2.4 transition metals Flashcards
what is a transition metal?
a metal that can form one or more stable ions with an incomplete d sub-level
which transition metals do you need to know?
period 4 in d-block (titanium to copper)
exceptions to normal electron configuration rules
chromium- prefers to have 1 electron in each orbital of 3d and 1 in 4s so more stable
copper- prefers to have full 3d and 1 in 4s so more stable
why arent Sc and Zn transition metals?
their stable ions dont have incomplete d subshell
Sc only forms Sc3+ which has empty d subshell
Sc is [Ar] 3d1 4s2 so Sc3+ is just [Ar]
zinc only forms Zn2+ which has full d subshell
Zn is [Ar] 3d10 4s2 so Zn 2+ is [Ar] 3d10
physical properties of transition metals
all have high density
all have high melting and boiling points
chemical properties of transition metals
can form complex ions
form coloured ions
good catalysts
exist in variable oxidation states
Ti oxidation states and colours
Ti2+ = +2, violet
Ti3+ = +3, purple
V oxidation states and colours
V2+ = +2, violet
V3+ = +3, green
VO^2+ = +4, blue
VO2^+ = +5, yellow
Cr oxidation states and colours
Cr3+ = +3, green
Cr2O7^2- = +6, orange
CrO4^2- = +6 yellow
Mn oxidation states and colours
Mn2+ = +2, very pale pink/ colourless
MnO4^2- = +6, green
MnO4^- = +7, purple
Fe oxidation states and colours
Fe2+ = +2, pale green
Fe3+ = +3, yellow
Co oxidation states and colours
Co2+ = +2, pink
Ni oxidation states and colours
Ni2+ = +2, green
Cu oxidation states and colours
Cu2+ = +2, pale blue
why do transition metals show variable oxidation states?
the energies of the 4s and 3d subshells are very similar so diff numbers of electrons can be gained/ lost using fairly similar amounts of energy
what is a complex ion?
a metal surrounded by co-ordinately bonded ligands
eg [Cu(H2O)6]2+
what is a co-ordinate bond?
covalent bond in which both electrons in the shared pair come from the same atom
what is a ligand?
an atom/ ion/ molecule that donates a pair of electrons to a central metal ion
what is a monodentate ligand?
a ligand that can only form one co-ordinate bond
eg NH3, Cl-, H2O
what is a multidentate ligand?
a ligand that can form more than one co-ordinate bond
eg EDTA^4-
what are bidentate ligands?
ligands that can form 2 co-ordinate bonds to a central metal ion
eg NH2CH2CH2NH2
how can you work out the oxidation state of a metal ion in a complex?
total charge of complex - sum of charges of ligands
what is co-ordinate number?
what does this determine?
number of co-ordinate bonds formed with central metal ion
determines the shape of the complex ion
what is the shape of a complex ion with 6 coordinate bonds?
octahedral
often hexaaqua (6 water ligands)
what is the shape of a complex ion with 4 coordinate bonds?
tetrahedral
but in a few, may be square planar
eg (PtCl2(NH3)2)
what is the shape of a complex ion with 2 coordinate bonds?
linear
eg [Ag(NH3)2]+
why can you fit 6 H2O or NH3 ligands around a central metal ion but only 4 Cl- ligands?
because H2O and NH3 ligands are small and Cl- ligands are larger
when does optical isomerism occur in complex ions?
when it can occur as 2 non-superimposable mirror images
happens in octahedral complexes where 3 bidentate ligands are attached to the central ion
what is cis-trans isomerism?
occurs in square planar complex ions that have 2 pairs of ligands
trans- 2 paired ligands are directly opposite each other
cis- 2 paired ligands are next the each other
what is cisplatin?
complex of platinum(II) with 2 chloride ions and 2 ammonia molecules in a square planar shape
what can cisplatin be used for?
treat some types of cancer
prevents cancer cells reproducing (more info in booklet?)
downside of using cisplatin?
prevents some healthy cells from reproducing eg blood and hair
causes hair loss, supressed immune system, kidney damage
how can octahedral complex ions show cis-trans isomerism?
octahedral complexes with 4 ligands of 1 type and 2 of another
trans- 2 odd ligands opposite each other
cis- 2 odd ligands next to each other
what happens to energy when ligands bond?
normally 3d orbitals of transition metals have same energy
ligand binds- some orbitals have more energy than others so 3d orbitals are split into diff energy levels
what is the energy gap (ΔE)?
energy absorbed when electrons jump from lower orbitals (ground state) to higher orbitals (excited state)
this energy comes from visible light
what is the equation for ΔE?
