catalysts (transition metal chemistry)

Question 1

why do transition metals act as catalysts (2 marks)

Answer 1

• transition metals are able to form variable oxidation states and temporally donate/accept electrons
• they provide an alternative reaction pathway with a lower activation energy, therefore more particles have energy greater than or equal to activation energy therefore more collisions result in a reaction

Question 2

define heterogeneous

Answer 2

catalyst is in different phase from reactants

Question 3

define catalyst

Answer 3

something that speeds up a chemical reaction without being used up

they provide an alternative reaction pathway with a lower activation energy, therefore more particles have energy greater than or equal to activation energy therefore more collisions result in a reaction

Question 4

where do reactions occur on a catalyst

Answer 4

reactions occur at active sites on the surface

these exist due to vacant d orbitals that can form weak temporary bonds

Question 5

what makes a catalyst suitable

Answer 5

must form strong enough bonds to adsorb reactants but weak enough that their products can de adsorb

Question 6

give the equations for the contact process and how vanadium acts as a catalyst

Answer 6

V2O5 (s) + SO2(g) —-> V2O4 (s) + SO3(g)

V2O4 (s) + 1/2 O2 (g) —–> V2O5 (s)

Question 7

what catalyst is needed for the Haber process

Answer 7

iron

Question 8

write an equation for the Haber Process in the presence of a iron catalyst

Answer 8

N2 (g) + 3H2 (g) —–> 2NH3 (g)

Question 9

define homogenous catalyst

Answer 9

catalyst and reactant are in the same phase

(an intermediate species is formed)

Question 10

write the overall equation for the reaction of iodide ions and peroxodisulfate ions

Answer 10

S2O8 2- + 2I- —–> 2SO4 2- + I2

Question 11

why does the reaction of iodide ions and peroxodisulfate ions have a high activation energy

Answer 11

reaction has high activation energy because both ions are negatively charged and repel in collisons

Question 12

write the 2 intermediate equations for the reaction of iodide ions and peroxodisulfate ions in presence of Fe2+

Answer 12

S2O8 2- + 2Fe 2+ —-> 2SO4 2- + 2Fe3+

2Fe 3+ + 2I- —-> 2Fe2+ + I2

Question 13

what is the catalyst in the reaction of iodide ions and peroxodisulfate ions

Answer 13

Fe2+

Question 14

how does Fe2+ act as a catalyst in the reaction of iodide ions and peroxodisulfate ions

Answer 14

Fe ions change oxidation state

either reaction can go first so Fe2+ or Fe3+ ions will catalyse the reaction

Question 15

give 3 factors that affect the colour of a transition metal

Answer 15

• oxidation state
• ligands
• coordination number

Question 16

Explain how the efficiency and cost effectiveness of V2O5
is increased in the contact process

Answer 16

used as a fine powder sprayed onto a wire mesh (increased
SA and low amount of an expensive material)

Question 17

Explain with the aid of equations how V2O5 acts as a
catalyst for the contact process

Answer 17

V2O5 + SO2 ⎯→ V2O4 + SO3

V2O4 + ½O2 ⎯→V2O5

overall SO2 + ½O2 ⎯→ SO3

Question 18

This question is about iron and its ions.

Discuss the role of iron as a heterogeneous catalyst in the Haber process.

3 H2 + N2 ⇌ 2 NH3

Your answer should include:

• the meaning of the term heterogeneous catalyst
• how iron acts as a heterogeneous catalyst
• the factors that affect the efficiency and lifetime of the catalyst.

Answer 18

heterogenous means reactant in different phase from catalyst

catalyst speeds up chemical reaction and is left unchanged or catalyst lowers activation energy

hydrogen and nitrogen (reactants) adsorb onto the surface/active sites of iron

bonds weaken/reaction takes place

products de adsorb from iron

large surface area of iron by using powder or pellets

catalyst poisoned by sulfur/ sulfur binds to catalyst

active sites are blocked

Question 19

Platinum acts as a heterogeneous catalyst in the reaction between ammonia
and oxygen. It provides an alternative reaction route with a lower activation
energy.

Describe the stages of this alternative route. [3 marks]

Answer 19

• reactants adsorb onto platinum surface/platinum provides active sites
• bonds breaking/ bonds making occurs on the surface
• de adsorption of product

Question 20

Write an equation for the reaction of peroxodisulfate ions S2O82- with iodide
ions

Answer 20

S2O82- + 2I- ⎯→ 2SO42- + I2

Question 21

Explain why the activation energy for this reaction is so high (therefore low
rate)

Answer 21

Both ions are negatively charged, therefore repel on collision

Question 22

What is an autocatalytic reaction?

Answer 22

Where one of the products of the reaction catalyses the reaction

Question 23

The reaction between C2O42– ions and MnO4– ions is autocatalysed.

write the overall equation for the reaction

Answer 23

Overall reaction

2MnO4- + 5C2O42- +16H+ ⎯→ 2Mn2+ + 10CO2 + 8H2O

Question 24

The reaction between C2O42– ions and MnO4– ions is autocatalysed.

write the 2 step reaction equation involving catalyst

Answer 24

5Mn2+ + MnO4- + 8H+ ⎯→ 5Mn3+ + Mn2+ + 4H2O

2Mn3+ + C2O42- ⎯→ 2Mn2+ + 2CO2

Question 25

outline the steps involved in Fe acting as a heterogenous catalyst in the Haber process

Answer 25

1- reactants adsorb to active sites on surface
2. weakens bonds in reactants
3. new bonds form in products
4. products deadsorb from surface

Question 26

explain how heterogenous catalysts can become poisoned and explain how this reduces effectiveness of catalysts

Answer 26

some impurities stick to enzyme and block active sites

(impurities may permanently adsorb and block active sites