Reaction Kinetics

Question 1

Define rate of a reaction

Answer 1

The rate of a chemical reaction is defined as the change in concentration of reactant or product with time

Question 2

Define order of reaction

Answer 2

Order of reaction with respect to a given reactant/catalyst is the power to which the concentration of that reactant/catalyst is raised in the experimentally determined rate equation.

Question 3

Define rate constant

Answer 3

Rate constant,k, is the proportionality constant in the rate equation.

Question 4

Define half life of a reaction

Answer 4

Half life of a reaction is defined as the time taken for the concentration of a reactant to reduce to half of its initial value.

Question 5

Define activation energy

Answer 5

The activation energy is the minimum energy which the colliding molecules must possess before a collision will result in a reaction.

Question 6

What is the Arrhenius equation?

Answer 6

k=Ae^-Ea/RT

Question 7

Define catalyst

Answer 7

It is a substance which increases the rate of a reaction by providing an alternative reaction pathway with lower activation energy without itself undergoing any permanent chemical change.

Question 8

Define heterogeneous catalyst

Answer 8

It is one that exists in a different phase from reactants and increases the rate of a reaction by providing a different reaction pathway with a lower activation energy without itself undergoing any permanent chemical change.

Question 9

Define homogeneous catalyst

Answer 9

It is one that exists in a same phase as the reactants and increases the rate of a reaction by providing a different reaction pathway with a lower activation energy without itself undergoing any permanent chemical change.

Question 10

Describe the mode of action of heterogeneous catalyst

Answer 10

• Reactant molecules are adsorbed onto the active site of the catalyst via weak temporary bonds.
• The reactant molecules are brought closer together, thus increasing their concentration at the catalyst surface.
• The bonds in the reactant molecules are weakened, lowering the activation energy.
• The molecules are orientated in the right postions for reaction, increasing the number of effective collisions.
• The product is desorbed from the catalyst surface, regenerating the active sites on the catalyst.

Question 11

Why does an increase in concentration increases the rate of reaction?

Answer 11

When concentration of a reactant increases, the number of particles per unit volume increases, hence increasing the frequency of effective collisions. According to the Collision Theory, rate of reaction increases.

Question 12

Why does an increase in temperature increases the rate of reaction?

Answer 12

When temperature increases, the average kinetic energy of reactant molecules increases. More molecules have energy greater or equals to the activation energy. Hence, frequency of effective collisions increases. Rate constant increases, hence rate increases.

Question 13

How does the addition of a catalyst increases the rate of reaction?

Answer 13

• When a catalyst is added, the catalyst provides an alternative reaction pathways with lower activation energy compared to the uncatalysed reaction.
• More molecules will have the energy greater of equals to the lowered activation energy.
• Frequency of effective collisions increases.
• Rate constant increases, rate increases