ΔE= hv = hc/λ
ΔE= energy absorbed (J)
h= planks constant (Js)
v= freq of light absorbed (Hz)
c= speed of light (ms-1)
λ= wavelength of light absorbed (m)
what affects the size of the energy gap/ energy needed?
- central metal ion
- oxidation state of ion
- ligands
- coordination number
state the origin of the colour of transition metal complexes
- d orbital electrons are excited from lower energy orbitals to higher energy orbitals
- the energy absorbed for this d-d transition is from the visible light range
what does the frequencies absorbed by transition metal ions depend on?
what happens to the other frequencies?
depends on the size of the energy gap
rest are reflected and combine to make the compliment of the absorbed frequencies (makes the colour we see)
what happens if there are no 3d electrons or the 3d sub-level is full?
no electrons jump so no energy absorbed so all light reflected so compound appears white
what can change the colour of a complex?
changes in oxidation state of the transition metal
changes in coordination (involves change of ligand too)
changing the ligand (even if oxidation state and coordination number stay the same)
how do you find the concentration of a substance using calorimetry?
add a ligand to intensify the colour
pick a filter of a complimentary colour
measure absorbance of solutions of varying known concs
measure absorbance of unknown solution
produce calibration curve of conc on x and absorbance on y
use calibration curve to find conc of solution of unknown conc
benefits of using spectroscopy to measure conc of coloured ions
easy to get lots of readings
work out conc quickly
measure very low concs
doesnt use up any of the substance/ interfere with any reactions
what is a ligand substitution reaction?
one ligand being swapped for another ligand
usually causes colour change
what happens in ligand substitution of similarly sized ligands?
eg H2O and NH3
coordination number of complex ion doesnt change
shape doesnt change
what happens in ligand substitution of different sized ligands?
change of coordination number
change of shape
eg [Cu(H2O)6]2+ + 4Cl- -> 6H2O = [CuCl4]2-
octahedral-> tetrahedral as Cl- is larger than H2O
what is partial substitution of ligands?
not all of the 6 H2O ligands are substituted
eg [Cu(H2O)6]2+ + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 4h2o
what is haemoglobin?
protein in blood that transports oxygen around body
contains Fe2+ ions which are hexacoordinated (6 lp donated to them to form 6 coordinate bonds)
4lps come from N atoms which form circle around Fe2+ (haem)
porphyrin- multidentate ligand that the 4 N atoms are part of
globin and either oxygen/ water mol binds to Fe2+ to form octahedral structure
what happens to haemoglobin in the body?
H2O and O2 bind to Fe2+ ions as ligands so can transport O2 then swap for H2O
lungs- high O2 conc, water ligand substituted for O2 to form oxyhaemoglobin
vice versa
why is carbon monoxide poisonous
haemoglobin can sub water ligands for CO ligands to form carboxyhaemoglobin
CO forms very strong bond w Fe2+ and doesnt readily exchange w O2/ H2O so haemoglobin cant transport O2
organs starved of oxygen- headaches, dizziness, death
can ligand substitution reactions be reversed?
yes unless new complex ion is much more stable than old one
if new ligands form stronger bonds w central metal ions than old ones, change less likely to reverse
eg CN- stronger bonds than H2O w Fe3+
do multidentate ligands form more or less stable complexes than monodentate ligands
more so ligand sub with bi/multidentate ligands are hard to reverse
eg with EDTA4-
is the enthalpy change for ligand exchange reactions usually big or small? why?
small
bonds are broken and formed
strength of bonds broken is similar to strength of bonds made
- delta H of bonds broken and formed is similar
- number of bonds broken and formed is similar
what is the chelate effect?
