst to oxidise SO2 to SO3
what is a heterogenous catalyst?
one that is in a different phase from the reactants
the reaction happens on active sites on the surface of the catalyst
how to increase rate of a heterogenous catalyst?
increasing surface area increases the number of molecules that can react at the same time, increasing rate of reaction
how do support mediums increase rate?
they are often used to make the area of a catalyst as large as possible
they help minimise the cost of reaction, because only small coating of catalyst needed to provide large SA
what are examples of a heterogenous catalyst?
iron in the Haber process for making ammonia
N2(g) + 3H2(g) –Fe(s)–> 2NH3(g)
vanadium (V) oxide in the contact process for making sulfuric acid
S02(g) + 1/2O2(g) –V2O5(s)–> SO3(g)
how do heterogenous catalysts work?
they adsorb reactants onto active sites located on their surfaces
what is catalyst poisoning?
impurities in the reaction mixture may also bind to the catalysts surface and block reactants from being adsorbed
what does catalyst poisoning do?
it reduces the surface area of the catalyst available to reactants, slowing down reaction
increases cost of chemical process because less product can be made in a certain time or with a certain amount of energy
catalyst may need replacing or regenerating - costs money
what is an example of a catalyst poison?
sulfur poisons the iron catalyst in the Haber process
what are homogenous catalysts?
they are in the same phase as the reactants, usually aqueous
how do homogenous catalysts work?
they combine with reactants to form an intermediate species which then reacts to form the products and re-form the catalyst
the catalyst always re-forms so it can keep catalysing the reaction
what does the enthalpy profile for a homogenously catalysed reaction do?
it has 2 humps, corresponding to the 2 steps in the reaction
activation energy to form intermediates is less than that to make products directly
why does the reaction between S2O8 2- and I - need to be catalysed?
they are both negative ions so repel each other, so are unlikely to collide and react
how is the reaction between S2O8 2- and I- catalysed?
S2O8 2- + 2Fe 2+ —> 2Fe 3+ + 2SO4 2-
2Fe 3+ + 2I- —> I2 + 2Fe 2+
Fe 3+ is the intermediate
what is an autocatalysis reaction?
when one of the products of the reactions acts as a catalyst for the reaction
e.g. Mn2+ in reaction between C2O4 2- and MnO4 -
what is the reaction between C2O4 2- and MnO4 -?
MnO4 - + 4Mn 2+ + 8H+ –> 5Mn3+ + 4H2O
2Mn 3+ + C2O4 2- —> 2Mn2+ + 2CO2
what is a lewis base?
lone pair donor (ligands)
what is a lewis acid?
lone pair acceptor (metal)
what are unidentate ligands?
they form 1 coordinate bond (H2O, OH-, NH3, CN-, Cl-)
what is cis platin?
very effective drug to fight cancer
binds to guanine in DNA to stop replication
(platinum centre with 2 NH3 ligands and 2 Chlorine ligands)
what uses do transition metals have?
testing for aldehydes - tollens reagent, Fehling’s solution
testing for alcohols - acidified K2Cr2O7
redox titrations - acidified manganate ions
catalysis - many reactions catalysed by transition metals
what factors affect how easily a transition metals oxidation can be changed?
pH
ligands
how does pH affect how easily a transition metals oxidation can be changed?
easier to oxidise a transition metal in alkaline conditions
easier to reduce a transition metal in acidic conditions
how do ligands affect how easily a transition metals oxidation can be changed?
[Co(H2O)6]2+ is easier to reduce than [Co(NH3)6]2+
when are ligands only partially substituted?
if [Cu(H2O)6]2+ is reacted with an excess of ammonia [Cu(NH3)4(H2O)2]2+ is formed
what happens when transition metal compounds dissolve in water?
water molecules form co-ordinate bonds with the metal ions.
this forms metal-aqua complex ions
6 H2O ligands
what happens when metal aqua ions with a charge are put in water?
a hydrolysis/ acidity reaction takes place
where an H2O is substituted for an OH and H3O+ is made
H+ ions are released = acidic solution formed
why do 3+ metal aqua ions form more acidic solutions than 2+ metal aqua ions?
