23.5 Transition metal catalysts

Question 1

What is a catalyst

Answer 1

A catalyst is something that speeds up a chemical reaction by providing an alternative reaction pathway with a lower activation energy, and it doesn’t get used up

Question 2

Why do transition metals make good catalysts

Answer 2

They can change oxidation states easily by gaining or losing electrons in their d orbitals, so they can transfer electrons to speed up reactions

Question 3

What is a heterogeneous catalyst

Give two examples of reactions using these

Answer 3

It is a catalyst that is in a different phase to the reactants, eg is in a different physical state

The contact process with V2O5
The Haber process with Fe

Question 4

How can you make a heterogenous catalyst more efficient

Answer 4

. Increase their surface area: The larger the surface area, the better the efficiency
. Spread catalyst onto support medium so increase surface to mass ratio so a little goes a long way.
Do that with expensive catalyst

Question 5

What is the contact process used for

what is the catalyst for this

Answer 5

It is used industrially to make sulfuric acid

Vanadium (V) oxide V2O5 catalyses this reaction

Question 6

What product are the reactants in the contact process forming.

How is this turned into sulfuric acid

Answer 6

SO2 is made into SO3

You can react SO3 with water (H2O) to make H2SO4

Question 7

How does the contact process occur

Answer 7

. Vanadium (v) oxide is the catalyst used, and it occurs in two steps:

• First the V2O5 oxidises SO2 to SO3 in the reaction, and itself is reduced :

V2O5 + SO2 —> SO3 + V2O4

This reduced catalyst is then oxidised back to V2O5 by oxygen to start the reaction again

V2O4 + 1/2 O2 —> V2O5

Question 8

What type of catalyst is vanadium (V) oxide in this reaction

Why is vanadium (V) oxide the catalyst used in this reaction

Answer 8

It is a heterogeneous catalyst because it is in a different phase to the reactants eg a different physical state

Because it has the right variable oxidation state needed

Question 9

Why can’t elements in other blocks of the periodic table, eg S block be catalysts in redox reactions

Answer 9

. They generally aren’t used to catalyse redox reactions because they don’t have an incomplete 3d sub shell, and don’t have variable oxidation states

Question 10

What is the overall reaction for the contact process

Answer 10

SO2(g) + 1/2O2(g) ———-> SO3(g)

With V2O5 catalyst used, over the arrow

Question 11

What is the Haber process making

Answer 11

Ammonia is made from reacting nitrogen and hydrogen

Question 12

Describe the Haber process

Answer 12

. Nitrogen and Hydrogen are reacted together

. The iron catalyst Fe(s) is used in the shape as lots of pea sized lumps to. increase surface area

Reaction:

N2(g) + 3H2(g) —> 2NH3(g)

With iron over the arrow

Question 13

How are the contact process and the Haber process using heterogeneous catalysts

How is the rate of reaction increased

Answer 13

The catalyst in both is a solid whilst the reactants are gases

So the reaction occurs on the surface of the catalyst

So increasing surface area of catalyst increases the number of molecules that can react at the same time, increasing the rate of the reaction

Question 14

What are catalytic converters and what transition metal is the catalyst

Answer 14

Rhodium is the catalyst in them

They clean up emissions from car engines, by converting nasty waste gases into less harmful products .

They are shaped as a ceramic lattice coated with a thin layer of rhodium on top to increase surface area of catalyst to minimise cost of it because only a small coat is needed

Question 15

What is catalyst poisoning

Answer 15

During a reaction, reactants are adsorbed onto the surface of heterogeneous catalysts

Impurities in the mixture may also bind to the catalyst surface, blocking the reactants from being absorbed

So it reduces the surface area available for the reactants so slows down the reaction

It can increase cost as less product is made with the certain amount of energy

Question 16

Why does the iron catalyst in the haber process have to be replaced every 5 years

Answer 16

There are sulfur impurities because the hydrogen used is from methane which is obtained from natural gas

So that sulfur can be adsorbed into the iron forming iron sulfide

Question 17

What is a homogeneous catalyst

Answer 17

. These are catalysts in the same physical state as the reactants.

