transitition metals Flashcards
Explain how heterogeneous catalysts work, give one example of a reaction
catalysed in this way and discuss why different catalysts have different activities (8 marks)
reactants brought together / increased concentration on surface
or increased collision frequency (1)
reactants must be correctly orientated (1)
reaction on the surface (1)
products desorbed (1)
example of a catalysed reaction (not a named process) (1)
a suitable catalyst for this reaction (1)
penalise incorrect second reactions and catalysts
If absorption too weak reactants not brought together (1)
e.g. silver (1)
If adsorption too strong products not desorbed (1)
e.g. tungsten (1)
Explain why the equilibrium in part (c)(i) is displaced almost completely to the
right to form the EDTA complex.
2 mol of reactants form 7 mol of products
Allow more moles/species of products
Allow consequential to (c)(i)
1
Therefore disorder increases
1
Entropy increases / +ve entropy change / free-energy change is negative
Iron is an important element in living systems. It is involved in redox and in acid-base
reactions.
(a) Explain how and why iron ions catalyse the reaction between iodide ions and S2O8
2–
ions. Write equations for the reactions that occur
two negative ions repel / lead to reaction that is slow / lead
to reaction that has high Ea
1
iron able to act because changes its oxidation state
allow iron has variable oxidation state
1
With iron ions have alternative route / route with lower
activation energy
You are provided with a 1.00 mol dm–3 solution of iron(III) ions and a visible-light
spectrophotometer (colorimeter). Outline a plan for experiments using this solution
and this apparatus which would enable you to determine the concentration of
iron(III) ions in a solution of unknown concentration.
Add an appropriate (or a given correct) ligand to intensify colour (1)
e.g. thiocyanate (CNS)—
or bipyridyl
Make up solutions of known concentration (1)
Measure absorption or transmission (1)
Plot graph of results or calibration curve (1)
Measure absorption of unknown and (1)
compare
State the electron configuration of a Ti(III) ion and that of a Ti(IV) ion. Explain,
in terms of electron configurations and electron transitions, why Ti(III) compounds
are usually coloured but Ti(IV) compounds are colourless
Ti(III) has a d electron that can be excited to a higher level
Allow idea that d electrons can be excited to another level (or
move between levels)
1
Absorbs one colour of light from white light
Allow idea that light is absorbed
1
Ti(IV) has no d electron so no electron transition with
energy equal to that of visible light
Allow Ti(IV) has no d electrons
give two reasons why ceramic support is used for the catalyst in a
- large surface area
- reduce cost or amount of catalyst
explain how lead poisons this catalyst
lead absorbed
lead not deabsorbed and so blocks the active site
state three important features in mechanism of heterogeneous catlaysis
feature 1: brings the reactants together on the surface
feature 2: weakens the bonds
feature 3: increase in surface concentration
what poisons the catalyst in the Haber process
sulfur
the candidate was told to dissolve the weighted sample in water and to add one further reagent before titrating against of 0.0200M KMnO4.
Identify a suitable reagent and state the colour change at the end-point of the titration
Reagent: dilute sulfuric acid
End - point: colourles to purple
a standard solution of the purple ion obtained from B can be used to determine the concentration of a solution of ethanedioate ions
- purple solution/ mangate (VII) in burette
- add dilute sulfuric acid to the flask
- pipette known quantity of ethanedioate into conical flask
- add manganate (VII) from burette until end point, then add dropwise - untill first purple colour
- repeat until concordant results
suggest why iron has two common oxidation states in its aqueous chemistry
it loses two 4s2 electrons
it can lose another to give half full shell
Explain how the properties of the species enable it to act as a catalyst in this
reaction.
Mn2+ OR Mn3+
If catalyst incorrect can only score M1 and M3
1
(Possible because) Mn can exist in variable oxidation states
1
Ea lowered because oppositely charged ions attract
These marks can be gained in any order
1
Mn3+ (reduced) to Mn2+ by C2O4
2− / equation
M5 may appear before M2
1
Mn2+ (oxidised (back)) to Mn3+ by MnO4
−
/ equation
M5 and M6 can be scored in unbalanced equations or in words showing: Mn3+ + C2O4 2− → Mn2+ Mn2+ + MnO4 − → Mn3+
other than having variable oxidation states explain why Fe3+ ions are good
the positive ions attract the negative ions in the process providing an alternate pathway for the reaction with a lower activation energy
