transition metals p1 Flashcards
transition element
an element which forms at least one stable ion with a partially full d-shell of electrons
where are the transition metals located in the periodic table?
middle from Ti to Cu- part of the d-block
what are some characteristic physical properties of transition metals?
metallic, good conductors of heat and electricity, hard, strong, shiny, high mp and bp, low reactivity
uses of iron?
vehicle bodies, reinforce concrete
uses of titanium?
jet engine parts
uses of copper?
water pipes
what are the characteristic chemical properties of transition metals?
variable oxidation states –> take part in many redox reactions
coloured compounds/ions in solution
good catalysts
form complex ions
complex ion
central transition metal ion surrounded by ligands (other ions/molecules) that are co-ordinated bonded to it
examples of transition metal catalysts and the process/reactions they catalyse
iron - Haber process
Vanadium oxide - Contact process
MnO2 - decomposition of H2O2
which electrons do transition metals lose first when forming ions?
4s
ligand
an ion or molecule with at least one lone pair of electrons, that donates them to a transition metal ion ton form a co-ordinate bond and thus a complex ion
monodentate ligands
a ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate)
bidentate ligand
ligand that forms two co-ordinate bonds to the metal ion (2 lone pairs to donate)
multidentate ligand
a ligand that forms three or more co-ordinate bonds to the central metal ion
examples of common monodentate ligands
H2O, NH3, OH-
how many co-ordinate bonds does EDTA4- form?
6
coordination number
the number of co-ordinate bonds the metal ion has formed to surrounding ligands
chelate effect
chelate complexes with multidentate ligands are favoured over monodentate ligands or ligands that form fewer coordinate 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? what shape is it?
aqua ion
6 H2O ligands around the central metal ion
octahedral shape
if a transition metal has 2 ligands, what shape is it usually?
linear
if a transition metal has 4 ligands, what shape is it usually?
tetrahedral
exception to the general rule that ions with 4 ligands are tetrahedral? what shape is it?
platin
square planar – > forms cisplatin
what shape is a complex ion if it has 6 ligands?
octahedral
how can complex ions display E-Z or cis-trans isomerism? what shapes of ion does this apply to?
ligands differ in the way in which they are arranged in space
2 ligands of the same type can be on the same side of the metal ion (next to each other), which forms the Z or cis isomer
2 ligands of the same type can be on opposite sides of the metal ion (not next to each other), which forms the E or trans isomer
applies to square planar and octahedral complex ions
what conditions are needed for a complex ion to display optical isomerism?
usually applies to octahedral molecules with 2 or more bidentate ligands, so that the mirror images are non-superimposable
what happens to Co2+, Cu2+ and Fe3+ ‘s coordination numbers when Cl- ligands replace NH3 or H2O ligands?
decreases from 6 to 4 as Cl- is much larger than H2O and NH3
what is haem - its metal ion, coordination number and ligands?
a molecule which makes up protein chains with an Fe 2+ central metal ion, which has a coordination number of 6
4 of these bonds are to a ring system called porphyrin. 1 is to the nitrogen of a globin (protein) molecule and one is to an oxygen in 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 with regards to the body biologically?
CO also coordinately bonds to the Fe2+, 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 so electrons are able to move between the d-orbitals
in compounds (when ligands coordinately bond to the ion), 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. energy of photon = energy difference between levels
energy of photon is related to frequency of light by E = hf
the 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
what affects the colour of a transition metal compound?
ΔE affects the frequency of absorbed photons, so determines the colour
ΔE is changed by oxidation state of the metal, number and type of ligands, shape, co-ordination number
why is there a lack of colour in some aqueous ions and other complex ions?
ions that have completely filled 3d energy levels (such as Zn2+) and ions that have no electrons in their 3d energy levels (such as Sc3+) are not coloured those that have partially filled 3d energy levels are coloured
what are the vanadium oxidation numbers and colours? VO2 +, VO2+, V3+, V2+
VO2^+, oxidation =5+, yellow
VO^2+, oxidation =4+, blue
V^3+, oxidation = 3+, green
V^2+, oxidation = 2+, violet
what can you use to reduce vanadium?
zinc
equation and observation of reduction of V3+ to V2+ using zinc
green to violet
V3+ + e- –> V2+
Zn2+ + 2e- –> Zn
as zinc has a more negative electrode potential than the vanadium half equation zinc will reduce down to V2+
2V3+ + Zn –> Zn2+ + V2+
what colour is Fe2+’s aqua ion?
green
what colour is Fe3+’s aqua ion?
yellow sol
what colour is Cr2+’s aqua ion?
blue
what colour is Cr3+’s aqua ion?
violet
what colour is Co2+’s aqua ion?
pink sol
what does a colorimeter do?
measures the the absorbance of a particular wavelength of light by a solution
how would you use colorimetry experimentally?
use solutions of known conc to create a calibration graph; find unknown conc
what information can a colorimeter give you?
the conc 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)? why?
