Reaction Kinetics

Question 1

Catalysts

Answer 1

increase the rate of reaction by providing an alternative pathway which has a lower activation energy

Question 2

Catalysts can be either

Answer 2

homogeneous or heterogeneous

Question 3

Homogeneous catalysts

Answer 3

are those that are in the same phase as the reaction mixture

Question 4

Heterogeneous catalysts

Answer 4

are those that are in a different phase from the rest of the reaction mixture

Question 5

In heterogeneous catalysis, where do the molecules react

Answer 5

the molecules react at the surface of a solid catalyst

Question 6

mode of action of a heterogeneous catalyst consists of the following steps

Answer 6

1. Adsorption (or chemisorption) of the reactants on the catalyst surface
2. Desorption of the products

Question 7

Explain how adsorption (or chemisorption) occurs during heterogeneous catalysis

Answer 7

1)The reactants diffuse to the surface of the catalyst
2)The reactant is physically adsorbed onto the surface by weak forces
3)The reactant is chemically adsorbed onto the surface by stronger bonds
4)Chemisorption causes bond weakening between the atoms of the reactants

Question 8

Explain how the desorption of products occurs in heterogeneous catalysis

Answer 8

The bonds between the products and catalyst weaken so much that the products break away from the surface

Question 9

Describe the mode of action of the iron catalyst in the Haber process

Answer 9

1. Diffusion of the nitrogen and hydrogen gas to the iron surface
2. Adsorption of the reactant molecules onto the iron surface by forming bonds between the iron and reactant atoms
   *These bonds are so strong that they weaken the covalent bonds between the nitrogen atoms in N2 and hydrogen atoms in H2, but they are weak enough to break when the catalysis has been completed
3. The reaction takes place between the adsorbed nitrogen and hydrogen atoms which react with each other on the iron surface to form NH3
4. Desorption occurs when the bonds between the NH3 and iron surface are weakened and eventually broken
5. The formed NH3 diffuses away from the iron surface

Question 10

Catalysts in catalytic convertors perform what function

Answer 10

They remove harmful oxides from the exhaust gases of car engines by speeding up the conversion of:
-Nitrogen oxides (NOy) into harmless nitrogen gas (N2)
-Carbon monoxide (CO) into carbon dioxide (CO2)

Question 11

structure of a catalytic converter

Answer 11

has a honeycomb structure containing small beads coated with platinum, palladium, or rhodium metals which act as heterogeneous catalysts

Question 12

mode of action of the catalysts in a catalytic converter

Answer 12

1)Adsorption of the nitrogen oxides and CO onto the catalyst surface
2)The weakening of the covalent bonds within nitrogen oxides and CO
3)Formation of new bonds between:
-Adjacent nitrogen atoms to form N2 molecules
-CO and oxygen atoms to form CO2 molecules
4)Desorption of N2 and CO2 molecules which eventually diffuse away from the metal surface

Question 13

Homogeneous catalysis often involves

Answer 13

-redox reactions in which the ions involved in catalysis undergo changes in their oxidation number
-as ions of transition metals can change oxidation number they are often good catalysts
-catalysts are used in one step and are reformed in a later step

Question 14

The iodine-peroxydisulfate reaction

Answer 14

a very slow reaction in which the peroxydisulfate (S2O82- ) ions oxidise the iodide to iodine
S2O82- (aq) + 2I- (aq) → 2SO42- (aq) + I2 (aq)

Question 15

what and why is the catalyst used in the iodine-peroxydisulfate reaction

Answer 15

-Fe3+ (aq) ions are used as a homogeneous catalyst
-Since both the S2O82- and I- ions have a negative charge, it will require a lot of energy for the ions to overcome the repulsive forces and collide with each other
-By reacting the reactants with a positively charged Fe ion, there are no repulsive forces, and the activation energy is significantly lowered

Question 16

catalysis of Fe3+ (aq) ions in the iodine-peroxydisulfate reaction

Answer 16

The catalysis involves two redox reactions:
1)First, Fe3+ ions are reduced to Fe2+ by I-
*2Fe3+ (aq) + 2I- (aq) → 2Fe2+ (aq) + I2 (aq)
2)Then, Fe2+ is oxidized back to Fe3+ by S2O82-
*2Fe2+ (aq) + S2O82- (aq) → 2Fe3+ (aq) + 2SO42- (aq)

The order of the two reactions does not matter i.e. So, Fe2+ can be first oxidised to Fe3+ followed by the reduction of Fe3+ to Fe2+

Question 17

How acid rain is formed

in terms of fossil fuels

Answer 17

As fossil fuels contain sulfur, burning the fuels will release sulfur dioxide which oxidises in the air to sulfur trioxide, and then dilute sulfuric acid (H2SO4) is formed by reaction with water. The result is the acidification of rain:
SO3(g) + H2O(l) → H2SO4(aq)

Question 18

mode of action of nitrogen oxides in acid rain

Answer 18

1)Nitrogen oxides can act as catalysts in the formation of acid rain by catalysing the oxidation of SO2 to SO3
*NO2(g) + SO2(g) → SO3(g) + NO(g)
2)The formed NO gets oxidised to regenerate NO2
*NO(g) + ½ O2(g) → NO2(g)
3)The regenerated NO2 molecule can again oxidise another SO2 molecule to SO3 which will react with rainwater to form H2SO4 and so on