Unit 5: Reaction Kinetics

Chemical reactions are processes in which mass and energy are constantly changing. These reactions can take
place at a range of speeds, from very fast to very slow. Fermentation is known to be a slow reaction, taking
several weeks to make enough products, and digestion is a slow reaction. Acid-base neutralization is a fast
reaction, taking only a few microseconds. Other reactions take place at a moderate speed, such as those that
contract muscles send impulses along nerves and capture photographic images. For industry, it is important to
understand the conditions under which a reaction will be most cost-effective. Kinetics, also known as chemical
kinetics, is the study of the rate of chemical reactions, their mechanisms, and the factors that influence them.

5.1 Rates of Reactions

The rate of a chemical reaction tells us how quickly the reactants are used up or how quickly the products are
formed over time. It is the change in the concentration of reactants or products in each amount of time.
Mathematically,
Chnage∈concentration of a substance
Rate=
Time taken for change

As a reaction happens, the amount of reactants decreases, and the amount of products increases. For example, we
measure concentration in moles per cubic decimeter (mol dm3) and time in seconds (s). So, the unit for the rate of
reaction is mol dm-3s.
To better understand this, we can use a graph. (See figure 5.1)

A graph with two curves-one for reactants showing decreasing concentration and one for products showing
increasing concentration over time. At the beginning of the reaction, the slope of the reactant curve is steep,
showing that the reactants are being used up quickly. At the same time, the product curve is steep, showing that
products are being formed quickly.
1. As time goes on, the curves become less steep. This means the reaction slows down because there are fewer
reactant particles to react.
2. Eventually, the curves become flat. This means the reaction has stopped because all the reactants are used up or
the system has reached equilibrium.
This shows that the rate of reaction is not constant. It changes as the reaction progresses.
Total Chnage∈concentration
Average r ate=
Total t ime taken

Consider a general reaction

A B
The rate of reaction can be expressed in terms of the rate of disappearance of reactant A or the rate of appearance
of product B.

−d [ A]
Rate ofreaction=
dt

+d [ B ]
Rate ofreaction=
dt

Rate of reaction
Rate of reaction
Where, d [A] and d [B] are the changes in the concentration of A and B respectively. The negative sign indicates a
decrease in the concentration of reactant A whereas the positive sign indicates the increase in the concentration of
product B.
When we talk about the rate of a chemical reaction, we're interested in how quickly or slowly a chemical reaction
happens. This is often measured by how fast the reactions are used up or how quickly the products are formed.
Suppose you are carrying out a chemical reaction where substance A reacts with another substance B to produce
substance C.

Interpreting Data
Suppose a student is carrying out a chemical reaction where substance A reacts with another substance B to
produce substance C. He followed the rate of reaction by determining the concentration of C produced at regular
intervals. The following data was obtained.
A+ B C

Time (minutes) Concentration of C (mol dm-3)

0.0 0.00
20 15
40 21
60 23
80 25
100 25
The rate of reaction can be found by plotting a graph of concentration of C against time.

From this graph you can determine the rate of reaction at any moment by determining the gradient at that
moment. Suppose you want to calculate the rate of reaction at 20 minutes. For this purpose, draw a tangent to the

curve at 20 minutes:
∆C
R ate of reaction(¿ gradient )=
∆t
−3
24 mol dm
R ate of reaction(o gradient )=
40 x 60 s
= 0.01 mol dm-3s-1
Thus, the rate of reaction at 20min = 0.01 mol dm-3s-1
Total amount of C produced
Average rate of reaction=
Total time taken
 −3
2 5 mol dm
Average rate of reaction=
10 0 x 60 s
= 0.00 42 mol dm-3s-1
Conclusion
From this experiment, you can see that the reaction rate isn't constant. The amount of substance C formed
increases more quickly at first and then slows down. This is a common pattern in chemical reactions, where the
rate can change as reactants are consumed.