metal 3+ ions are small but have a big charge = high charge density
2+ ions have lower charge density
3+ are more polarising than 2+
so they attract electrons from oxygen more strongly, so its more likely that H+ is released
what happens when complexes with water ligands are added to water?
an OH- ion is substituted for an H2O and H3O+ is formed
as more OH- is added this keeps happening until the charges balance and an insoluble uncharged metal hydroxide is formed (precipitate)
how do metal hydroxides act as bases?
they dissolve in acid
they accept H+ ions
reverses hydrolysis to form water ligand complex
how do metal hydroxides act as acids?
some metal hydroxides are amphoteric
they can donate H+ ions in the presence of of a base, forming a soluble compound with OH- ligands
when do metal ion complexes with water ligands form precipitates?
in the presence of:
OH- ions
ammonia
sodium carbonate
what happens when ammonia dissolves in water?
NH3 + H2O ———> NH4 + + OH
what happens when ammonia is added to a complex with water ligands?
it has the same effect as adding OH
it acts as a base
[M(H2O)6] + 3NH3 —>[M(OH)3(H2O)3] + 3NH4 +
what happens when excess ammonia is added to [Cu(H2O)6]2+?
[Cu(H2O)6]2+ + 4NH3 –> [Cu(NH3)4(H2O)2]2+ + 4H2O
what happens when sodium carbonate reacts with metal 2+ ions?
they form insoluble metal carbonates
[M(H2O)6]2+ +CO3 2- —-> MCO3 + 6H2O
what happens when sodium carbonate reacts with metal 3+ ions?
they are stronger acids, so there is a higher concentration of H+ in solution, they react with carbonate ions
CO3 2- + 2H+ CO2 + H2O
so [M(OH)3(H2O)3] precipitate and CO2 formed
what happens to the metal aqua ions when sodium hydroxide is added?
all 4 form precipitates
aluminum hydroxide will dissolve in excess hydroxide because it is amphoteric
[Al(H2O)6]3+ +3OH- —> [Al(OH)3(H2O)3] +3H2O
[Al(OH)3(H2O)3] + 3OH- —> [Al(OH)6]3- + 3H2O
what happens to the metal aqua ions when ammonia is added?
all 4 form precipitates
copper hydroxide precipitate will dissolve in excess ammonia, because it undergoes ligand substitute
[Cu(H2O)6]2+ + 2NH3 —> [Cu(OH)2(H2O)4] + 2NH4 +
[Cu(OH)2(H2O)4] + 4NH3 —> [Cu(NH3)4(H2O)2]2+ + 2OH- + 2H2O
what happens to the metal aqua ions when sodium carbonate is added?
all 4 metal ions form precipitates with sodium carbonate.
solutions containing Al3+ or Fe3+ will also form bubbles as CO2 is formed
2[Al(H2O)6]3+ +3CO3 2- —> 2[Al(OH)3(H2O)3] + 3CO2 + 3H2O
what happens in the test tube reactions with [Cu(H2O)6]2+?
[Cu(H2O)6]2+ = blue solution NH3/OH- = blue precipitate excess OH- = no change excess NH3 = deep blue solution CO3 2- = green-blue precipitate conc. HCl = green solution
what happens in the test tube reactions with [Fe(H2O)6]2+?
[Fe(H2O)6]2+ = green solution NH3/OH- = green precipitate excess OH- = no change excess NH3 = no change CO3 2- = green precipitate conc. HCl = yellow solution
what happens in the test tube reactions with [Al(H2O)6]3+?
[Al(H2O)6]3+ = colourless solution NH3/OH- = white precipitate excess OH- = colourless solution excess NH3 = no change CO3 2- = white precipitate + CO2 bubbles conc. HCl = colourless solution
what happens in the test tube reactions with [Fe(H2O)6]3+?
[Fe(H2O)6]3+ = yellow solution NH3/OH- = brown precipitate excess OH- = no change excess NH3 = no change CO3 2- = brown precipitate + CO2 bubbles conc. HCl = yellow solution
what happens when Fe(OH)2(H2O)4 is left in the air?
the green precipitate will turn brown
