Transition metals - catalys

Question 1

Define Hetergenous Catalysts

Answer 1

when the catalyst is in a different phase to the reactants. they act by proving an alternate pathway with a lower activation emergy. this speeds up rate of reaction and they don’t get used up.

Question 2

Define Homogenous Catalysts

Answer 2

when the catalyst is in the same phase as the reactants. they act by proving an alternate pathway with a lower activation emergy. this speeds up rate of reaction and they don’t get used up.

Question 3

define catalyts

Answer 3

they increase the rate of reaction without getting used up, by providing an alternate pathway with a lower activaton energy. they never get used up and a re-formed at the end. saves time, money and energy so good for the environment.

Question 4

what states are heterogenous catalysts usually in

Answer 4

they are usually in solids, whereas the reactants and products are gasaous or in solution.

Question 5

what are the steps in hetergenus catalysis.

Answer 5

1. Reactants form bonds with atoms at the active sites on the surface of the catalyst. - they are adsorbed onto the surface. this adsorption leads to catalytic activity.
2. As a result the bonds in the reactants are weakened/or the bonds being held in a more reactive configuration, resulting in it breaking.
3. New bonds form between the reactants held close together on the catalysts surface.
4. This in turn weakens the bonds between product and catalyst and product leaves - the product deabsorbs.

Question 6

what is used to make the weak bonds on the atoms of the metals with the reactants

Answer 6

Transition metals can use the partially filled 3d and 4s electrons to make the weak bond.

Question 7

what factor increases the rate of reaction at the adsoroption stage.

Answer 7

there is a higher concentration of reactans at the surface of the solid catalyst = leading to a higher collision frequency.

Question 8

what helps determine the effectiveness of catalytic activity.

Answer 8

strength of adsorption. some metals have too strong adsorption and rhe producsta can’t be released easily. some metals like Ag have too weak adsorption, and the reactants do not absorb in high enough concentration.

Question 9

what increases the effectiveness of a solid catalyst

Answer 9

increasing the surfae area, this can be done by using a support medium. this maximisies the surface area and minimises the cost, eg - Rh on a ceramic support in catalytic converter.

Question 10

why are TMs with variable oxidation states good catalysts

Answer 10

electrons in the d-orbitals as they are very close to each other and need the same enrgy to reveive and lose them easily, which speeds up reactions.

Question 11

give examples of heterogenous catalysts and what proces they are used in

Answer 11

V2O5/vanadium (V) = in the contact process = making H2SO4
Fe = the Haber process = making ammonia

Question 12

what are ther two steps to make H2SO4 in the contact process

Answer 12

1. V2O5 oxidises SO2 to SO3
   V2O5 + SO2 -> V2O4 + SO3
2. V2O4 is oxidised by oxygen in the air and V2O5 is reformed.
   V2O4 + 1/2O2 -> V2O5
   this SO3 is bubbles through water to make H2SO4.
   overall equation =
   SO2 + 1/2O2 __V205___> SO3

Question 13

Whats a problem with hetergenous catalysts

Answer 13

they can get poisoned, this is when the active sites on the catalyst gets blocked by impurities. this reduces the surface area of the catalyst for the reactants to bind to nd slows down the reaction.

Question 14

give an example where poisoning becomes a problem

Answer 14

in the haber process = N2(g) + 3H2(g) ___Fe___> 2NH3 (g)
here the hyfrogen comes from methane. methane contains sulfur impurities. Any sulfur that isnt removed will absorb to the surface of iron and frm iron sulfide which reduces the surface aea and hence reduces the efficiency of iron = recusing the rate.

Question 15

what effects does a poisoned catalyst have on yeild and cost

Answer 15

• less product made.
• the catalyst would need to be replaced.
• and the cost of the chemical process will also increase.

Question 16

what is the difference in activation energy with homogenous catalysts. and normal catalysts

Answer 16

homogenous catalysts energy profiles have 2 activation energy bumpes. the dip in betweeen both is where the intermediates are producesd, these then react to form products . and the catalyst is reformed again.

Question 17

give an example of a proces that uses a homoegenous catalyst

Answer 17

oxidation of iodide ions using peroxodisulfate (S2O4^2-), with Fe2+ as a catalyst.
the reaction is very slow to start with as here we are reacting 2 negative ions, so there is very high repulsion. as a result it has high activation energy.
S2O4^2- + 2I2^- –> I2 + 2S2O4^2-

Question 18

explain what happens at the 2 steps of the oxidation of iodide ions using peroxodisulfate (S2O4^2-).

Answer 18

Using Fe2+ as a catalyst, lowers the activation energy and speed up the reaction.
1. first the peroxodisulfate (S4O4^2-) is reduced.
S2O4^2-(aq) + 2Fe2+(aq) –> 2Fe3+(aq) + SO4^2-(aq)
- here the Fe2+ is oxidised to Fe3+ and the S2O4^2- is reduced to SO4^2-.
2. the Fe3+ as the intermediate product is then used to form the I2
2Fe3+(aq) + 2I-(aq) -> 2Fe2+(aq) + I2(aq)
- here the Fe3+ ions can also act as the catalyst cos the two steps can occur in any order.

Question 19

what conditions are neccessary for smth to act as a homogenous catalyst

Answer 19

it’s electrode potential must lie in between the electrode potentials of the two reactants. it will reduce the more postive elecrtrode potential first and then oxidises the more negative electrode potnetial (NO PRoblem in the order of OILRIG).
- using electrode potential values to find a catalyst only shows that catalysis is possible but it doesnt garantee that the rate of reaction will be increased.

Question 20

Autocatalysis and give an example

Answer 20

another form of homogenous catalst where the product acts as the catalyst. eg= where Mn2+ is the catalyst in a reaction between C2O4^2- and MnO4^-
2MnO4^-(aq) + 5C2O4^2-(aq) + 16H+(aq) –> 2Mn^2+(aq) + 10CO2(g) + 8H2O(l)
- this reaction is very slow as we are reacting two negatively chanrged ions, so have a greater activation energy.

Question 21

Describe the steps in reaction where Mn2+ catalyses the reaction between C2O4^2- and MnO4^-

Answer 21

1. 4Mn2+ + MnO4- + 8 H+ -> 5Mn3+ + 4 H2O
   the Mn2+ catalyses the reaction by converting MnO4- into Mn3+
2. 2Mn3+ + C2O42- –> 2Mn2+ + 2 CO2
   then the Mn3+ formed reacts with C2O42- and Mn2+ ions are reformed.

Question 22

following the rate of an autocatalysis reaction.

Answer 22

by removing samples at set times and titrating to work out the concentration of MnO4- .
It could also be done by use of a spectrometer measuring the intensity of the purple colour. This method has the advantage that it does not disrupt the reaction mixture, using up the reactants and it leads to a much quicker determination of concentration.