Chemical Kinetics

Question 1

Define Rate of a chemical reaction

Answer 1

This is the change in the concentration (amount) of reactants or products per unit time.

Question 2

What is Initial rate of a reaction?

Answer 2

This is the instantaneous rate at the start of the reaction when just a very little amount of the reactants has been used up.

Question 3

What is the Rate equation

Answer 3

This is a mathematical equation that expresses the rate of reaction as a function of the concentration of reactants raised to their appropriate powers.

Question 4

Define ‘Rate constant’, K

Answer 4

This is the ratio of the rate of a chemical reaction to the products of the molar concentrations of the reactants raised to the appropriate powers as in the rate equation.

Question 5

What is the Rate determining step?

Answer 5

This is the slowest stage of a reaction through which reactants are converted to products.

Question 6

Define the term Order of a reaction

Answer 6

This is the sum of the powers to which the molar concentration of a reactants are raised in the experimental rate equation.

Question 7

What is order of reaction with respect to a reactant?

Answer 7

This is the power to which the molar concentration of the reactant is raised in the experimental rate equation.

Question 8

True or false
The order of reaction dependent on the stoichiometric coefficients of the reactants.

Answer 8

False
Order of reaction is experimentally determined and is not dependent on the stoichiometric coefficients of the reactants.

Question 9

What is Molecularity of reaction?

Answer 9

This the number of reactant species (ions, molecules or atoms) involved in the rate determining step.

Question 10

What are the differences between order and molecularity of a reaction?

Answer 10

ORDER
1. Sum of the powers to which the molar concentrations of the reactants are raised in the experimental rate equation.
2. Experimentally determined and not based on the stoichiometry of the reaction.
3. It can be a whole number, zero or a fraction.
4. It is obtained from the rate for the overall reaction.
5. It provides important information about the reaction mechanism.

MOLECULARITY
1. Number of reactant species at the rate determining step.
2. Theoretical concept based on the rate determining step.
3. Always a whole number.
4. It is calculated based on only the rate determining step.
5. It provides no information about the reaction mechanism.

Question 11

Define an Elementary reaction

Answer 11

This is a reaction in which the reactants are converted to products in only one step.

Question 12

True or false
For elementary reactions, the molecularity and order of reaction is the same.

Answer 12

True

Question 13

Describe an experiment to measure the rate of reaction between marble chips and dilute hydrochloric acid using the volume of carbon dioxide released against time

Answer 13

Procedure
- A known mass of marble chips is put in a conical flask connected to a gas syringe by means of a delivery tube. The volume of gas inside the gas syringe is initially set at volume zero by pushing the plunger up the zero mark.
- A known volume of dilute hydrochloric acid is quickly added to the marble chips using a measuring cylinder. The flask is immediately corked and a stop clock started quickly started.
- The acid reacts with the marble chips and releases carbon dioxide according to the following equation: CaCO3 (s) + 2HCl(aq) → CaCl2 (aq) + CO2 (aq) + H2 O(l)
- The volume of carbon dioxide liberated is recorded at regular time intervals until the reaction is complete when the volume of the gas released does not increase any more.
- The results are then be tabulated as shown below.
- A graph of volume of the gas released against time taken is plotted.
- Note: If the initial rate of reaction is needed, construct a tangent at point (0,0) and calculate its gradient.
- The rate of reaction at a specific time (x) is obtained by calculating the gradient of the tangent to the curve at that time.
- Rate at time, X = Gradient of the tangent at time X = 𝐵𝐶/𝐴𝐵

Question 14

Describe an experiment to measure the rate of reaction between marble chips and dilute hydrochloric acid using the change in mass of reactants against time

Answer 14

Procedure
- About 100cm3 of dilute hydrochloric is put acid into a conical flask. The flask is rested onto an electronic balance and the mass of the flask and its contents is recorded.
- To the acid is added a known mass of marble chips, a stop clock immediately started and a loose plug of cotton wool is quickly inserted in the neck of the flask to prevent the escaping carbon dioxide gas from carrying acid sprays with it.
- The mass of the flask and its contents is noted at regular time intervals until the reaction is complete and results recorded in the table below.
- A graph of mass of flask and its contents against time is plotted.
- The rate of reaction at time x is obtained by constructing a tangent to the curve at that time and calculating its gradient.
- Rate at time, X = Gradient of the tangent at time X = 𝑃𝑄/ RQ

Question 15

Why is the rate of reaction is highest at the start of the reaction and decreases as the reaction proceeds?

Answer 15

The concentration of the reactants is highest at the start and decreases with time causing a reduction in the frequency of successful collisions.

The graph eventually flattens showing that the reaction is complete.

Question 16

What factors affect the rate of reaction

Answer 16

a) Concentration of the reactants.
b) Particle size (surface area)- for solid reactants.
c) Temperature of reactants.
d) Pressure (for gaseous reactants).
e) Presence of a catalyst.
f) Light intensity (for photochemical reactions).

