Kinetics

Question 1

When asked to draw the Maxwell-Boltzmann Distribution curve, what are the key things to include?

Answer 1

1. Origin
2. Axes
3. Legend / key
4. Shading

Question 2

Explain the effect of increase in temperature on the rate of a reaction.

Answer 2

1. At higher temperature, average kinetic energy of reactant particles increases.
2. No. of reactant particles with energy greater than or equals to Ea increases (shaded area is larger).
3. Frequency of effective collisions in the reaction increases.
4. Since the rate of reaction is proportional to the frequency of effective collisions, the rate of reaction increases.

Question 3

What is a catalyst?

Answer 3

A catalyst is a substance that increases the rate of reaction by providing an alternative pathyway with LOWER activation energy. It remains chemically UNCHANGED at the end of a reaction.

Question 4

Explain the effect of a catalyst on the rate of reaction.

Answer 4

1. In the presence of a catalyst, activation energy is lowered.
2. No. of reactant particles with energy more than or equals to Ea’(cat) increases (shaded area is larger).
3. Frequency of effective collisions in the reaction increases.
4. Since rate of reaction is proportional to frequency of effective collisions, the rate of reaction increases.

Question 5

Explain the effect of increasing concentration of a reactant on the rate of reaction.

Answer 5

1. No. of reactant particles per unit volume increases.
2. Frequency of effective collisions in the reaction increases.
3. Since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases.

Question 6

Half-life of an overall first-order reaction does not depend on the reactant concentration. T/F?

Answer 6

T. This is only false if it’s an overall pseudo first-order reaction, where half life depends on concentration of the reactant in large excess.
(recall: half life = ln2/k’, where k’=k[reactant in large excess]

Question 7

What is a heterogeneous catalyst?

Answer 7

A catalyst that acts in a different phase from the reactant. It is usually a transition metal that has vacant, low-lying/energetically accessible partially filled 3d orbitals able to accept electrons from reactant molecules. It lowers activation energy by means of adsorpption.

Question 8

What is the general mode of action of heterogeneous catalysts?

Answer 8

1. ADSORPTION
   Reactant molecules diffuse towards the catalyst surface and are adsorbed onto the active sites at the surface. Weak chemical bonds are formed between the molecules on the active sites, thereby weakening the bonds in the adsorbed reactant molecules. Adsorption increases the concentration of reactant molecules on the catalyst surface.
2. REACTION AT THE SURFACE.
   Intramolecular bonds WITHIN reactant molecules are weakened, thus lowering the activation energy.
3. DESORPTION
   Product molecule desorbs and diffuses away from catalyst surface.
   This frees up the active sites on catalyst surface for adsorption with new reactant molecules.

Question 9

What is the definition of ‘activation energy’?

Answer 9

Activation energy is the minimum amount of energy, which the reactant particles must possess to overcome the energy barrier in order for effective collisions to result in a chemical reaction.