Eg an aqueous catalyst for a reaction between two aqueous solutions

Question 18

How does a homogeneous catalyst work

Answer 18

It forms an intermediate species by the reactants combining with the catalyst

This intermediate species then reacts to form the products and reform the catalyst

So these two reactions make the enthalpy profile for a homogeneously catalysed reaction have two humps in it

The energy needed to form the intermediates (and to form the products from the intermediates) is less than the energy needed to create the products directly from the reactants

Question 19

What two reactions catalysed by homogeneous catalysts do we need to know

Answer 19

. Fe2+ catalysing the reaction between S2O8 2- ions and I- ions

. Mn2+ autocatalysing the reaction between MnO4- and C2O4 2-

Question 20

Describe the reaction between S2O82- (peroxodisulfate) and I- ions

Answer 20

The redox reaction is catalysed by iron (Fe2+)
The overall reaction is shown

S2O8 2- + 2I- —> I2 + 2SO42-

Question 21

why would the reaction between peroxodisulfate ions and iodide ions be really slow without the iron catalyst

Answer 21

Because the reactants are both negatively charged so will repel each other so it is unlikely they will collide and react

Question 22

How does adding the Fe2+ ion help speed up the reaction between the S2O82- and 2I-

Give the reactions

Answer 22

Because it is positively charged so the reaction will now involve a positive and negative ion

First the Fe2+ ions are oxidised to Fe3+ ions by the S2O8 2- ions:

S2O8 2-(aq) + 2Fe2+ —>
2Fe3+ + 2SO4 2-

The newly formed intermediate Fe3+ ions can now easily oxidise the I- ions to iodine, and the catalyst is regenerated

2Fe3+ + 2I- —->I2 + 2Fe2+

Question 23

What is the catalyst in the reaction between MnO4- and C2O4 2-

Answer 23

Mn2+ is the catalyst in this reaction, and it is an autocatalysis reaction because Mn2+ is a product and also acts as a catalyst

This means that as the reaction progresses, more product is formed so it speeds up over time

Question 24

What is the redox equation for the reaction between MnO4- and C2O4 2-

Answer 24

2MnO4- + 5C2O42- + 16H+ —–>
2Mn2+ + 8H2O + 10CO2

The activation energy at the beginning is very high as there is no catalyst

There are two negative ions as reactants which repel eachother

Question 25

How does the Mn2+ catalyst work in the reaction between MnO4- and C2O4 2-

Answer 25

When some of the Mn2+ is formed, it reacts with the MnO4- ions to make Mn3+ ions and water

4Mn2+ + MnO4- + 8H+ —> 5Mn3+ + 4H2O

These Mn3+ ions are intermediate, so then react with C2O42- ions to make CO2 and reform the Mn2+ catalyst

2Mn3+ + C2O42- –> 2Mn2+ + 2CO2

So rate of reaction increases as time increases in the reaction

Question 26

What happens if you add ammonia solution to an aqueous solution of cobalt(ii) ions

Answer 26

. First a precipitate is formed by reaction with OH- ions from the ammonia which is alkaline
[Co(H2O)6]2+ + 2OH- –> Co(H2O)4(OH)2(s) + 2H2O(l)

Then the precipitate dissolves in excess ammonia
Co(H2O)4(OH)2(s) + 6NH3 –> [Co(NH3)6]2+ + 2OH- + 4H2O

The resulting complex is oxidised by oxygen in the air to the yellow solution it forms [Co(NH3)6]3+

To get that overall reaction, do two half equations:
[Co(NH3)6]2+ —> [Co(NH3)6]3+ +e-
and
O2 + 2H2O +4e- —> 4OH-

To get
4[Co(NH3)6]2+ + O2 + 2H2O –>
4[Co(NH3)6]3+ + 4OH-