5: Catalysis

Question 1

What is a catalyst?

Answer 1

A substance that speeds up rate of reaction without being used up, by providing an alternative reaction pathway with a lower activation energy

Question 2

What is a heterogenous catalyst?

Answer 2

A catalyst that is in a different state to the reactants

Question 3

What are homogenous catalysts?

Answer 3

Catalysts that are in the same state as reactants

Question 4

In heterogenous catalysis, what state are reactants and catalysts in?

Answer 4

Solid catalyst, gaseous reactant

Question 5

Name the heterogenous catalyst used in the Haber process.

Answer 5

Iron

Question 6

Give an equation for the Haber process.

Answer 6

N2 (g) + 3H2 (g) = 2NH3 (g)

Question 7

Name the catalyst used in the contact process.

Answer 7

V2O3

Question 8

Give an equation for the contact process.

Answer 8

SO2 (g) + 0.5 O2 (g) -> SO3 (g)

Question 9

Where do heterogenous catalysts work during a reaction?

Answer 9

The reactant binds to the catalysts surface.

Question 10

Explain the process of how heterogenous catalysts work.

Answer 10

1. Reactants adsorb onto reactant surface
2. Reaction take places
3. Products desorbed

Question 11

Give 2 issues that could happen if the heterogenous catalyst is too strong.

Answer 11

Reactants cannot move around the surface.
Products cannot desorb

Question 12

Give 1 thing that could happen if the heterogenous catalyst is too weak.

Answer 12

Reactants may never adsorb.

Question 13

Explain how catalytic converters are made efficient.

Answer 13

The catalyst is heterogenous, so is spread across the surface. Inside the catalytic converter is a honeycomb structure, increasing the surface area for attachment of the catalyst.

Question 14

What is catalyst poisoning?

Answer 14

The active site of the catalyst becomes blocked, preventing the catalyst from binding to reactants, ruining its ability to speed up rate.

Question 15

Describe how catalysts become poisoned.

Answer 15

Active site becomes blocked by substances that will not desorb.

Question 16

Name an element that could poison the iron catalyst in the Haber process.

Answer 16

Sulfur

Question 17

Name an element that could poison the catalyst in a catalytic converter.

Answer 17

Lead

Question 18

What state are homogenous catalysts usually?

Answer 18

Aqueous

Question 19

Briefly explain how homogenous catalysts work.

Answer 19

They form an intermediate species which reacts with products and then regenerates the original catalyst.

Question 20

Describe the shape of an energy diagram showing the use of a homogenous catalyst.

Answer 20

The first peak shows the activation energy for the main reaction. The second peak shows the activation energy for reaction of intermediates.

Question 21

Explain why the reaction of the equation shown is slow.
2I- + S2O8 2- -> I2 + 2SO4 2-

Answer 21

Both reactants have a negative charge and repel each other, making it unlikely that they will collide and react.

Question 22

Give the ionic equation when you add an Fe2+ catalyst to this reaction.
Explain how this catalyst works to speed up rate of reaction.
2I- + S2O8 2- -> I2 + 2SO4 2-

Answer 22

2Fe2+ + S2O8 2- -> 2Fe3+ + 2SO4 2-
Addition of a positive ion means that there is no repulsion so particles collide and react.

Question 23

Give the equation involving 2I- and 2Fe 3+ in the second stage of homogenous catalysis of S2O8 2-.
Explain what happens.

Answer 23

2I- + 2Fe 3+ -> 2Fe 2+ + I2
The iodide ions react with the new catalyst intermediate to produce iodine.

Question 24

What is autocatalysis?

Answer 24

Where the product of a reaction acts as a catalyst.

Question 25

How does rate of reaction change over progression of a reaction using autocatalysis?

Answer 25

As the reaction progresses, more product is produced. As more product is produced, this means that more catalyst is produced. Therefore, as the reaction goes on, it becomes faster as more and more catalyst is formed.

Question 26

Give a redox equation for the autocatalysis involving 5MnO4- and 5C2O4 2-

Answer 26

2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2

Question 27

Give an equation for the formation of Mn3+ ion intermediates during autocatalysis of this reaction.
2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2

Answer 27

4Mn 2+ + MnO4- + 8H+ -> 5Mn3+ + 4H2O

Question 28

Give an equation for the reaction of Mn3+ intermediates with C2O4 2- ions during autocatalysis of this equation.
2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2

Answer 28

2Mn 3+ + C2O4 2- -> 2CO2 + 2Mn 2+

Question 29

What is an autocatalyst?

Answer 29

The product of a reaction acts as a catalyst

Question 30

A student determines the concentration of ethanedioate ions in an acidified solution by
titration with potassium manganate(VII) solution.
2 MnO4
– + 5C2O4
2– + 16H+ → 2 Mn2+ + 10CO2 + 8H2O
The mixture is warmed before the addition of potassium manganate(VII) solution
because the reaction is slow at first. When more potassium manganate(VII) solution
is added, the mixture goes colourless quickly due to the presence of an autocatalyst.
Explain the meaning of the term autocatalyst.
Explain, using equations where appropriate, why the reaction is slow at first and then
goes quickly

Answer 30

M1 autocatalyst: product of the reaction catalyses the reaction
M2 slow: negative ions repel / ions of same charge repel
M3 high Ea
M4 attraction between oppositely charged ions / negative reactant
ion(s) and positive catalyst / Mn2+ / Mn3+
M5 4 Mn2+ + MnO4
– + 8 H+ → 5 Mn3+ + 4 H2O
M6 2 Mn3+ + C2O4
2– → 2 Mn2+ + 2 CO2

Question 31

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.

Answer 31

Reactant(s) adsorbed onto the
platinum surface
Reaction occurs on the surface (bond making and breaking)
Product desorbed