monodentate ligands are substituted by bidentate/multidentate ligands to produce a much more stable complex as no of particles increases- more particles= greater entropy
reactions that increase entropy are more likely to occur
oxidation states of vanadium and colours
VO2^+, +5, yellow
VO^2+, +4, blue
V^3+, +3, green
V^2+, +2, violet
how can vanadium(V) be reduced?
add to zinc metal in acidic solution
equations for reducing Vanadium
2VO2^+ +Zn +4H+ -> 2VO^2+ + Zn^2+ + 2H2O
yellow- blue (vanadium V-> vanadium IV)
2VO^2+ +Zn +4H+ -> 2V^3++ + Zn^2+ + 2H2O
blue- green (vanadium IV-> vanadium III)
2V^3+ +Zn -> 2V^2++ + Zn^2+
green- violet (vanadium III-> vanadium II)
what does redox potential tell us?
how easily an ion or atom is reduced to a lower oxidation state
same as electrode potentials
more + = less stable so more likely to be reduced
is redox potential always the same as standard electrode potential?
no
depends on environment that the ion is in
how do ligands affect redox potential?
standard electrode pot is measured in aq so and ions are surrounded by water ligands
diff ligands change redox pot depending on how well they bind to metal ion in a particular oxidation state
is redox potential more positive in more acidic or alkaline solutions?
acidic
because the ion is more easily reduced
how does tollen’s reagent work?
silver is a transition metal that is most commonly found in +1 oxidation state. easily reduced to silver metal
prepared by adding ammonia to silver nitrate to form [Ag(NH3)2]+
add to aldehyde, aldehyde oxidised to carboxylate anion and Ag+ reduced to silver
RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag + 4NH3 +2H2O
cant oxidise ketones so no silver mirror forms
why are transition metals useful in titrations?
they change colour- easy to spot end point
theyre often present in reducing/ oxidising agents
how to carry out titration to find how much oxidising agent is needed to react with reducing agent?
- measure quantity of reducing agent using pipette and put in conical flask
- add 20cm3 dilute H2SO4 (in excess so dont need to be exact) (ensures theres plenty of H+ ions
- add oxidising agent using burette whilst swirling
- oxidising agent reacts with reducing agent until all reducing agent used
- next drop added will give the mixture the colour of the oxidising agent
- stop when mixture becomes tainted with colour of oxidising agent and record vol added
- repeat until concordant results (0.1 cm3)
equation for titration of Fe2+ with MnO4- in aqueous potassium manganate (VII)
MnO4- (aq) +8H+ (aq) + 5Fe2+ (aq) -> Mn^2+ (aq) + 4H2O (l) + 5Fe^3+ (aq)
equation for titration of C2O4^2- with MnO4- in aqueous potassium manganate (VII)
2MnO4- (aq) +16H+ (aq) + 5C2O4^2- (aq) -> 2Mn^2+ (aq) + 8H2O (l) + 10CO2(g)
is MnO4- an oxidising or reducing agent?
oxidising
what is a catalyst?
something that speeds up the rate of a reaction by providing an alternative reaction pathway with a lower activation energy and doesnt get used up
why do transition metals make good catalysts?
they can change oxidation states by gaining/ losing electrons in their d orbitals so they can transfer electrons to speed up reactions
what is the contact process?
makes sulphuric acid
SO2+1/2O2 -> SO3 (V2O5 catalyst)
two steps to V2O5 speeding up reaction:
V2O5+SO2 -> V2O4 + SO3
V2O4+1/2O2 -> V2O5
(explanation on page 155)
why cant elements from other blocks?
they dont have an incomplete 3d subshell and dont have variable oxidation states
what is a heterogeneous catalyst?
a catalyst that is in a different phase (state) from the reactants
what are the 2 examples of heterogeneous catalysts to know?
iron in the haber process
N2 + 3H2 -> 2NH3
Vanadium oxide in the contact process
SO2 + 1/2O2 -> SO3
both are a solid catalyst with gaseous reactants
what is a support medium?
when a heterogeneous catalyst is used, reaction happens on surface of catalyst so increasing sa increases no of moles that can react at the same time (increases ror)
support medium is used to make catalyst as big as poss
eg the honeycomb structure in catalytic converters
what is catalyst poisoning?
