Catalysts

Question 1

State what is meant by a heterogeneous catalyst.

Answer 1

the catalyst is in a different phase from the reactants

Question 2

State what is meant by a homogenous catalyst.

Answer 2

→ same phase from reactants
→ usually form intermediate species

Question 3

Describe how a heterogenous catalyst works.

Answer 3

→ reactants bind to active sites on surface of catalyst (adsorption)
→ increase in frequency of collisions
→ bonds in reactants are weakened & break
→ new bonds are formed between reactants
→ catalyst releases product, bonds between catalyst & product becomes weak (desorption)
→ product leaves active site

Question 4

Explain the effects of a poisoned catalyst.

Answer 4

→ impurities strongly adsorb to surface
→ active site is blocked
→ catalyst is less efficience

Question 5

Give three examples of a heterogenous catalyst and the processes they’re involved in.

Answer 5

V₂O₅ → contact process
Fe → Haber process
Cr₂O₃ → manufacture of methanol

Question 6

Give the two equations of the catalyst involved in the contact process.

Answer 6

V₂O₅ + SO₂ → V₂O₄ + SO₃

V₂O₄ + 0.5O₂ → V₂O₅

Question 7

Explain why leaded petrol cannot be used in cars fitted with a catalytic converter.

Answer 7

Lead strongly adsorbs onto the surface of the catalyst.

Question 8

Explain why the uncatalysed between iodide and persulfate ions is slow.

Answer 8

→ both ions are negative
→ very few collisions due to repulsion
→ high activation energy

Question 9

Give the two equations for the catalysed reaction between iodide and persulfate ions. (catalysed by Fe2+)

Answer 9

2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻
2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

Question 10

Give an example of an autocatalysis reaction.

Answer 10

Titration reaction between manganate and ethanedioate ions

Question 11

Explain the concentration time graph for an autocatalysed reaction.

Answer 11

→ initially, rate is slow as reaction is uncatalysed
(high Ea as both ions are negative)
→ rate increases as reaction is catalysed
→ rate decreases as concentration of reactants decrease

Question 12

State which species acts as the catalyst during the autucatalysis of manganate and ethanedioate ions.

Answer 12

Mn²⁺

Question 13

Give the two equations for the catalysed reaction of manganate with ethanedioate ions.

Answer 13

Step 1:
4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O
Step 2:
2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂

Question 14

Suggest how poisoning of a catalyst, used in an industrial process, can be
minimised.

Answer 14

purify reactants

Question 15

Give two reasons why the use of a spectrometer is the most appropriate method for measuring concentration of coloured ions.

Answer 15

→ rapid process
→ easy to get many readings

Question 16

Explain how Mn2+ acts as an autocatalyst using equations.

Answer 16

Mn²⁺ + MnO₄⁻ → Mn³⁺
Mn³⁺ + C₂O₄²⁻ → Mn²⁺
Ea is lowered as opposite ions attract.