Transition Metals Flashcards
Define a transition element
an element which forms at least one stable ion with a partially full d-shell of electrons
some characteristic physical properties of transition metals
metallic, good conductors of heat and electricity, hard, strong, shiny, high melting point, high boiling point, low reactivity
some uses of iron
vehicle bodies, to reinforce concrete
some uses of titanium
jet engine parts
some uses of copper
water pipes
what are the characteristic chemical properties of transition metals (there’s 4)
variable oxidation states (takes place in many redox reactions)
coloured compounds/ions in solution
good catalysts
form complex ions
define the term complex ion
central transition metal ion surrounded by ligands that are co-ordinate bonded to it
transition metal catalyst in the Haber process
iron
transition metal catalyst in the contact process
vanadium (V) oxide
transition metal catalyst in the decomposition of H2O2
MnO2
which electrons do transition metals lose first when forming ions
4s
define ligand
an ion or molecule with at least one lone pair of electrons that donates them to a transition metal ion to form a co-ordinate bond and thus a complex ion
define monodentate ligands
forms one co-ordinate bond to central metal ion (one lone pair to donate)
define bidentate ligand
forms two co-ordinate bonds to central metal ion (two lone pairs to donate)
define multidentate ligand
forms 3+ co-ordinate bonds to central metal ion
4 common monodentate ligands
Cl-
H2O
NH3
CN-
what kind of ligand is ethanedioate
bidentate
-:O—C===O
I
-:O—C===O
what kind of ligand is benzene-1,2-diol
bidentate
what kind of ligand is benzene-1,2-diol
bidentate
what is the full name of en
ethane-1,2-diamine
what kind of ligand is en
bidentate
how many coordinate bonds does EDTA 4- form?
six
define the term coordination number
number of co-ordinate bonds the metal ion has formed to surrounding ligands
what is the chelate effect
chelate complexes with multidentate ligands are favoured over monodentate ligands or ligands that form fewer co-ordinate bonds per molecule
explain the chelate effect in terms of entropy and the reaction that is occurring
number of molecules increases when multidentate ligands displace ligands that form fewer co-ordinate bonds per molecule
significant increase in entropy –> Gibbs’ free energy change <0 –> feasible reaction
a more stable complex ion is formed
what ion is usually formed when a transition metal compound is dissolved in water and what shape is it
aqua ion
6 H2O ligands around central metal ion
octahedral complex ion formed
if a transition metal ion has 2 ligands, what shape is it usually
linear
if a transition metal ion has 4 ligands, what shape is it usually
tetrahedral
name an exception to the general rule that ions with 4 ligands is generally tetrahedral. what shape is it?
platin is square planar (forms cisplatin)
what shape is a complex ion if it has 6 ligands
octahedral
what conditions are needed for a complex ion to display optical isomerism
usually applies to octahedral molecules with 2 or bidentate ligands, so that the mirror images are non-superimposable
what happens to Co2+ , Cu2+ and Fe3+ coordination numbers when Cl- ligands replace NH3 or H2O ligands
decreases from 4 to 6 as Cl- is a much larger ligand than H2O and NH3
what is haem - its metal ion, coordination number and ligands
a molecule which makes up protein chains
Fe2+ central metal ion, which has a coordination number of 6
4 of these bonds are to ring system called porphyrin
1 is to the nitrogen of a globin (protein) molecule
1 is to an O2 molecule
how does haemoglobin transport oxygen
O2 forms a weak coordinate bond to the metal ion, then is transported around the body. the bond breaks when haemoglobin reaches cells and oxygen is released
why is CO toxic
CO also coordinately bonds to the Fe 2+, and is a better ligand, so bonds more strongly than O2
stops O2 from bonding to haemoglobin, so O2 cannot be transported around the body
why are transition metal compounds coloured
- partially filled d-orbitals and electrons are able to move between the d-orbitals
- in compounds, the d-orbitals split into different energy levels
- electrons can absorb energy in the form of photons to become excited and move to a higher energy level (excited state)
- energy of photon = energy different between levels
- energy of photo is related to the frequency of light by E = hf
- colour corresponding to the frequency of the energy change is missing from the spectrum, so we see a combination of all the colours that aren’t absorbed
how do you calculate ∆E from f and/or λ
∆E = hf = hc / λ
what affects the colour of a transition metal compound
∆E affects the frequency of absorbed photons, so determines the colour
∆E is changed by oxidation state of metal, number and type of ligands, shape, co-ordination number
VO2 + oxidation number and colour
5+
yellow
VO 2+ oxidation number and colour
4+
blue
V 3+ oxidation number and colour
3+
green
V 2+ oxidation number and colour
2+
violet
what can you use to reduce Vanadium
Zinc
what colour is Fe 2+’s aqua ion
green
what colour is Fe 3+’s aqua ion
pale brown
what colour is Cr 2+’s aqua ion
blue
what colour is Cr 3+’s aqua ion
red/violet
what colour is Co 2+’s aqua ion
brown
what colour is Co 3+’s aqua ion
yellow
what does a colorimeter do
measures the absorbance of a particular wavelength of light by a solution
how would you use colorimetry experimentally
use solutions of known concentration to create a calibration graph; find unknown concentration
what information can a colorimeter give you
concentration of a certain ion in the solution
why can transition metals have variable oxidation states
they have partially filled d-orbitals so can lose 4s and 3d electrons
