3.2.5 Transition metals Flashcards
What is the definition of a transition metal?
Element with an incomplete d sub-shell in atoms/ions
What are the elements of the first transition series?
Titanium → copper
What are the electron configurations of the following transition metals:
a) Copper
b) Chromium
a) [Ar] 4s^1 3d^10
b) [Ar] 4s^1 3d^5
What are the 4 characteristic properties of transition metals?
- Form coloured compounds
- Act as catalysts
- Variable oxidation states
- Form complexes
What do the following terms means:
a) Complex
b) Ligand
c) Co-ordination number
a) Central metal ion that is surrounded by ligands
b) The molecules which donate a lone pair of electrons to the transition metal ion to form a co-ordinate bond
c) The number of co-ordinate bonds to the central metal ion
Give an example of the following types of ligands:
a) Monodentate
b) Bidentate
c) Polydentate
a) Cl-, NH3, H2O, CN-, OH-
b) 1,2-diaminoethane (en) (H2N-CH2-CH2-NH2)
ethandioate ion (ox) (C2O4 ^2-)
c) EDTA ^4-
What property of transition metals allows them to form complexes?
Variable oxidation states
What causes a change in coordination number/geometry of the complex during a ligand substitution?
Change in size of ligand
eg. NH3 to Cl-
Why would a ligand substitution be incomplete?
- Energetics of the reaction and stability of the products are not favourable
- Different size ligands
What is the most common arrangement when a transition element ion is in solution?
Hexaaqua complex ion
(6 water ligands)
Explain how a haem complex is formed, what are the positives/negatives of the weak co-ordinate bond?
- Oxygen atoms form co-ordinate bond with iron(II)
- Allows transport of oxygen around body
- However, oxygen atoms aren’t very good ligands and the co-ordinate bond is weak so oxygen is easily removed from the iron to enter cells
- Carbon monoxide is a better ligand however, it forms stronger complexes with iron (II)
What is the Chelate effect?
When a monodentate ligand is replaced by either a polydentate/bidentate ligand in complex ions.
What 4 shapes of molecules can complexes form?
(Include the bond angle)
- Linear (180°)
- Tetrehedral (109.5°)
- Octahedral (90°)
- Square planar (90°)
Why is the Chelate effect energetically favourable?
∆G = ∆H - T∆S
- Relatively small ∆H, reaction between aqueous ions
- Positive ∆S, chelation produces a net increase in the number of particles
- A small ∆H + relative large positive ∆S generally ensures that the overall ∆G is negative
Why is ∆H negligible in most ligand substitution reactions?
- Bonds formed have similar enthalpy to bonds broken
- Same number of bonds broken and made
What shape of complex is cisplatin an example of?
Square planar
(Cl)2-Pt-(NH3)2
90° bond angle
What is the use of cis-platin in medicine, why is trans-platin not used in this way?
- Cancer treatment, binds to DNA in cancer cells
- Trans-platin has a different arrangement so cannot bind to DNA in the same way
What shape complex is formed when Cl- ions form a complex?
Tetrehedral
What shape complex is formed when there are 6 co-ordinate bonds?
Octahedral complex
Which shape complexes form cis-trans isomerism?
Explain the difference between the two isomers?
Octahedral and square planar
Cis = Z isomer (same side) or adjacent to eachother
Trans = E isomer or opposite eachother
How do optical isomers form in ligand complexes?
Octahedral complexes with bidentate ligands
How do you calculate the amount of energy absorbed by electrons in a transition element complex?
ΔE = h x v
h = Planck’s constant
v = frequency
Explain electron promotion and how this produces a coloured complex.
- Electron absorbs light energy which excites it from a 3d orbital of lower energy (ground state) to one of higher energy (excited state)
- The other frequencies of light which are not absorbed by the electrons combine to make the complimentary colour
What do the terms degenerate and non-degenerate orbitals mean?
Degenerate = transition ion not bonded to ligands
- 3d orbitals all have equal energy
Non-degenerate = transition ion with co-ordinate bonds to ligands
- 3d orbitals are split into two sets of non-degenerate orbitals with different energy
- This difference in energy is ΔE
What factors change the colour of a complex and how?
