Reaction Kinetics

Question 1

Define rate of reaction

Answer 1

Change in concentration of reactant per unit time.

Question 2

Define rate constant

Answer 2

Rate constant, k, is a constant of proportionality in an experimentally determined rate
equation.

Question 2

Define order of reaction

Answer 2

The power to which
the concentration of a reactant is raised to.

Question 2

Define rate equation

Answer 2

Shows exact dependence of rate of reaction on concentration of all reactants.

Question 3

Define half-life of a reaction

Answer 3

Half-life of a reaction is the time taken for the concentration of a reactant to decrease to
half its initial value.

Question 3

Define activation energy

Answer 3

Activation energy is the minimum amount of energy that reactants must possess in order
for a successful reaction to take place.

Question 4

Define catalyst

Answer 4

A catalyst is a substance which increase the rate of a reaction by providing an
alternate reaction pathway of lower activation energy and remains chemically
unchanged at the end of the reaction.

Question 5

Define first order reaction, and its units for k

Answer 5

The reaction rate is directly proportional to the concentration of a single reactant. units of k = s^-1

Question 5

Define second order reaction, and its units for k

Answer 5

The reaction rate is proportional to the product of the concentrations of 2 reactants.
OR
to the concentration of a single reactant raised to the power of 2.
Units of k = mol^-1dm3s^-1

Question 5

Define zero order reaction, and its units for k

Answer 5

The reaction rate is independent of the concentration of the reactant. Rate = k. units of k = mol/dm^3s

Question 6

Formulas for calculations involving half-life

Answer 6

t1/2 = ln2/k

final/initial conc of reactant = (1/2)^n where n is number of half lives

Question 7

State the collision theory

Answer 7

The collision theory is important in explaining the rate of reaction in terms of particle collisions and energy profile of a chemical reaction,

Question 7

How does concentration of reaction increase rate of reaction?

Answer 7

When concentration increases, the number of reactant particles per unit volume increases, frequency of collisions increases. Hence, the frequency of effective collisions also increases. The increased frequency of effective collisions leads to an increase in reaction rate.

Question 8

How does a more finely divided solid reactant react faster in a reaction?

Answer 8

The more finely divided a solid reactant, the greater its surface area per unit volume. The number of sites of collisions increases, frequency of effective collisions increases. Frequency of effective collisions taking place in the reaction increases, thus, the rate of reaction
increases.

Question 9

How does increasing temperature increase rate of reaction?

Answer 9

The average kinetic energy of the particles increases. There is an increase in the fraction of particles with energy equal to or greater than the activation energy (this is as shown by the larger shaded area at a higher temperature in a diagram). This result in an increase in the frequency of effective collisions, hence the rate of reaction
increases, with an increase in rate constant k.

Question 10

How does catalyst increase rate of reaction?

Answer 10

Catalyst provides an alternative reaction pathway which lowers the activation energy of reaction.
Thus, the fraction of particles with energy equal to or greater than the activation energy of
the catalysed reaction increases (this is as shown by the larger shaded area for the
catalysed reaction in a diagram).
This results in an increase in frequency of effective collisions and hence an increase in the rate of the reaction, as well as an increase in rate constant, k.

Question 11

Features of a catalyst

Answer 11

A catalyst is usually unchanged in quantity but may change physically at the end of reaction.

A catalyst generally increases the rate constant, k, for a reaction.

Question 12

What is heterogenous catalysis?

Answer 12

In heterogeneous catalysis, the catalyst and the reactants are in different phases. The catalyst is
usually in the solid phase while the reactants are usually liquids or gases.

Question 13

Define Adsorption

Answer 13

Reactant molecules become adsorbed onto the active sites of the catalyst surface by means of attractive forces. This increases the local concentration of reactants and also weakens the covalent bonds in the molecules.

Question 14

What do catalytic convertors in cars remove and convert into?

Answer 14

Cars are fitted with catalytic converters to remove three main pollutants (CO, NOx and unburnt hydrocarbons) from exhaust gases. The harmful gases are converted into CO2, N2 and H2O.