TM Catalysts

Q1.Transition metal compounds have a range of applications as catalysts.
(a)     State the general property of transition metals that allows the vanadium in vanadium(V) oxide to act as a catalyst in the Contact Process.
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(1)
(b)     Write two equations to show how vanadium(V) oxide acts as a catalyst in the Contact Process.
Equation 1
 
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Equation 2
 
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(2)
(c)     In the Contact Process, vanadium(V) oxide acts as a heterogeneous catalyst.
(i)      Give the meaning of the term heterogeneous.
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(1)

(d)     The oxidation of C2O42− ions by MnO4− ions in acidic solution is an example of a reaction that is autocatalysed.
(i)      Give the meaning of the term autocatalysed.
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(1)
(ii)     Identify the autocatalyst in this reaction.
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(1)

(iii)    Write two equations to show how the autocatalyst is involved in this oxidation of C2O42− ions.
Equation 1
 
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Equation 2
 
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(2)
(Total 9 marks)

Q2.An acidified solution of potassium manganate(VII) was reacted with a sample of sodium ethanedioate at a constant temperature of 60 °C. The concentration of the manganate(VII) ions in the reaction mixture was determined at different times using a spectrometer to measure the light absorbed.

(a)     Write an equation for the reaction between manganate(VII) ions and ethanedioate ions in acidic solution.
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 (2)

(b)     By considering the properties of the reactants and products, state why it is possible to use a spectrometer to measure the concentration of the manganate(VII) ions in this reaction mixture.
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(2)

(c)     This reaction is autocatalysed. Give the meaning of the term autocatalyst.
Explain how the above curve indicates clearly that the reaction is autocatalysed.
Meaning of autocatalyst ................................................................................
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Explanation ....................................................................................................
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(3)
 (d)     Identify the autocatalyst in this reaction.
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(1)

(e)     Write two equations to show how the autocatalyst is involved in this reaction.
 
Equation 1 ......................................................................................................
 
Equation 2 ......................................................................................................
(2)
(Total 10 marks)

Q3.          Three characteristic properties of transition metals are complex formation, coloured ions and catalytic activity.
(a)     State the feature of transition metals that gives rise to these characteristic properties.
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(1)
(b)     State a fourth characteristic property of transition metals.
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(1)
(c)     For each of the following shapes of complex, identify an appropriate example by drawing its structure.
(i)      a linear complex

 
 
(1)

(ii)     a square planar complex

 
 
(1)

(iii)     a tetrahedral complex

 
 
(1)

(d)     The chemical industry makes use of the catalytic activity of transition metal compounds. For example, vanadium(V) oxide is used as a heterogeneous catalyst in the Contact Process.
(i)      Write an equation for the overall reaction in the Contact Process.
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(1)

(ii)     Explain the meaning of the term heterogeneous as applied to a catalyst.
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(1)
 (iii)     Write two equations to illustrate how vanadium(V) oxide acts as a catalyst in the Contact Process.
Equation 1 ...........................................................................................
Equation 2 ...........................................................................................
(2)
 (iv)    Suggest what is done to a heterogeneous catalyst such as vanadium(V) oxide to maximise its efficiency and how this is achieved.
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(2)
 (e)     The porphyrin ring is a multidentate ligand that is found in living systems complexed with iron(II) ions in haemoglobin and with cobalt(II) ions in vitamin B12
(i)      Give the meaning of the term multidentate.
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(1)
 (ii)     A porphyrin ring can be represented by the symbol PR. It reacts with aqueous iron(II) ions as shown in the equation below.
The enthalpy change for this reaction is approximately zero.
PR(aq) + [Fe(H2O)6]2+(aq) → [FePR(H2O)2]2+(aq) + 4H2O(l)
Explain why the free-energy change for this reaction is negative.
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(2)

(iii)     In vitamin B12 the cobalt(II) ion is co-ordinated to a porphyrin ring, a cyanide
(CN–) ion and an additional unidentate ligand. The cyanide ion is very toxic.
Predict the co-ordination number of the cobalt ion in vitamin B12
Suggest why vitamin B12 is not toxic.
Co-ordination number .....................................................................................
Reason why vitamin B12 is not toxic
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(2)
(Total 16 marks)
Q4.          This question is about the use of transition metals as catalysts.
(a)     State how a catalyst speeds up a chemical reaction.
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(2)
(b)     State the characteristic property of transition metals that enables them to act as catalysts in redox reactions.
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(1)
(c)     In the Contact Process for the conversion of sulfur dioxide into sulfur trioxide, vanadium(V) oxide acts as a heterogeneous catalyst.
(i)      Write two equations to show how the catalyst is involved in this reaction.
Equation 1 …......................................................................................
Equation 2 ..........................................................................................
(2)

 

Q5.          (a)     Explain why the reaction between sodium ethanedioate, Na2C2O4, and potassium manganate(VII) in acidified aqueous solution is initially slow but gradually increases in rate. Write equations to illustrate your answer.

