catalysts

Question 1

Why do transition metals make good catalysts>

Answer 1

Because of their variable oxidations states

• they can also transfer electrons to speed up reactions

Question 2

vanadium (V) -> vanadium (IV)

CONTACT PROCESS

Answer 2

V2O5 + SO2 -> V2O4 + SO3

-vanadium OXIDISES SO2 to SO3
and itself is reduced

Question 3

Vanadium (IV) -> vanadium (V)

CONTACT PROCESS

Answer 3

V2O4 + 1/2o2 -> V2O5

• reduced catalyst (V2O4) is then oxidised by oxygen gas back into its original state ( V2O5 catalyst)

^heterogenous catalyst

Question 4

What is a heterogenous catalyst?

Answer 4

A catalyst that is in a different phase from the reactants

Question 5

What are support mediums used for?

Answer 5

Often used to make an area of a catalyst bigger

• help minimise cost of reaction
• small coating of a catalyst provides a large surface area.

Question 6

What are the 2 examples of heterogenous catalysts you need to know?

Answer 6

1) Iron in the Haber Process

             Fe N2 + 3H2 ->  2NH3

2) Vanadium(V) oxide in the contact process whilst making sulfuric acid.

                V2o5 SO2 +1/2o2 ->          SO3

• both Fe and V2O5 are in (s) solid state, whilst everything else is in (g)

Question 7

What is catalyst poisoning

Answer 7

When impurities in a mixture bind to the surface of the catalyst blocking the reactants from being adsorbed.

This reduces the surface area of the catalyst and therefore slows down the rate of reaction.

Question 8

Why is catalyst poisoning bad for the industry?

Answer 8

Increases cost, as less product is being made in a certain time or with a certain amount of energy

catalyst poisoning could also cause you to have to regenerate or replace the catalyst, which increases costs

Question 9

What poisons the iron catalyst in the Haber process?

Answer 9

Sulfur

• the hydrogen gas comes from methane
• the sulfur comes from the methane

Any sulfur that is not removed is adsorbed onto the iron, forming iron sulfide, stopping the iron from catalysing the reaction

Question 10

What is a homogenous catalyst?

Answer 10

A catalyst which is in the same physical state as the reactants

Question 11

How does a homogenous catalyst work ?

Answer 11

The homogenous catalyst binds to reactants, forming an intermediate species.

This intermediate species then reacts, to form products and reforms the catalyst.

Question 12

What is special about the enthalpy profile for a homogenous catalyst?

Answer 12

Two curves
The activation energy needed to form the intermediate species is LOWER than the activation energy needed to make products directly from the reactants.

Question 13

peroxodisulfate

Answer 13

S2O8 2-

Question 14

A redox reaction with 2 negatively charged ions…

Answer 14

causes the ions to repel each other, so they are UNLIKELY to collide and react.

• the 2 negative charges is what causes the reaction to have high activation energy

Question 15

Redox reaction between S2O8 2- and iodide ions EQUATION

Answer 15

S2o8 2- + 2I- -> I2 + 2SO4 2-

• all (aq)
• however Fe2+ speeds things up…

Question 16

Fe 2+ speeding things up by reacting with peroxodisulfate first and then Fe3+ reacts with iodide to reform Fe2+ homogenous catalyst:

Answer 16

peroxidisulfate oxidises Fe2+ into Fe3+

S2O8 2- + 2Fe2+ -> 2Fe3+ + 2SO4 2-

• then the newly formed Fe3+ reacts with iodide ions, oxidising them to form iodine and reform the Fe2+ catalyst

2Fe3+ + 2I- -> I2 + 2Fe2+

Question 17

What is autocatalysis?

Answer 17

When a product acts as a catalyst

• this means as a reaction progresses, more product is formed so reaction speeds up.
  e. g Mn2+