during a reaction, reactants absorbed onto active sites on surfaces of heterogeneous catalysts
impurities also bind so catalysts surface and block reactants from being absorbed
reduces sa available for reactants so decreases ror
increases cost of chemical process as less product made in certain time/ with certain amount of energy. catalyst may need replacing
examples of catalyst poisoning
lead poisoning catalyst in catalytic converter- covers surface of catalyst- use unleaded petrol
sulphur poisoning catalyst in Haber process- hydrogen used is from methane which is from natural gas which contains impurities eg sulphur compounds. sulphur can be absorbed onto the iron to form iron sulphide
how can you reduce catalyst poisoning?
purify the reactants to remove impurities which would otherwise poison the catalyst
what is a homogeneous catalyst?
in the same physical state as reactants
usually aqueous
how does a homogeneous catalyst work?
forms an intermediate species
reactants combine to form intermediate which then reacts to form products and reform catalyst
what does the enthalpy profile for a homogeneous catalysed reaction look like
it has 2 humps (2 reactions)
the activation energy to form the intermediates and the products from the intermediates is lower than needed to make the products directly from the reactants
what are the 2 reactions catalysed by homogeneous catalysts that you need to know?
Fe2+ catalysing S2O8^2- and I-
Mn2+ catalysing MnO4^- and C2O4^2-
Fe2+ catalysing S2O8^2- and I-
S2O8^2- + 2I- -> I2 + 2SO4^2-
happens very slow as both ions are - so repel
adding Fe2+ means theres a + and - in each reaction
S2O8^2- + 2Fe^2+ -> 2Fe^3+ + 2SO4^2-
(Fe2+ oxidised to 3+ using S2O8^2-)
2Fe3+ + 2I- -> I2 + 2Fe^2+
(intermediate Fe3+ oxidise I- to I and catalyst regenerated)
Mn2+ catalysing MnO4^- and C2O4^2-
autocatalysis- Mn2+ is a product and acts as a catalyst so as reaction progresses it speeds up
2MnO4^- + 16H+ + 5C2O4^2- -> 2Mn^2+ + 8H2O + 10CO2
(initially slow as no Mg2+, high Ea)
4Mn2+ + MnO4^- + 8H+ -> 5Mn^3+ +4H2O
(some Mn2+ made so reacts with MnO4^- to make Mn^3+)
2Mn3+ + C2O4^2- -> 2Mn2+ + 2CO2
(Mn3+ intermediate reacts with C2O4^2- to make CO2 and reform catalyst)
conc-time graph looks unusual as rate increases over time (page 158)
what happens when a metal compound dissolves in water?
water molecules form coordinate bonds with the metal ions
this forms metal-aqua complex ions
in general, 6 H2O molecules form coordinate bonds w each metal ion by donating a non-bonding pair of electrons from their oxygen
acidity of metal-aqua ion solutions
in a solution containing metal aqua 2+ ions, theres a hydrolysis/ acidity reaction between metal aqua ion and water
the metal-aqua 2+ ions release H+ ions so acidic solution is formed. only slight dissociation though so solution is only a weak acid
same with 3+ ions but they make more acidic solutions
see equations in transition metals booklet
why is M^3+ more acidic than M^2+
M3+ is smaller and has a greater charge than M2+
M3+ is more polarising than M2+
So O-H bond in water ligand is weakened and breaks more easily
learn table of reactions of metal aqua ions
can also see page 160 onwards on textbook for explanations/ steps
how does iron act as a homogeneous catalyst?
Hydrogen and nitrogen/reactants adsorb onto the surface/
active sites of the iron
Bonds weaken/reaction takes place
Products desorb/leave from the surface (of the iron)
Explain why complexes formed from transition metal ions are coloured.
absorb some wavelengths of (visible light)
to excite electrons in d-orbitals
complimentary wavelengths of light reflected to give colour seen
how do you reverse the hydrolysis reactions of metal aqua ions with NaOH as shown in table?
all metal hydroxide ppts will dissolve in acid
they act as bronsted-lowry bases and accepts H+ ions
what does amphoteric mean?
can act as acids and bases
will dissolve in excess of base as well as acid
required practical 11
tests to identify metal ions
