Catalysts

Question 1

What is a catalyst?

Answer 1

A substance that increases the rate of reaction by providing an alternative reaction pathway which has a lower activation energy without being used up after the reaction

Question 2

Heterogenous catalyst

Answer 2

A catalyst that is in a different phase which reaction occurs at the surface of the catalyst

Question 3

Examples of heterogenous catalysts

Answer 3

Fe catalyst in haber process
Ni in hydrogenation of Alkene to alkane
Catalytic converters

Question 4

What does the contact process manufacture?

Answer 4

Sulfur trioxide gas which is used to make Sulphuric acid

Question 5

Catalyst used in the Contact process?

Answer 5

V₂O₅

Question 6

What gases are used in the contact process?

Answer 6

2 moles of sulfur dioxide to 1 mole of oxygen

Question 7

First step of the contact process

Answer 7

Sulfur dioxide and oxygen are passed over the surface of the V2O5 catalyst
Sulfur dioxide + V2O5 ——> sulfur trioxide + V2O4

Question 8

Second step of the contact process

Answer 8

Reformation of the V2O5 catalyst
2V2O4 +O2 makes 2V2O5

Question 9

Change in V oxidation state during the contact process

Answer 9

+5 in V2O5 to +4 in V2O4
Then reforms to +5 in V2O5

Question 10

How does V2O5 act as a catalyst?

Answer 10

Provides an alternative reaction pathway with a lower activation energy
By reducing V5+ to V4+

Question 11

What do catalytic converters do?

Answer 11

Convert carbon monoxide and nitrogen monoxide from internal combustion engines to less harmful gases N2 and CO2

Question 12

Stages of catalytic converter reaction on surface

Answer 12

Adsorption of NO and CO to surface of catalyst
Weakening of bonds in a chemical reaction and formation of new bonds
Desorption of CO2 and N2 from surface of catalyst by temporary bonds breaking

Question 13

Homogenous catalyst

Answer 13

Is in the same phase as the reactant molecules

Question 14

Example of homogenous catalyst

Answer 14

Fe2+ or Fe3+ in solution to the reaction between
I- and S₂O₈²⁻ ——> 2SO₄²⁻ + I2

Question 15

Fe2+ catalysis of I- and S₂O₈²⁻

Answer 15

2Fe2+ + S₂O₈²⁻ ——> 2SO₄²⁻ + 2Fe3+
2Fe3+ + 2I- ——> I2 +2Fe2+
So Fe2+ is reformed

Question 16

Fe3+ catalysis of I- and S₂O₈²⁻

Answer 16

2Fe3+ + 2I- ——> I2 + 2Fe2+
2Fe2+ + S₂O₈²⁻ ——> 2SO₄²⁻ + 2Fe3+

Question 17

Stages of heterogenous catalysis

Answer 17

Adsorption of molecules on the active sites of the surface of a catalyst forming weak bonds
Reaction where bonds in reactants are broken, new bonds in product formed
Desorption where weak bonds to surface between reactant is broken - reactant is released

Question 18

How does adsorption on a transition metal as a heterogenous catalyst work?

Answer 18

metal atoms on surface have partially filled 3d subshell- electrons inside form temporary bonds

Question 19

Adsorption strength: determining whether a transition metal can act as heterogenous catalyst?

Answer 19

Too strong adsorption = products cant be released
Too weak adsorption = reactants cant form temp bonds to surface so reaction not catalysed

Question 20

Catalyst poisoning

Answer 20

Occurs when some substances (eg impurities in a reaction mixture) permanently adsorb to the catalyst surface, blocking active sites and reducing the available surface area of the catalyst.

Question 21

How to improve rate of catalysed reaction

Answer 21

Increase surface area of the catalyst
May require a support medium

Question 22

Advantages of heterogenous catalysts

Answer 22

Heterogeneous catalysts can be filtered off and are
easy to separate from any liquid or gaseous
products.
They are also suited to continuous
processes rather than batch processes

Question 23

Does oxidation number of V change in contact process?

Answer 23

Yes: from +5 to +4
Then reformed back to +5 when reacted with oxygen

Question 24

Catalytic converters

Answer 24

Remove CO, NOx and unburned hydrocarbons from exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O.

Question 25

Metal used in catalytic converter

Answer 25

Platinum

Question 26

Steps of catalytic converter

Answer 26

CO and NO molecules form bonds with atoms at active sites on the surface of the catalyst - adsorbed onto the surface As a result bonds in the reactants are weakened and break New bonds form between the reactants held close together on catalyst surface Desorption of CO2 and N2 product molecules from the surface of the catalyst

Question 27

Homogeneous catalyst

Answer 27

In the same phase as the reactants and appreciate that the catalysed reaction will proceed via an intermediate species

Question 28

Oxidation number change of a homogeneous catalyst

Answer 28

The intermediate will have a different oxidation state to the original transition metal At the end of the reaction the original oxidation state will reoccur when remade

Question 29

Why can transition metals acts as homogeneous catalysts?

Answer 29

Have different oxidation states so, their intermediate is in different oxidation state to original transition metal used = need different oxidation states

Question 30

What catalyses the reaction between I- and persulphate ions (S2O8²⁻)

Answer 30

Fe 2+ ions or Fe 3+ ions

Question 31

Overall reaction between S2O8 ²⁻ + I- ions

Answer 31

S₂O₈²⁻ + 2I⁻ -----> I₂ + 2SO₄²⁻

Question 32

Catalysed route of Fe2+ on reaction with persulfate and I-

Answer 32

Fe²⁺ + S₂O₈²⁻ -----> Fe³⁺ + 2SO₄²⁻ Fe³⁺ + 2I⁻ ----> I₂ + Fe²⁺

Question 33

Why can both Fe 2+ and Fe 3+ act as the catalyst between persulfate and I-

Answer 33

Because either stage of the reaction can occur in any order as long as the catalyst changes oxidation state then changed back

Question 34

Why does the catalyst Fe2+/Fe3+ actually lower the activation energy of the reaction between persulfate and I-?

Answer 34

Because S₂O₈²⁻ + 2I⁻ are both negative ions so high repulsion, few successful collisions occur in a given time so high Ea In the catalysed reaction, both stages are between neg ions and pos ions so have lower Ea

Question 35

How to write mechanisms for catalysts

Answer 35

Half equations for original reaction - same no. of electrons Half equations for oxidation and reduction of catalyst Combine - same no. of electrons Combine reduced catalyst half equation with oxidised original half equation etc

Question 36

Autocatalysis

Answer 36

When a reaction is catalysed by one of its own products

Question 37

Example of autocatalysis

Answer 37

Mn²⁺ catalyses the oxidation of ethanedioate by acidified manganate (7) MnO₄⁻ + C₂O₄²⁻ + H+ -----> Mn²⁺ + H2O + CO2

Question 38

Why is reaction between Manganate (7) and ethanedioate ions slow at first?

Answer 38

Between 2 negative ions that repel, activation energy high, few successful collisions per unit time Low conc of Mn2+ catalyst because not much product made

Question 39

Why does reaction between Manganate (7) and ethanedioate ions speed up?

Answer 39

More Mn2+ catalyst produced = increases rate of reaction

Question 40

Concentration time graph of autocatalysis

Answer 40

Slope suddenly becomes very steep to show massive increase in rate of reaction (gradient) when catalyst is produced