Catalysts

Question 1

What is the most important feature in the mechanism of homogenous catalysts?

Answer 1

Change of oxidation state

Question 2

Reduction of S2O82- by I-

Answer 2

S2O82- + 2I- -> 2SO + I2

Question 3

Heterogenous catalysts

Answer 3

Present in a reaction in a different phase than the reactants. Usually present as solids, while the reactants may be gasses or liquids. Catalytic action occurs on the solid surface.

Question 4

How do we make catalysts more efficient?

Answer 4

1. Increase surface area

2. Spread the catalyst onto an inert support medium -> Increases surface-to-mass ratio

Question 5

Why don’t catalysts last forever?

Answer 5

1. Surfaces may become covered with impurities

2. Finely divided catalyst may gradually be lost from support medium

Question 6

What catalyst is used in the Haber process?

Answer 6

Iron

Question 7

2 steps of the Contact process

Answer 7

1. Vanadium (v) oxide oxidises sulfur dioxide to sulfur trioxide and is itself reduced to vanadium (iv) oxide
   SO2 + V2O5 -> SO3 + V2O4
2. The vanadium (iv) oxide is then oxidised back to vanadium (v) oxide by oxygen
   2V2O4 + O2 -> 2V2O5

Question 8

What catalyst is used for production of methanol

Answer 8

Cr2O3

Question 9

Synthesis gas

Answer 9

Made from methane, present in natural gas and steam. It’s a mixture of CO and H2 and is used to make methanol.

Question 10

Homogenous catalysts

Answer 10

When the catalyst is in the same phase as the reactant, an intermediate species is formed.

Question 11

Autocatalysis

Answer 11

One of the products of the reaction is a catalyst for the reaction

Question 12

How does Mn2+ act as a catalyst?

Answer 12

Reacts with MnO4- ions to form Mn3+ as an intermediate species which then reacts with C2O42- ions to reform Mn2+

Question 13

Why does tungsten have low catalytic efficiency?

Answer 13

Absorption too strong

Question 14

Why does silver have low catalytic efficiency?

Answer 14

Absorption too weak

Question 15

What catalysts are used in a catalytic convertor?

Answer 15

Rhodium

Question 16

Catalytic convertor equation

Answer 16

2CO + 2NO -> 2CO2 + N2

Question 17

Haber process equation

Answer 17

N2 + 3H2 -> 2NH3

Question 18

Iron and HCl

Answer 18

Fe + 2H+ -> Fe2+ + H2

Question 19

Reagent for conversion of [Cr(OH)6]3- into CrO42-

Answer 19

H2O2

Question 20

3 essential steps in the mechanism of heterogeneous catalysis

Answer 20

1. Surface absorption
2. Reaction on surface
3. Products released from surface

Question 21

Identify a substance which acts as a poison for a catalyst and explain how poisoning occurs

Answer 21

Sulfur, Blocks active sites

Question 22

2CrO42-

Answer 22

Yellow

Question 23

CrO72-

Answer 23

Orange

Question 24

What are the reagents, conditions and equation for the conversion of potassium dichromate(VI) to Cr3+

Answer 24

H2O2 + dilute H2SO4

Cr2O7 + 14H+ + 6Fe2+ -> Cr3+ +7H2O +6Fe3+

Question 25

What are the reagents, conditions and equation for the conversion of potassium dichromate(VI) to Cr2+

Answer 25

HCl + absence of air

Cr2O7 +14H+ +4Zn -> Cr2+ + 7H2O + 4Zn2+

Question 26

Why is the strength of absorption of reactants and products onto the surface of a catalyst important factors in determining catalytic activity?

Answer 26

Too strong -> can’t be desorbed from surface

Too weak - reactants not brought together

Question 27

Other than strength of absorption, why would the absorption onto a catalyst surface not result in a reaction?

Answer 27

Reactants not correctly orientated

Question 28

3 characteristic features of a catalyst

Answer 28

1. Speeds rate
2. Unchanged at the end
3. New route with lower Ea

Question 29

What 2 measures can be taken to maximise catalytic efficiency?

Answer 29

1. Increase S.A.

2. Remove catalyst poisons from reactants

Question 30

Why does iron have 2 common oxidation states in aq chemistry?

Answer 30

Lose 2 4s electrons so it can lose another to give half full shell