5.2 Collision Theory and Activation Energy

In a chemical reaction, bonds between atoms are broken and new bonds are formed. Collision theory helps us
understand how these reactions happen. For a reaction to occur, the atoms or molecules must collide with each
other. However, not all collisions lead to a reaction. The success of a collision depends on two things:

1. Energy
The colliding particles must have enough energy to overcome the repulsion between their outer electrons.

2. Orientation
The particles must be aligned in a way that allows the necessary atoms to come together and form new bonds.
The smallest amount of energy needed for a reaction to happen is called activation energy. If the energy of the
colliding particles is less than this activation energy, the reaction will not occur.
Reactions happen faster when more particles have enough energy to collide effectively. If the activation energy is
high, fewer particles will have enough energy, and the reaction will be slower. If the activation energy is low,
more particles can collide effectively, and the reaction will happen faster.
For example, when hydrogen (H2) and chlorine (Cl2) react to form hydrogen chloride (HCI), the molecules need
enough energy to break their bonds and form new ones. When an effective collision occurs, the particles form a
temporary, high-energy state called the activated complex (or transition state). This complex quickly breaks down
to form the final products. (Figure 5.2)

You can think of activating energy like a hill. The reacting molecules need to climb this hill (using energy) before
they can roll down the other side and form products. If the molecules don't have enough energy to reach the top,
they roll back and remain unchanged.
5.3 Catalysts and their Role in Reaction Kinetics
In industries, reactions are often carried out at high temperatures to
produce products quickly. However, high temperatures can be
dangerous and may damage certain chemicals. An alternative way
to speed up reactions is to use a catalyst.

A catalyst is a substance that speeds up a chemical reaction without

being used up itself. It works by providing an easier pathway for
the reaction to happen, which has lower activation energy. With
lower activation energy, more particles can collide effectively, and
the reaction happens faster.
It's important to note that a catalyst doesn't change the overall
energy of the reaction or its outcome. It only makes the reaction happen faster. Catalysts cannot make a reaction if
it is not already possible based on the reaction's energy changes.
Concept Assessment Exercise 5.1
1. How does the presence of a catalyst alter the activation energy of a reaction?
2. Why is it important for a catalyst to remain unchanged at the end of a reaction?

5.3.1 Physical Parameters that Affect the Rate of Reaction

1. Change in Mass
During a chemical reaction, the mass of the reactants decreases because they are used up, while the mass of the
products increases as they are formed.
2. Formation of Gas
In some reactions, a gas is produced. If the gas escapes (in an open system), the total mass of the reaction mixture
decreases. In a closed system where gas cannot escape, the pressure increases as more gas is formed because the
volume stays the same.
3. Temperature
When the temperature of a reaction increases, the particles move faster, causing them to collide more often. This
increase in collision frequency results in a faster reaction rate.

5.3.2 Factors Affecting Rate of Reactions

Several factors influence how fast a reaction happens by changing the number of successful collisions between
particles. Some of the important factors are:
1. Concentration of Reactants
The more reactant particles there are in a given space, the more likely they are to collide. This means that
increasing the concentration of reactants makes the reaction happen faster.
 For example, acid rain damages marble more quickly when the acid is stronger.
 Two antacid tablets neutralize stomach acid faster than one tablet because there are more reacting
particles.
  Hydrogen and chlorine gases react twice as fast if their concentrations are doubled.
2. Surface Area
When solid reactants have more surface area exposed, the reaction happens faster. This is because there are more
opportunities for particles to collide.
 Powdered zinc reacts with diluted HCL faster than a solid chunk of zinc because the powder has more
exposed surface area.
 Similarly, powdered aluminum reacts quickly with NaOH, while aluminum foil reacts more slowly.
3. Temperature
Raising the temperature increases the speed of particles, leading to more frequent and energetic collisions.
However, for a collision to cause a reaction, the particles must:
 Have enough energy (called activation energy).
 Be oriented correctly when they collide.
At normal temperatures, only a small number of particles have enough energy to react. A graph called the
Maxwell-Boltzmann curve shows how energy is distributed among particles. Most particles have medium energy,
while only a few have the activation energy needed for a reaction. When the temperature increases, more particles
gain enough energy to overcome the activation energy barrier, leading to faster reactions.
The curve shows the distribution of available energy between the molecules of a gas at constant temperature
(Figures 5.4 and 5.5). It clearly shows that most of the molecules do not have the necessary activation energy.
When temperature increases (say from T1 to T2), the energy of the molecules also increases. Thus, the proportion
of molecules that have required energy activation increases.

