Transition Elements

Question 1

CATALYST AND VARIABLE OXIDATION STATE QUESTIONS

Answer 1

CATALYST AND VARIABLE OXIDATION STATE QUESTIONS

Question 2

Explain what is meant by the term ‘heterogenous’

Give an example of a reaction where a heterogenous catalyst is used? [2]

Answer 2

different phase/ state to the reactants 1
any one from:
Haber Process 1
Contact Process

Question 3

In the Haber Process, for the manufacture of ammonia a catalyst is required
Identify the catalyst used in this reaction? [1]

Answer 3

Iron / (Fe) 1

Question 4

Heterogenous catalysts can become poisoned by impurities in the reactants
Give an example of an impurity, and explain how it poisons the catalyst? [3]

Answer 4

sulfur compound ( allow sulfur ) 1
it strongly adsorbs onto catalyst active sites 1
or (it blocks the reactants from being adsorbed)

poison reduces the surface area of the catalyst 1

Question 5

Explain what is meant by the term ‘homogeneous’ Suggest why transition metals are good catalysts? [2]

Answer 5

same phase / state to the reactants 1

they have variable oxidation states 1

Question 6

In aqueous solution S2O8[2-] ions are reduced to SO4[2-] ions using I- ions
Write an equation for this reaction? [1]

Answer 6

S2O8 [2-] + 2I- ——–> 2SO4[2-] + I2 1

Question 7

Suggest why the reaction between S2O8 [2-] and I- is very slow without using a catalyst? [1]

Aqueous iron ions can be used to catalyse this reaction
Write two equations to show the role of the catalyst? [2]

Answer 7

both ions are negative, so they repel each other 1

S2O8 [2-] + 2Fe[2+] ——> 2SO4 [2-] + 2Fe[3+] 1

2I- + 2Fe[3+] ——-> I2 + 2Fe[2+] 1

Question 8

Describe what is meant by the term ‘autocatalysis’? [1]

Answer 8

when one of the products catalyses the reaction 1

Question 9

Describe and explain why the reaction between sodium ethanediote (Na2C2O4 ) and potassium manganate (VII) is initially slow but the overtime increases?
Write equations to show the reaction [6]

Answer 9

(slow initially) as both the ions are negative 1
so ions repel each other 1
(overtime) Mn[2+] produced acts as an autocatalyst 1

2MnO4 - +5C2O4[2-] +16H+ > 2Mn[2+] +8H20 +10CO2 1

MnO4 - + 4Mn[2+] + 8H+ —–> 5Mn[3+] + 4H2O 1

2Mn[3+] + C2O4[2-] ——-> 2Mn[2+] + 2CO2 1

Question 10

Describe and explain why the reaction between sodium ethanediote (Na2C2O4 ) and potassium manganate (VII) is initially slow but the overtime increases?
Write equations to show the reaction [6]

Answer 10

(slow initially) as both the ions are negative 1
so ions repel each other 1
(overtime) Mn[2+] produced acts as an autocatalyst 1

2MnO4 - +5C2O4[2-] +16H+ > 2Mn[2+] +8H20 +10CO2 1

MnO4 - + 4Mn[2+] + 8H+ —–> 5Mn[3+] + 4H2O 1

2Mn[3+] + C2O4[2-] ——-> 2Mn[2+] + 2CO2 1

( NB: If struggling work out both half equations first then combine them to help with balancing! )

Question 11

i) What is meant by the term ‘active sites’ in relation to catalysts? [1]
ii) Describe how the number of active sites can be increased for a given mass of catalyst? [1]
iii) Describe how the efficiency of a catalyst can be reduced? [1]

Answer 11

i) region on catalyst where the reactants are (adsorbed) onto the surface 1
ii) use a support medium / powdered 1
iii) catalytic poisoning / another species is adsorbed onto surface(irreversibly) 1

Question 12

When petrol is burned in cars, CO, CO2, oxides of nitrogen and water are produced. Catalytic converters can be used to reduce the emissions of toxic gases

Write an equation for the reaction which occurs in a catalytic converter? [1]
Identify a transition metal that could be used in catalytic converters? [1]