(6)

M1.(a)    Variable / many oxidation states
1
(b)     V2O5 + SO2 → V2O4 + SO3
Equations can be in either order
Allow multiples
1
V2O4 + ½O2 → V2O5
1
(c)     (i)      In a different phase / state from reactants
1
 (d)     (i)      The catalyst is a reaction product
1
(ii)     Mn2+ / Mn3+ ion(s)
1
(iii)    4Mn2+ + MnO4− + 8H+ → 5Mn3+ + 4H2O
Equations can be in either order
1

2Mn3+ + C2O42− → 2Mn2+ + 2CO2
1
[9]

M2.(a)    2MnO4– + 16H+ + 5C2O42–  2Mn2+ + 8H2O + 10CO2
For all species correct / moles and species correct but charge incorrect
1

For balanced equation including all charges (also scores first mark)
1
(b)     Manganate(VII) ions are coloured (purple)
1
All other reactants and products are not coloured (or too faintly coloured to detect)
Allow (all) other species are colourless
Allow Mn2+ are colourless / becomes colourless / pale pink
1

(c)     The catalyst for the reaction is a reaction product
1
Reaction starts off slowly / gradient shallow
1
Then gets faster/rate increases / gradient increases
Allow concentration of MnO4– decreases faster / falls rapidly
1
(d)     Mn2+ ions
Allow Mn3+ ions
1
(e)     MnO4– + 8H+ + 4Mn2+  5Mn3+ + 4H2O
Allow multiples
1
2Mn3+ + C2O42–  2Mn2+ + 2CO2
1
[10]
M3.          (a)     Incomplete (or partially filled) d orbitals/sub-shells
Do not allow d shell
1
(b)     Variable oxidation states
1
(c)     (i)      [H3N–Ag–NH3]+
Allow [Cl–Ag–Cl]– or similar Cu(I) ion
Allow compounds in (i), (ii) and (iii) (eg Cl-Be-Cl)
Allow no charge shown, penalise wrong charge(s)
1
(ii)     Cis platin drawn out as square planar
Allow NiX42– etc
1
(iii)     [CuCl4]2– drawn out as tetrahedral ion
Or [CoCl4]2– drawn out
1

(d)     (i)      SO2 + 1/2O2 → SO3
Allow multiples
Allow SO2 + 1/2O2 + H2O → H2SO4
ignore state symbols
1
(ii)     In a different phase/state (from the reactants)
1
(iii)     V2O5 + SO2 → V2O4 + SO3
can be in either order
1
V2O4 + 1/2O2 → V2O5
(iv)    Surface area is increased
1
By use of powder or granules or finely divided
Allow suspending/spreading out onto a mesh or support
1

(e)     (i)      Forms two or more co-ordinate bonds
Allow more than one co-ordinate bond or donates more than 1 electron pair.
Do not allow “has more than one electron pair”
Allow uses more than one atom to bond (to TM)
1
(ii)     Number of product particles > Number of reactant particles
Allow molecules/entities instead of particles
Penalise incorrect numbers (should be 2→5)
1
Disorder increases or entropy increases
(or entropy change is positive)
Allow ΔG must be negative because ΔH = 0 and ΔS is +ve
1
(iii)     6
1
Cyanide strongly bound to Co (by co-ordinate/covalent bond)
1
[16]

M4.          (a)     Alternative route
Allow mechanism outlined
allow forms intermediate species
1
Lower activation energy
1
(b)     Variable oxidation state
allow changes oxidation states
1
(c)     (i)      SO2 + V2O5 → SO3 + V2O4
allow 2VO2 instead of V2O4
1
O2 + 2V2O4 → 2V2O5
1
7]
M5.          (a)     (Initially slow) because reaction is between two negative ions
(or between two negative reactants or two negative species)
1
Which repel each other
1
Then Mn2+ (or Mn3+) (ions) are formed acting as an autocatalyst
(QOL mark)
(or answer such as Mn2+ ions formed in the reaction act as a catalyst)
1
2MnO4– + 16H+ + 5C2O42– → 2Mn2+ + 8H2O + 4CO2
1
MnO4– + 4Mn2+ + 8H+ → 5Mn3+ + 4H2O
1
C2O42– + 2Mn3+ → 2Mn2+ + 2CO2
1
(Note these equations may gain credit if they have spectator ions and/or be written as half equations)