Question 17

What is the effect of concentration on the rate of reaction?

Answer 17

The rate of reaction increases with increase in concentration of the reactants. The higher the concentration, the greater the number of reactant particles per unit volume and the high be er the frequency of successful collisions. This increases the rate of conversion of reactants to products.

Question 18

True or false
The change in concentration affects the rate of reaction but has no effect on the activation energy of the reaction and the magnitude of the rate constant

Answer 18

True

Question 19

Describe an experiment to investigate the effect of concentration on the rate of reaction between dilute hydrochloric acid and sodium thiosulphate solution

Answer 19

The reaction between aqueous sodium thiosulphate and dilute hydrochloric acid forms a pale- yellow precipitate of sulphur according to the following equation:
SO2-(aq)+2H+(aq) →S(s)+SO(g)+H2O(l)

Procedure
- 50cm3 of 0.2M sodium thiosulphate is placed in a 200 cm3 glass beaker. To the solution in the beaker is added 10cm3 of 2M dilute hydrochloric acid and immediately a stop clock started.
- The beaker is carefully swirled a couple of times and placed on white piece of paper with a small cross marked on it.
- By looking down through the solution from above, the stop clock is stopped as soon as the pale- yellow sulphur precipitate completely obscure the cross and the time taken (t) is recorded.
- The concentration of the thiosulphate solution is varied by taking 40, 30, 20 and 10cm3 of the original thiosulphate solution and making the total volume 50cm3 by topping up with distilled water. To each of the resultant solutions is added 10cm3 of 2M dilute hydrochloric acid and time taken for the cross to become invisible is recorded in each case.
- A graph of 1/t against volume of sodium thiosulphate is plotted
- The graph gives a straight line through the origin with a positive gradient implying that rate of reaction is proportional to the concentration of the reactants since the reciprocal of the time taken for the cross to disappear is proportional to the rate of reaction and the volume of sodium thiosulphate consumed is proportional to the concentration.

Question 20

What is the effect of surface area or particle size of solid reactants on rate of reaction?

Answer 20

The larger the surface area (smaller the particle size) of solid reactants the greater the area open for the reaction. Hence increase in surface area increases the rate of reaction.

(However increase in surface area has no effect on the activation energy and the value of the rate constant)

Question 21

Describe an experiment to investigate the effect of surface area on the rate of reaction between dilute hydrochloric acid and calcium carbonate

Answer 21

Calcium carbonate and dilute hydrochloric acid react according to the following equation:
CaCO3 (s) + 2HCl(aq) → CaCl2 (aq) + CO2 (g) + H2O(l)

Procedure
- 40cm3 of 2M hydrochloric acid is put in a conical flask. The mass of the flask and its contents is weighed.
- To the acid a known mass of marble chips (e.g. 20g) is added, a stop clock immediately started and quickly a loose plug of cotton wool is inserted in the neck of the flask to prevent the escaping carbon dioxide from carrying acid sprays with it.
- The mass of the flask and its contents is noted at regular time intervals until no further change in the mass of the flask.
- The experiment is repeated with 20g of calcium carbonate powder.
- A graph of mass of flask and its contents against time is plotted on the same axes for both sets of results.

• The graph provides three pieces of evidence that the rate of reaction is greater for calcium carbonate powder than marble chips.
• The reaction completes in a shorter time for calcium carbonate powder than marble chips.
• the curve is steeper for calcium carbonate powder than marble chips.
• within the same period of time t (before completion of the reaction), the mass of unreacted calcium carbonate is greater for marble chips than for the powder.

Question 22

What is the effect of temperature on the rate of reaction?

Answer 22

• At a higher temperature, a greater fraction of reactant particles attains the required activation energy to be converted to products and this increases the reaction rate.
• Also at a higher temperature, the reactant particles attain greater kinetic energy and successful collisions between the reactant particles are more frequent resulting into an increase in the reaction rate.
  (On average the rate of reaction doubles every 10 degrees centigrade temperature rise.)

Question 23

What is the effect of pressure on the rate of reaction?

Answer 23

Increase in pressure brings gas molecules closer together and consequently increasing their concentration. As a result the gas molecules collide more frequently and react more rapidly

Question 24

How do catalysts work?

Answer 24

• Catalysts provide an alternative pathway for which the activation energy for the reaction is lower than that of uncatalysed reaction.
• This enables a larger proportion of reactant particles to attain the required activation energy to be converted into products.
• Catalysts become temporarily involved in the reaction but reform at the end of the reaction.
• They are highly specific in that a suitable catalyst for a particular reaction is not necessarily a catalyst for another reaction.

Note: Although a catalyst affects both the rate of reaction and activation energy, it has no effect on the rate constant.

Question 25

What is a catalyst?

Answer 25

A catalyst is a substance that increases the rate of chemical reaction without itself being used up during the reaction.

Question 26

Define Activation energy

Answer 26

This is the minimum energy required by reactant particles to form products.

