DM - Transition metal catalysts

Question 1

Why do transition metals and their compounds make good catalysts?

Answer 1

Because they can change oxidation states by gaining or losing electrons within their d-orbitals. This means they can transfer electrons to speed up reactions.

Question 2

What are catalysts good for?

Answer 2

Industry and the environment as they allow reactions to happen faster and at lower temperatures and pressures, reducing energy usage.

Question 3

Why can using transition metal catalysts be bad?

Answer 3

As they can pose health risks as many of the metals and their compounds are toxic - for example, long term exposure to copper can damage the liver and kidneys, and exposure to manganese can cause psychiatric problems.

Question 4

What is a heterogeneous catalyst?

Answer 4

A catalyst in a different phase (state) from the reactants.

Question 5

What state/phase are the reactants usually in heterogeneous catalysis?

Answer 5

Gases or solutions.

Question 6

What state/phase is the catalyst usually in heterogeneous catalysis?

Answer 6

Solid.

Question 7

Why do transition metals make good heterogeneous catalysts?

Answer 7

Because they can use their s- and d-orbitals for bonding to the reactant molecules.

Question 8

Explain the 3 stages of heterogeneous catalysis

Answer 8

1) The reactant molecules are attracted to the surface of the catalyst and stick to it - this is called adsorption.
2) The surface of the catalyst activates the molecules so they react more easily.
3) The product molecules leave the surface of the catalyst making way for fresh reactant to take their place - this is called desorption.

Question 9

What is it called when the reactant molecules stick to the surface of the catalyst?

Answer 9

Adsorption.

Question 10

What is adsorption?

Answer 10

When the reactant molecules stick to the surface of the catalyst.

Question 11

What is is called when the product molecules leave the surface of the catalyst?

Answer 11

Desorption.

Question 12

What is desorption?

Answer 12

When the product molecules leave the surface of the catalyst.

Question 13

How does the surface of the catalyst activating the molecules cause them to react more easily (heterogeneous catalysis step 2)?

Answer 13

Interaction with the catalyst weakens the bonds in the molecule making them easier to break and reform as the products.

Question 14

For something to be a good heterogeneous catalyst it must what?

Answer 14

1) Attract reactant molecules strongly enough so that they are held to the surface long enough to react.
2) Not attract the product molecules too strongly - otherwise the molecules won’t desorb and will block the catalyst from fresh reactants.

Question 15

Explain how iron catalyses the reaction of nitrogen with hydrogen to make ammonia in the haber process

Answer 15

Both N2 and H2 are adsorbed on to the surface of the iron. This has the effect of weakening the bonds in the molecules, which allows the N2 and H2 molecules to split into N and H atoms more easily. The N and H atoms are then able to bond together to form NH3, which desorbs from the iron.

Question 16

What process does iron catalyse?

Answer 16

The haber process (combining nitrogen and hydrogen to form ammonia).

Question 17

What reaction does the transition metal catalyst CuSO4 catalyse?

Answer 17

The reaction of zinc with acids.

Question 18

What reaction does the transition metal catalyst MnO4 catalyse?

Answer 18

The decomposition of hydrogen peroxide.

Question 19

What are homogeneous catalysts?

Answer 19

In the same state as the reactants.

Question 20

What physical state are homogeneous catalysts in?

Answer 20

Aqueous (for a reaction between two aqueous solutions).

Question 21

How do homogeneous catalysts work?

Answer 21

By combining with the reactants to form an intermediate species which then reacts to form the products and reform the catalyst.

The activation energy needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants.

Question 22

How do homogeneous catalysts work in terms of activation energy?

Answer 22

The activation energy needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants.

Question 23

What happens to the homogeneous catalyst after the reaction?

Answer 23

It is always reforms so it can carry on catalysing the reaction.

Question 24

Why is the rate of the uncatalysed reaction between iodide and peroxodisulfate ions so slow?

Answer 24

Because both ions are negatively charged so they repel each other, so it is unlikely they’ll collide and react.

Question 25

Explain how homogeneous catalysis works in peroxodisulfate ions oxidising iodide ions

Answer 25

The redox reaction between iodide ions (I-) and peroxodisulfate ions (S2O8^2-) is slow as both ions are negatively charged so they repel each other making it unlikely that they’ll collide and react (also makes the activation energy really high).

But if Fe2+ ions are added (a homogeneous catalyst), the reaction is sped up because each stage of the reaction involves a positive and negative ion, so there’s no repulsion.

1) First, the Fe2+ ions are oxidised to Fe3+ ions by the S2O8^2- ions.
2) The newly formed intermediate Fe3+ ions now easily oxidise the I- ions to iodine, and the catalyst is regenerated.