Developing Metals 2: Catalytic Activity

Question 1

Definition of catalyst

Answer 1

• Substance which increases the rate of a chemical reaction without undergoing any permanent chemical change its self
• lowers activation energy and provides an alternative route/pathway

Question 2

Definition of heterogeneous catalyst

Answer 2

Catalyst is in a different physical state to the reactants

eg. Solid catalyst with the reactants in gas liquid or aqueous phase

Question 3

Definition of homogeneous catalyst

Answer 3

Catalyst is in the same physical state to the reactants

eg. All in aqueous solution

Question 4

Explain how transition metals can act as heterogeneous catalysts ( 3 marker )

Answer 4

1. Transition metals can use d and s electrons
2. To form weak bonds with reactants
3. Providing a rate of lower activation energy

Question 5

What property of transition metals allows them to act as a homogeneous catalysts

Answer 5

• ability to have more than one oxidation state

-act as an intermediary in a redox reaction
speed up reactions involving transfer of electrons i.e. redox reactions

Question 6

Explain how a transition metal is able to increase the rate of a redox reaction ( I.e. how acts as a homogeneous catalyst ) ( 4 marker )

Answer 6

1. Transition metal ion reacts with one of the reactants ( transition metal reacts to form an intermediate )
2. Oxidation state of the transition metal ion changes
3. New iron (intermediate )then reacts to reform ( regenerate ) original transition metal ion
4. Activation energy for this reaction is lower compared to if the reaction were to happen without the transition metal ion

Question 7

Give examples of day-to-day situations that involve transition metals acting as heterogeneous catalysts

Answer 7

1. Harber process = iron is catalyst

2. Car exhausts = platinum , rhodium & palladium catalysts

Question 8

What property allows transition metals to act as heterogeneous catalysts

Answer 8

Unfilled d orbitals allows them to accept electrons

Question 9

Describe the process of heterogeneous catalysis in terms of transition metals ( 4 + 2 internal steps in step 2 )

Answer 9

1. gases adsorbed onto surface of transition metal
2. Transition metal ions form weak bonds with reactants

= reactants in closer proximity when adsorbed =increase concentration of reactants = increase rate of reaction

= Bonds that form onto the catalyst may withdraw electron density from the molecules internal bonds= weakening them

3. Products deadsorbed from catalyst surface
4. Leading active sites available for more gasses ( reactants )

Question 10

Why is the strength of adsorption essential

Answer 10

If bonds to strong
= products remain fixed to catalyst
=prevent further catalysis
= catalyst poisoning

Question 11

In what other AS topic does homogeneous catalysis occur

Answer 11

Ozone = Cl• = catalyst O3 depletion = reactants and catalysts all in gas state = homogeneous

Cl• = regenerated = catalyst

In O3 + Cl• —> ClO• + O2
ClO• + O ——> Cl• + O2

= O3 + O —> 2 O2