which oxidation states do all transition metals have (except Sc) and why
+2 due to loss of electrons from 4s orbital
when oxidation state is high, do the transition metals exist as simple ions
no, after oxidation state of about III, metal ions covalently bond to other species
what is the use of the complex [Ag(NH3)2]+ ion
tollens’ reagent to test for aldehydes/ketones (silver mirror formed with aldehyde, no visible change with ketone)
what colour is MnO4-
deep purple
what colour is Mn 2+
pink
write half equation for the reduction of MnO4 - to Mn 2+
MnO4 - + 8H+ + 5e- —> Mn2+ + 4H2O
why are redox titration with transition metal compounds said to be self-indicating
they usually involve a colour change as the metal is changing oxidation state; sometimes an indicator is still needed/useful
what colour is Cr2O7 2-
orange
what colour is Cr 3+
green
write a half equation for the reduction of Cr2O7 2- to Cr 3+
CrO7 2- + 14H+ + 6e- —-> 2Cr 3+ + 7H2O
what happens to aqua metal ions in acidic conditions
they get reduced
what happens to aqua metal ions in alkaline conditions
they get oxidised
what happens to aqua metal ions in neutral conditions
no change
what decides whether reduction/oxidation occurs and the readiness of the reaction depend on
E values
what can change E values
pH and ligands involved
define a catalyst
substance that increases the rate of a reaction without being chemically changed at the end of the reaction
how do catalysts usually work
provide an alternative reaction pathway with a lower activation energy
why are transition metals good catalysts
they can exist in variable oxidation states, so can provide alternative pathways easily
why are group 1, 2 and 3 metals not as good catalysts
only exist in one oxidation state
what are advantages of using a catalyst for a reaction
allows reactions to proceed at lower temperature and pressure –> save valuable energy and resources
what metals are used in a catalytic converter and which reactions do they catalyse
Pt, Rd, Pd
CO, NO –> CO2, N2 and CxH(2x+2) –> H2O, CO2
define a heterogeneous catalyst
catalysyt that is present in the reaction in a different phase to the reactants
catalytic activity occurs on the solid surface as the reactants pass over it
what is an advantage of using a heterogeneous catalyst
no need for separation of products from catalyst
how do heterogeneous catalysts work
reactants adsorb to the catalysts’ surface at active sites. this weakens bonds within the reactants, holds reactants close together on the surface and/or in the correct orientation to react. once the reaction has occurred, products desorb from the active sites
what properties does the catalyst need to have to make it a good catalyst
can’t adsorb too strongly, otherwise the products will not desorb
can’t adsorb too weakly as reactant would not be held in place for long enough and bonds would not be sufficiently weakened
need a good balance between desorption and adsorption
how can you increase the efficiency of heterogeneous catalysts
- increase the surface area to increase the number of active sites that are present
- also spread onto an inert support medium, e.g. ceramic, to increase the surface/mass ratio. use ceramic honeycomb matrix/mesh/sponge
what is catalyst poisoning
unwanted impurities adsorb to the catalysts’ active sites and do not desorb
this blocks the active sites on the catalyst’s surface
what effect does catalyst poisoning have on catalytic activity
decreases the effectiveness of the catalyst over time
how else can a catalyst be degraded aside from catalyst poisoning
finely divided catalysts can be gradually lost from their support medium
what is the Haber process and what catalyst is used
N2 (g) + 3H2 (g) –> 2NH3 (g)
makes ammonia
uses iron (Fe) catalyst
what size/shape is the catalyst for the Haber process
pea sized lumps to increase surface area
how long does the catalyst last for the Haber process? what is it poisoned by?
approx 5 years
poisoned by sulfur impurities in the gas streams
what is the contact process? what is the catalyst ?
makes H2SO4 -
catalysed by vanadium (V) oxide V2O5
2SO2 (g) + O2 (g) —> 2SO3 (g)
what are the two reactions that are involved in the contact process
SO2 + V2O5 —> SO3 + V2O4
V2O4 + 1/2 O2 —> V2O5
why is V a good catalyst in the case of the contact process
can change oxidation state from 5+ to 4+ and back to 5+ (so it can be used again)
define homogeneous catalyst
catalyst that is in the same phase as the reactants
how do homogeneous catalysts work
form intermediates to give a different reaction pathway with a lower activation energy
what is the reaction between S2O8 2- ions and I- ions
S2O8 2- + 2I- —> 2SO4 2- + I2 (all aq)
why does the reaction between S2O8 2- ions and I- ions have a higher activation energy in normal conditions
two negative ions are reacting
they repel each other so activation energy is high
which transition metal ions catalyse the reaction between S2O8 2- and I- ions?
give 2 reactions that do
Fe 2+
S2O8 2- + 2Fe 2+ —> 2Fe 3+ + 2SO4 2-
2Fe 3+ + 2I - —> 2Fe 2+ + I2
define the term autocatalysis
when the product of a reaction is also a catalyst for that reaction
concentration of reactant against time graph for autocatalysed reactuon
- initially slow, uncatalysed as not much of catalyst has been formed
- rate increases as catalyst is made; catalysed reaction is faster
- slows down as reactants are used up
write a half equation for the conversion of C2O4 2- ions into CO2
C2O4 2- —-> 2CO2 + 2e-
write an equation for the reaction between Cr2O4 2- ions and MnO4 - ions
how does Mn 2+ autocatalyse this reaction
2MnO4 - + 16H+ + 5C2O4 2- —-> 10CO2 + 2Mn 2+ + 8H2O
1st stage
MnO4 - + 4Mn 2+ + 8H+ —> 4H2O + 5Mn 3+
2nd stage
2Mn 3+ + C2O4 2- —> 2CO2 + 2Mn 2+