Question 27

What is a Homogeneous catalyst?

Answer 27

A homogeneous catalyst is a catalyst in the same physical phase or state of matter as the reactants.

An example is the acid hydrolysis of ethylmethanoate; both the reactants and the catalyst in liquid state.

Question 28

What is a Heterogeneous catalyst?

Answer 28

A heterogeneous catalyst is a catalyst in a different phase or state of matter from that of reactants.

(During the reaction, reactant molecules get absorbed on the surface of the solid catalyst resulting into the weakening of the reactants’ bonds via coordinate bonding and complex formation.
This allows the products to be formed more quickly and increase the reaction rate)

An example of reactions involving heterogeneous catalysts is; Hydrogenation of ethene in presence of nickel catalyst; the reactants are gaseous while the catalyst is solid.

Question 29

What are Auto catalysts?

Answer 29

An auto catalyst is a catalyst generated as one of the products of the reaction.

Examples of reactions involving auto catalysts include: Reduction of acidified potassium permanganate by ethanedioate ions.

The reaction is catalyzed by the manganese(II) ions generated by the reaction.

The rate of reaction increases at the beginning as the manganese(II) ion is being generated but later decreases as the concentration of the reactants decreases.

Question 30

What is the effect of light on rate of reaction?

Answer 30

Light energy may split covalent bonds homolytically forming free radicals which initiate the reaction.

Question 31

What are photochemical reactions?

Answer 31

Reactions which are started by the absorption of light

Examples include: Chlorination of methane

Question 32

Define a Zero-order reaction

Answer 32

A zero order reaction is a reaction whose rate is constant and independent of the concentration of the reactants.

𝑹𝒂𝒕𝒆 = 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕, k

Question 33

What is a first order reaction?

Answer 33

A first order reaction is a reaction whose rate is directly proportional to the concentration of the reactants.

𝑹𝒂𝒕𝒆 = 𝒌[𝒓𝒆𝒂𝒄𝒕𝒂𝒏𝒕𝒔]

Question 34

What is a second-order reaction

Answer 34

A second order reaction is a reaction in which the rate of reaction is proportional to the square of the molar concentration of the reactants.

𝑹𝒂𝒕𝒆 𝜶 𝒓𝒆𝒄𝒕𝒂𝒏𝒕𝒔 or
𝑹𝒂𝒕𝒆 = 𝒌𝒓𝒆𝒄𝒕𝒂𝒏𝒕𝒔;
𝒌 = 𝒓𝒂𝒕𝒆 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕

Question 35

Which one of these would be a first order reaction and a second order reaction?

Answer 35

• A curve of concentration of reactants against time in which the next half-life doubles the previous one
• An exponential curve of concentration against time with constant half-life

Question 36

Define half-life of a chemical reaction

Answer 36

Half life is the time taken for the concentration of reactants to reduce to half it’s original value during a chemical reaction

Question 37

What are the experimental methods of determining order of reaction?

Answer 37

a) Volumetric analysis:
In this method a known volume of the reaction mixture is removed at various time intervals, rapidly quenched and the concentration of one of the substances present in the reaction mixture determined by a suitable titrant

b) Pressure changes:
If a reaction is carried out in gaseous phase results in change in pressure, it can be followed by recording pressure at various time intervals.

c) Electrical conductivity measurements:
This is possible if the electrolytic conductivity changes sufficiently during the course of the reaction.

d) Spectroscopy:
If one of the species in a reaction absorbs radiation in a convenient part of the electromagnetic spectrum, the reaction may be followed by recording the intensity of absorption.

e) Chromatography:
A portion of the reaction mixture is removed at varying time intervals and then analyzed using chromatography.

f) Colorimetry:
This method is only applicable to reaction mixtures that show a steady colour change as the reaction as the proceeds. The concentration of reactants is proportional to the intensity of light shining through a coloured reaction mixture.

Question 38

What type of reaction is the hydrolysis of primary halogen alkanes by aqueous sodium hydroxide or potassium hydroxide to primary alcohols?

Answer 38

• Bimolecular nucleophilic substitution reaction
• It is first order with respect to each of the reactants

Question 39

What type of reaction is the hydrolysis of tertiary halogen alkanes by aqueous sodium hydroxide or potassium hydroxide to tertiary alcohols?

Answer 39

• Unimolecular nucleophilic substitution reaction
• It is first order with respect to the tertiary halogen alkane but zero order with respect to hydroxide ions

Question 40

In a graph of log 10k against 1/T how does one find activation energy?

Answer 40

Ea= -(2.303 x R x slope)

Where R is the universal gas constant 8.31

Question 41

In a graph of lnk against 1/T how does one find activation energy?

Ea= -(R x slope)

Where R is the universal gas constant 8.31

Question 42

In a graph of concentration against time in a zero order reaction, how does one find rate constant, k?

Answer 42

Slope= -k

Where R is the universal gas constant 8.31