+2 due to loss of electrons from 4s orbital
when oxidation state is high, why 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 Mn2+?
pink
write a half equation for the reduction of MNO4 - to Mn2+?
MnO4 - + 8H+ + 5e- –> Mn2+ + 4H2O
why are redox titrations 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 Cr3+?
green
half equation for the reduction of Cr2O7 2- to Cr3+?
Cr2O7 2- + 14H+ + 6e- —> 2Cr3+ + 7H2O
what happens to aqua metal ions in acidic conditions?
they get reduced
equation and observation of the reduction of Cr3+ and Cr2+ ions using zinc in acidic conditions?
orange to blue
Cr2O7 2- + 14H+ + 4Zn —> 2Cr2+ + 7H2O + 4Zn2+
what happens to aqua metal ions in alkaline conditions?
they get oxidised
equation and observation Cr3+ ions using hydrogen peroxide in alkaline conditions
green to yellow
reduction: H2O2 + 2e- –> 2OH-
oxidation: [Cr(OH)6]3- + 2OH- –> CrO4 2- + 3e- + 4H2O
2[Cr(OH)6]3- + 3H2O2 –> 2CrO4 2- + 2OH- + 8H2O
equation for the chromate/dichromate equilibrium including the colour changes
2CrO4 2- + 2H+ ⇌ Cr2O7 2- + H2O
yellow –> orange
what happens to aqua metal ions in neutral conditions?
no change
what does whether reduction/oxidation occurs and the readiness of the reaction depend on?
E^o values
what can change E^o values?
pH, ligands involved
catalyst
a 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 temperatures and pressures –> saves valuable energy and resources
how do catalytic converter decrease carbon monoxide and nitrogen monoxide emissions from internal combustion engines?
adsorption of CO and NO molecules onto the surface of the catalyst
weaken of bonds and chemical reaction
desorption of CO2 and N2 product molecules from the surface of the catalyst
what metals are used in a catalytic converter and which reactions do they catalyse?
Pt, Rd, Pd
catalyse CO, NO –> CO2, N2 and CnH2n+2 –> H2O, CO2
heterogeneous catalyst
a catalyst that is present in the reaction in a different phase to the reactants (usually a solid with gas/liquid 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 catalyst’s 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 a catalyst need to have to make it a good catalyst?
can’t adsorb too strongly, otherwise the products will not desorb
can’t desorb 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 catalyst’s 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?
finely divided catalysts can be gradually lost from their support medium
what is the Haber process? what catalyst is used?
N2(g) + 3H2(g) –> 2NH3(g)
makes ammonia, uses iron catalyst
homogeneous catalyst
a 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 Ea
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 high Ea in normal conditions?
two negative ions are reacting
they repel other so Ea is high
which transition metal ions catalyse the reaction between S2O8 2- ions and I- ions?
two equations
Fe2+
S2O8 2- + 2Fe 2+ —> 2Fe 3+ + 2SO4 2-
2Fe 3+ + 2I- –> 2Fe 2+ + I2
autocatalysis
when the product of a reaction is also a catalyst for that reaction
what does a conc of reactant against time graph for an autocatalysed reaction look like?
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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
half equation for the conversion of C2O4 2- ions into CO2
C2O4 2- –> 2CO2 + 2e-
equation for the reaction between C2O4 2- ions and MnO4 - ions
how does Mn2+ 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+
how can you monitor the conc of MnO4 - ions?
using a colorimeter
colour of [Fe(H2O)6]2+ with…
few drops NaOH
few drops NH3
(few drops/dilute)
aqua ion - pale green solution
few drops NaOH = green ppt
few drops NH3 = green ppt (brown in sitting)
colour of [Co(H2O)6]2+ with…
few drops NaOH
few drops NH3
excess OH-
excess NH3
HCl
aqua ion - pink solution
few drops NaOH = blue ppt
few drops NH3 = blue ppt
excess OH- = no reaction
excess NH3 = brown/yellow
HCl = blue solution
colour of [Cu(H2O)6]2+ with…
few drops NaOH
few drops NH3
excess OH-
excess NH3
HCl
aqua ion - blue solution
few drops NaOH = pale blue ppt
dilute NH3 = pale blue ppt
excess OH- = no reaction
excess NH3 = dark/royal blue solution
HCl = yellow solution
colour of [Fe(H2O)6]3+ with…
few drops NaOH
few drops NH3
aqua ion - yellow solution
NaOH (ppt) = brown ppt
dilute NH3 (ppt) = brown ppt
colour of [Cr(H2O)6]3+ with…
few drops NaOH
few drops NH3
excess OH-
excess NH3
aqua ion - green/violet solution
few drops NaOH = green ppt
few drops NH3 (ppt) = green ppt
excess OH- = green solution
excess NH3 (ppt) = purple solution
