Homogeneous catalysis

Question 1

a homogeneous catalyst is

Answer 1

one that is in the same phase as the reactants

Question 2

autocatalysis occurs when

Answer 2

a reaction product acts as a catalyst for the reaction

Question 3

a key feature of homogeneous catalysis is

Answer 3

the formation of an intermediate species for which a specific formula can be written

Question 4

the formula for the persulfate ion (also known as the peroxodisulfate ion) is

Answer 4

S2O8 2-

Question 5

the S2O8 2- acts as a……………….. agent in the reaction with iodide ions
the equation for the reaction is

Answer 5

oxidising agent

S2O8 2- + 2I- → 2SO4 2- + I2

Question 6

one reason for why the following reaction is slow at room temperature is:
S2O8 2- + 2I- → 2SO4 2- + I2

Answer 6

the two reactant ions are both negatively charged and so repel each other

Question 7

the reaction below can be catalysed using…………….

S2O8 2- + 2I- → 2SO4 2- + I2

Answer 7

Fe2+ ions

this, along with the reactants and product, is in the aqueous phase, making this a homogeneous catalyst

Question 8

show the two steps in the reaction between S2O8 2- and I- ions when catalysed by Fe2+ ions in the aqueous phase:

Answer 8

1) The Fe2+ are not repelled by the S2O8 2- ions because they have the opposite charge
S2O8 2- + 2Fe2+ → 2SO4 2- + 2Fe3+
2) The Fe3+ ions formed in step 1) now react with the I- ions which also have the opposite charge
2Fe3+ + 2I- → 2Fe2+ + I2

the iron(II) ions used in step 1) are regenerated i step 2), so the two steps can repeat continuously

Question 9

show the two steps in the reaction between S2O8 2- and I- ions when catalysed by Fe3+ ions in the aqueous phase (this is an alternative mechanism):

Answer 9

1) The Fe3+ ions formed in step 1) now react with the I- ions which have the opposite charge
2Fe3+ + 2I- → 2Fe2+ + I2
2) The Fe2+ are not repelled by the S2O8 2- ions because they also have the opposite charge
S2O8 2- + 2Fe2+ → 2SO4 2- + 2Fe3+

the iron(III) ions used in step 1) are regenerated i step 2), so the two steps can repeat continuously
(this involves the two same steps as with the Fe2+ ions, but in a different order)

Question 10

in equation for the oxidation of ethanedioate ions by potassium manganate(VII) is:

Answer 10

2MnO4 - + 5C2O4 2- + 16H+ → 2Mn2+ + 5CO2 + 8H2O

Question 11

what can you deduce from the oxidation of ethanedioate by KMnO4 using the following information:

2MnO4 - + 5C2O4 2- + 16H+ → 2Mn2+ + 5CO2 + 8H2O

the reaction starts slowly, then the rate increases rapidly, then slows down at near completion

Answer 11

the reaction starts slowly because the two reacting ions have negative charges, so repel each other

as more MnO4 - is added, the rate increases because more Mn2+ is produced
in this reaction, the Mn2+ is an autocatalyst which explains why the rate increases rapidly
the rate then decreases as the concentration of C2O4 2- decreases as it is used up

Question 12

on a graph, you can recognise an autocatalysis reaction through:

Answer 12

a slow decrease in the reactant concentration at the beginning, then a sharper fall in conc., then a slower decrease at the end