ΔE change caused by change in:
- Co-ordination number
- Ligand
- Oxidation state
How can you determine the concentration of coloured ions in a solution?
Colorimeter
- Produce a calibration curve, absorption against concentration, by measuring absorption of a set of standard solutions
What are the 4 possible oxidation states of vanadium and what colour compounds do they form?
+5
VO2^+, yellow
+4
VO^2+, blue
+3
V^3+, green
+2
V^2+, purple
How can you illustrate the variable oxidation states of vanadium?
(Write out equations for the oxidation of the Vanadium ions and the reduction of zinc and then combine)
Oxidation by zinc in acidic conditions
What is the transition metal complex in Tollen’s reagent?
How does it react with aldehydes to produce a silver mirror?
[Ag(NH3)2]+
[Ag(NH3)2]+ + e- → Ag (s) + 2NH3 (aq)
What are the two common transition metal ion redox titrations which are ‘self-indicating’?
Give the reduction equation and colour change.
- manganate(VII)
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
purpe → colourless - dichromate(VI)
Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O
orange → green
What is the definition of a heterogeneous and homogeneous catalyst?
Heterogeneous = catalyst is in a different physical state (phase) from the reactants
Homogeneous = catalyst is in the same physical state (phase) as the reactants
Why are transition elements often used as catalysts?
- Able to form ions with more than one stable oxidation state
- Vacant d orbitals
How do you minimise the cost and maximise the efficiency of a transition metal catalyst (heterogeneous)?
- Increase the SA
- Only use a thin coat of the catalyst over an inert surface medium
Identify a transition metal used in a catalytic converter and state how the converter is constructed to maximise the effect of the catalyst.
- Platinum
- Deposited on a ceramic honeycomb/mesh which increases the surface area of the catalyst
What is the Contact Process and what type of catalysis does it make use of?
Include the equations that involve the catalyst and its regeneration
Manufacturing of sulfuric acid
Heterogenous catalysis (V2O5)
- SO2 (g) + V2O5 (s) → V2O4 + SO3 (g)
- O2 (g) + 2V2O4 (s) → 2V2O5 (s)
Describe the steps of catalysis for a hetereogenous catalysit.
- Adsorption- molecules adhere to the surface of the catalyst
- Surface reaction- new bonds form between reactants
- Desorption- molecules move away from catalyst surface
Describe the steps of catalysis in the Haber Process.
Iron = heterogenous catalyst
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
- Nitrogen/hydrogen gas diffuse to the surface of the iron
- Adsorption to the surface takes place
- The reaction takes place in steps, new bonds form between nitrogen and hydrogen
- The ammonia molecule desorbs from the surface
What are the limitations of a heterogenous catalyst?
- Impurities can bind to the catalyst active sites and prevent further catalytic action (poisoning of the catalyst)
- Thin coating will be lost overtime, reducing the efficiency of the catayst
What is an example of a homogenous catalyst?
Show the overall equation and seperate steps.
S2O8^2- + 2I- → I2 + 2SO4^2-
- S2O8^2- + 2Fe^2+ → 2SO4^2- + 2Fe^3+
- 2I- + 2Fe^3+ → I2 + 2Fe^2+
Explain why and how iron ions catalyse the reaction between iodide ions and S2O8^2- ion.
- 2 negative ions repel eachother which leads to a high Ea and slow reaction
- Iron able to act because of changes in oxidation state
- Iron ions provide alternative reaction pathway with lower Ea
What is autocataylsis?
Product of a reaction can act as a catalyst in the same reaction
Give an example of autocatalysis.
reaction between manganate(VII) ions and ethandioate ions
Mn2+ = catalyst
MnO4- + 8H+ + 4Mn^2+ → 5Mn^3+ + 4H2O
2Mn^3+ + C2O4^2- → 2Mn^2+ + 2CO2
What happens to [MnO4-] during the autocatalysis reaction of manganate(VII) ions and ethandioate ions.
- [MnO4-] decreases as rate of reaction increases due to increase in catalyst Mn2+, which is being produced in the same reaction
- Rate decreases overtime as more reactents are used up
