Mechanisms of Catalysis - The Surface Adsorption Theory

Question 1

The surface adsorption theory is used to describe most of what type of catalysis?

Answer 1

Heterogeneous

Question 2

What is absorption?

Answer 2

When one substance moves into another

Question 3

What is an example of absorption?

Answer 3

Water moving into a sponge

Question 4

What is adsorption?

Answer 4

The accumulation of substances only at the surface of another substance

Question 5

Give an example of adsorption.

Answer 5

Gas molecules accumulating on the surface of a solid

Question 6

Give an example of a reaction which involves adsorption

Answer 6

The reaction between hydrogen and oxygen to form water in the presence of a platinum catalyst

Question 7

How many main steps are there to adsorption?

Answer 7

Three

Question 8

What are the three main steps of adsorption called?

Answer 8

1. Adsorption Stage
2. Reaction On The Surface
3. Desorption Stage

Question 9

Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the first step of Adsorption.

Answer 9

1. The H and O molecules diffuse towards the surface and settle on the surface of the platinum
2. The O and H molecules are held on the surface of the platinum by temporary bonds formed between these molecules and the platinum
3. The vacant d orbitals of platinum help form temporary bonds with the molecules i.e. the molecules are chemisorbed on the platinum

Question 10

Define chemisorbed

Answer 10

Chemically attached

Question 11

What holds the O and H molecules to the surface of the platinum?

Answer 11

Temporary bonds formed using the vacant d orbitals of platinum hold the molecules to the surface

Question 12

Why does platinum have vacant d orbitals?

Answer 12

As it is a transition metal

Question 13

Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the second step of Adsorption.

Answer 13

There is a high concentration of O and H molecules adsorbed to the surface of the platinum which makes it more likely that molecules will collide with each other
These collisions result in bonds being broken and new ones being formed (chemical reactions occur to form water molecules)

Question 14

Why is collision more likely to occur on the surface of a catalyst?

Answer 14

As there is a high concentration of reactants adsorbed to it

Question 15

What helps the chemical reactions which occur when two reactants collide at a catalyst ?

Answer 15

The covalent bonds holding the atoms together are weakened by the formation of new bonds between the reactants and the surface of the catalyst

Question 16

Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the third step of Adsorption.

Answer 16

The products (water molecules) leave the surface of the catalyst and the process begins again