2.5 - Transition Metals Flashcards
Define a transition element
An element which forms at least one stable ion witha partially full d-shell of electrons
Where are the transition metals located in the periodic table?
In the middle 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 M.P and B.P
- 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 chemical properties of transition metals (4)?
- Variable oxidation states –> take part in many redox reactions
- Coloured compounds/ ions in solution
- Good catalysts
- Form complex ions
Define the term complex ion
Central transition metal surrounded by ligands that are co-ordinated bonded to it
Give some example of transition metals catalysts and the processes/ reactions they catalyse (3)?
Iron - Haber process
Vanadium (V) oxide - contact process
MnO2 - decomposition of H2O2
Which electrons do transition metals lose first when forming ions?
4s
Define the term 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 the term monodentate ligands
A ligand that forms one co-ordinate bond to the central metal ion
1 lone pair to donate
Define the term bidentate ligand
A ligand that forms 2 co-ordinate bonds to the central metal ion
2 lone pairs to donate
Define the term multidentate ligand
A ligand that forms 3 or more co-ordinate bonds to the central metal ion
Give some common examples of monodentate ligands (4).
Cl-, H20, NH3, CN-
How many co-ordinate bonds does EDTA4- form?
6
Define the term coordination number
The number of co-ordinate bonds the metal ion has fromed 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 from fewer co-ordinate bonds per molecule
Significant increase in entroy -> 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, 6H20 ligands around the central metal ion. Octahedral complex ion is formed
If a transition metal has 2 ligands, what shape is it usually?
Linear
If a trasition metal ion has 4 ligands, what shape is it usually?
Tetrahedral
Name an exception to the general rule that ions with 4 ligands is generall tetrahedral. What shape is it?
Platin is 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 shaped of ion does this apply to?
- Ligands differe in the way in which they are arranges 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 conditons are needed for a complex ion to display to optical isomerism?
Usually applies to octahedral molecules with 2 or mor bidentate ligands, so that the mirror images are non-superimposable
What happens to Co2+, Cu2+ and Fe3+ ‘s coordination number when Cl- ligands replace NH3 or H20 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 is made up of protein chains, with an Fe2+ central metal ion, which has a CN of 6.
4 of these bonds are to a ring system called porphyrin. 1 is o the nitrogen of a globin (protein) molecules 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 aroud the body. The bond breaks when haemoglobin reaches cells and oxygen is released.
Why is CO toxic?
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?
- They have partially filled d-orbitals and 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 (excited state). 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.
How do you calculate ΔE from f and/or λ?
ΔE = hf = hc ÷ λ
What effect does 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.
What can you use to reduce Vanadium
Zinc
What colour is Fe2+’s aqua ion?
Green
What colour is Fe3+’s aqua ion?
Pale Brown
What colour is Cr2+’s aqua ion?
Blue
What colour is Cr3+’s aqua ion?
Red/ violet
What colour is Co2+’s aqua ion?
Brown
What colour is Co3+’s aqua ion?
Yellow
What does a colorimeter do?
Measures the absorbance of a particular wavelength of light by a solution
How woukd you use colorimetry experimentally?
Use solutions of known concentration to create a calibration graph; find unknown concentration.
What information can a colorimeter give you?
The concentration of a certain ion in the solution
Why can transition metas 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? 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 OS of about III, metal ions covalently bond to other species
What colour is MnO4-?
Deep purple
What colour is Mn2+?
Pink
Why are redox titrations with transition metal compound said to be self- indicating?
They usually involve a colour change as the metal is changing oxidation state; sometimes an indicator is still needed
What colour is Cr2O7 2-?
Orange
What colour is Cr3+?
Green
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 does whether reduction/ oxidation occurs and the readiness of the reaction depend on?
E values
What can change these E values?
pH, ligands involved
Define a catalyst.
A substance that increases the rate o a reaction without being chemically changes 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 OS, 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
What metals are used in a catalytic converter?
Pt, Rd, Pd
Define a heterogeneous catalyst
A catalyst 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 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 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 heterogenous 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 mesh.
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 size/ shape is the catalyst for the Haber process?
Pea sized lumps to increase surface area
What is the Haber process? What catalyst is used?
N2 (g) + 3H2 (g) —> 2NH3 (g)
Makes ammonia, uses iron (Fe) catalyst.
How long does the catalyst last for the Haber process? What is it poisoned by?
About 5 years. Poisoned by sulfur impurities in the gas streams.
Define homogeneous catalyst
It is in the same phase as the reactants.
How do homogeneous catalysts work?
Form intermediates to give a different reaction pathway with lower EA.
Why does (S2O8)-2 ions and I- ions have a high EA in normal conditions?
Two negative ions are reacting. they repel each other so EA is high.
Define the term autocatalysis
When the product of a reaction is also a catalyst for that reaction.
How can you monitor the concentration of MnO4- ions?
Using a colorimeter