Answer 12

2CO + 2NO ——> 2CO2 + N2 1

Pt / Rh / Pd 1

Question 13

Explain why transition metals that bind strongly to the active sites are not good catalysts? [1]

Answer 13

reactants cannot move, OR products not (desorbed) 1

Question 14

Suggest a reason why the reaction between I- (iodide ions) and S2O8 [2-] (persulfate ions), has a very high activation energy? [1]

Answer 14

Both ions are negative OR ions repel each other 1

Question 15

i) Write an equation to show the reaction of I- ions and S2O8 [2-] ions? [1]
ii) Write two equations to illustrate the effect of adding an Fe [2+] catalyst to the reaction? [2]

Answer 15

i) S2O8[2-] + 2I- ——–> 2SO4[2-] + I2 1

ii) S2O8[2-] + 2Fe[2+] ———> 2SO4[2-] + 2Fe[3+] 1
2I- + 2Fe[3+] ———> I2 + 2Fe[2+] 1

Question 16

State two reasons why an inert medium is often used to support heterogeneous catalysts? [2]

Answer 16

1) to increase the surface area 1

2) to minimise the costs 1

Question 17

State three important feature needed for the function of a heterogeneous catalyst? [3]

Answer 17

has active sites OR allows adsorption onto surface 1
weakens bonds of reactants 1
increases surface concentration 1

Question 18

The Haber process uses a heterogeneous catalyst to produce ammonia
Write a different equation for a process that uses a heterogeneous catalyst? [1]
Identity the catalyst used for this reaction and state the name of the process? [2]

Answer 18

2SO2 + O2 ——–> 2SO3 1

V / vanadium, allow (V2O5) 1
Contact Process 1

Question 19

Describe and explain how catalysts are used in the Contact Process, with the aid of equations? [2]

Give one use of SO3 produced by this reaction? [1]

Answer 19

V2O5 + SO2 ——–> V2O4 + SO3 1
V2O4 + 1/2 O2 ———-> V2O5 ( catalyst reformed) 1

used to make sulfuric acid / H2SO4 1

Question 20

Vanadium ions can have variable oxidation states
For each of the following identify the colour of the ion produced? [2}

a) VO2 [2+] b) VO [2+] c) V [3+] d) V [2+]

Answer 20

a) yellow
b) blue 1 mark for each two
c) green correct answers
d) violet, (DO NOT allow purple)

Question 21

Vanadium can be reduced by experiment

State a reagent and a condition needed? [2]

Answer 21

reagent: zinc 1
condition: acidic conditions/ add HCl 1

Question 22

FORMATION OF COLOURED IONS QUESTIONS

Answer 22

FORMATION OF COLOURED IONS QUESTIONS

Question 23

State three changes to a transition metal that would result in a change in colour? [3]

Answer 23

change in oxidation state 1
change in coordination number / number of ligands 1
change in type of ligand 1

Question 24

Colourimetry can be used to find the concentrations of transition metal ions
Suggest why a coloured filter is used? [1]
Suggest why colourimetry may be chosen as a preferred method over titration? [1]

Answer 24

To select the colour / wavelength that is (most strongly) absorbed (by the sample) 1

(Faster) to analyse extracted samples than by titration 1

Question 25

Explain why transition metal complexes are coloured?[2]

Answer 25

Energy is absorbed from(visible light) 1

when electrons are promoted to a higher energy level 1

Question 26

Briefly describe how you would construct a calibration curve? [3]

Answer 26

prepare a range of solutions with known concentrations          1
Measure absorbance   1
Plot graph( concentration against absorption )     1

Question 27

Frequency absorbed can be determined by the equation E= hv

State the meaning of E and the change it represents? [1]

Answer 27

(E) is the energy absorbed, when electron is promoted to higher energy level OR
moved to exited state 1

Question 28

Give the noble gas electron configuration of the following
Cu +
Cu [2+]

State which ion will produce a coloured ion, and why? [4]

Answer 28

Cu + [Ar] 3d10 1
Cu [2+] [Ar] 3d9 1

Cu [2+] 1
Has a partially filled d-sub shell 1