Reaction Kinetics

Question 1

What is an important feature of first row transition elements (and their compounds) which allow them to function effectively as heterogeneous catalysts

Answer 1

The availability of partially filled 3d electrons for bond formation with the reactant molecules or the availability of low lying vacant orbitals which can accept electron pair from the reactant molecule

Question 2

What are the important features of transition elements that enable them to function effectively as heterogeneous catalysts

Answer 2

(a) Their ability to exist in different oxidation states
(b) The relative ease with which they can be converted from one oxidation state to another

Question 3

Mechanism of Heterogeneous Catalyst

Answer 3

(1) Approach:
Reactant molecules diffuse towards the catalyst surface

(2) Adsorption:
Reactant molecules adsorbed onto the active sites. This adsorption leads to an increase in reaction rate since reducing the activation energy due to weakening of the bond

(3) Chemical Reaction:
Adjacent reactant molecules are brought into close contact with proper orientation for reaction

(4) Desorption:
This is the reverse of adsorption. The product molecule eventually breaks free from the catalyst surface. The product molecule diffuses away from the surface. The active sites are now vacant and available for adsorption of other reactant molecules

Question 4

How to tell if the reaction is first order with respect to [reactant] from the graph

Answer 4

regardless of axes of graph, just find the half-life of the [reactant]/[product], if it is consistent, doesn’t have to be exactly the same, as long as it is around the same it is first order.

Question 5

Explanation for Boltzmann graph on both Temperature and Catalyst

Answer 5

(a) Temperature:
As temperature increases, the number of particles with energy greater or equal to the activation energy increases. This increases the frequency of effective collisions. Given that the rate of reaction is directly proportional to the frequency of effective collisions, rate of reaction increases

(b) Catalyst
In the presence of a catalyst, the reaction will proceed with a different mechanism with a lower activation energy compared to the uncatalysed reaction. With that, the number of reactant particles with energy greater or equal to the catalysed lower activation energy increases. This increases the frequency of effective collisions, and in turn the rate of reaction will increase.

Question 6

How to spot pseudo-first order reactions

Answer 6

(1) One of the reagents is in excess (>10x)

(2) Presence of a Catalyst, k’ = k[catalyst]

make sure to write out the half-life equation to determine which conc. actually affects the rate.