Transition Metals Flashcards
Why is Ag a bad catalyst? (2m)
- Adsorption too weak
- Reactants do not absorb in high enough concentrations
Why is W a bad catalyst? (2m)
- Too strong adsorption
- Products cannot be released
Explain why FeCl4 has a different shape to the hexaaqua ion? (2m)
- Cl- is a bigger ligand
- Not as many ligands can therefore fit around the central transition metal ion
Explain why a non-transition metal compound appears white? (2m)
- Full d sub-shell
- No d-d transitions
Recall the alternative pathway through which heterogenous catalysts act? (3m)
- Reactants adsorbed onto the active site
- Bond breaking / making occurs
- Desorption of products occurs
Explain how the chelate effect works?
- Heat enthalpy change is approximately the same as the same amount of bonds broken are formed
- Overall entropy is more positive due to an increased amount of particles on the right hand side
- Gibbs free energy is more negative so the forward reaction is more feasible
What is the chelate effect?
Bidentate and polydentate ligands replace monodentate ligands from complexes resulting in a more stable complex.
Why is carbon monoxide toxic?
- It replaces the oxygen ligand co-ordinately bonded to Fe(II) in haemoglobin
- This prevents the Hb from transporting oxygen to oxygen deprived areas
- CO is a strong ligand not easily displaced leading to hypoxia
Recall the substitution reaction by chloride for iron(III)
[Fe(H2O)6]3+ + 4Cl- <—> [FeCl4]- + 6H2O
Recall the substitution reaction by chloride for cobalt (II)
[Co(H2O))6]2+ + 4Cl- <—> [CoCl4]2- + 6H2O
Recall ligand substitution by Cl- ions.
Use [Cu(H2O)6]2+ as an example and include observations.
- Adding concentrated HCl will replace the H2O with Cl-
- Cl- is larger hence the coordination numbers changes from six to four, the shape also changes from octahedral to tetrahedral
- From a blue to yellow solution
[Cu(H2O)6]2+ + 4Cl- <—> [CuCl4]2- + 6H20
Why is there no change in co-ordination number when NH3 and H2O undergo in ligand substitution?
Similar size
Recall the reaction between copper (II) and excess ammonia
- Excess NH3: ligands are substituted
- Substitution is incomplete
From a blue solution to a deep blue solution
[Cu(H2O)6]2+ + 4NH3 <—> [Cu(H2O)2(NH3)4]2+ + 4H2O
Recall the reaction between copper (II) and a ammonia added drop-wise
Ammonia acts as a base removing H+
From a blue solution to a blue precipitate
[Cu(H2O)6]2+ + 2NH3 <—> [Cu(H2O)4(OH)2] + 2NH4+
Explain with the use of equations how Mn2+ ions autocatalyse the reaction between C2O42- and MnO4^-
Half equations:
4MnO-(aq) + 8H+(aq) + 5e- —> 4Mn2+(aq) + 4H2O(l)
C2O4^2-(aq) —> 2CO2(g) +2e-
Full equation:
2MnO4- + 16H+ + 5C2O4^2- —> 2Mn2+ + 8H2O +10CO2
Mechanism:
4Mn2+ + MnO4^- + 8H+ —> 5Mn3+ + 4H2O
2Mn3+ + C2O4^2- —> 2Mn2+ + 2CO2
What is autocatalysis?
Give an example
Catalysis being a product within the reaction
E.g. Magnate (VII) + ethandioate
Explain, with the aid of equations, how Fe2+ ions catalyse the reactions between I- and S2O8^2-
S2O8^2-(aq) + 2Fe2+(aq) —> 2SO4^2-(aq) + 2Fe3+(aq)
2Fe3+(aq) + 2I-(aq) —> 2Fe2+(aq) +I2(aq)
Recall homogenous catalysis?
- Catalyst in the same physical state as the reactants
- Reaction progresses through an intermediate species
What property of transition metals allows them to be good catalysts?
Variable oxidation states
What can act as a poison in a catalytic converter?
Lead, Pb
What can act as a poison in the haber process?
Sulphur, S
Recall the cost implication of catalyst poisoning?
- Reaction is slower so takes longer to make product
- Catalyst may need replacing
Recall what catalyst poising is?
- Impurities can block active sites
-Consequentially the catalyst has reduced efficiency
Give an example of a heterogenous catalyst
Fe and V2O5