Figure 5.4: Maxwell-Boltzmann curve of kinetic energy Figure 5.4: Maxwell-Boltzmann curve of kinetic energy at 2

temperature

Hence an increase in kinetic energy of reactant molecules increases the collision frequency i.e. the number of
effective collisions and hence the reaction rate.
1. Pressure of gases
Pressure increases the concentration of gases for a given volume. There will be more collisions, which will
increase in reaction rate. For instance, a mixture of H2 molecules and Cl2 molecules will react twice as quickly
when the partial pressure of one of the H2 molecules is doubled compared to the partial pressure of the other.
2. Catalyst
A detailed account of the influence of catalysts on reaction rates is given in section 5.3
Concept Assessment Exercise 5.2
1. In what way does increasing the surface area of a solid reactant affect the rate of reaction?
2. How does temperature affect the rate of reaction?
3. Discuss the effect of changing the concentration of a reactant on the rate of reaction.

5.3.4 Enzymes
Most of the chemical reactions that occur in living organisms are regulated by molecules called enzymes. These
are biochemical catalysts, i.e., substances that increase the rate of chemical reactions within living things.
Enzymes like catalysts are not consumed during chemical reactions. Virtually all reactions in living cells are
catalyzed by enzymes. An enzyme is a specialized protein that catalyzes specific biochemical reactions. Each
enzyme catalyzes only one reaction. Most of the enzymes are found inside the cells. However, some are found in
extracellular fluids such as saliva, gastric juice, etc. Enzymes may speed up reactions by a factor of 10 30. Enzymes
have specific shapes. They hold the reactant molecules in the right orientation necessary for successful collisions.
This causes an increase in reaction rates.

5.4 Role of Chemical Kinetics in the Food Industry

When it comes to the food industry, chemical kinetics plays an important role. Some of its key roles include:
 Identifying the optimal time to harvest and transport products, so that it has the best taste, texture, and
nutritional content.
 Identifying factors that cause degradation during transportation.
 Estimating the optimal time of harvest so that the product reaches the market of its best quality.
Minimizing losses due to ripeness during transportation
 Developing methods for storage and transportation that help maintain nutritional value.
 Identifying methods for slowing down specific reactions by controlling temperature and humidity
increasing the shelf life of the product.

KEY POINTS
 The rate of a chemical reaction is a change in the concentration of the reactant or product in a given time.
 The rate of reaction between two specific time intervals is called the average rate of reaction.
 The rate of a chemical reaction depends upon the activation energy for the reaction.
 The minimum amount of kinetic energy that the interacting particles must have is known as activation
energy.
 A substance that accelerates the chemical reaction but does not change its chemical structure at the end is
called a catalyst
 Reaction rates are influenced by the catalyst, which changes the mechanism of the reaction by decreasing
the energy of activation.
 Enzymes are catalysts in living organisms.
References
 Silberberg, Chemistry The Molecular Nature of Matter and Energy
 Bonderand Pardue, Chemistry and Experimental Science 2/e
 Uno Kask and J. David Rawn, General Chemistry
 Graham Hill and John Holman, Chemistry in Context
 John M. Deman, Principles of Food Chemistry

EXERCISE
1. Multiple Choice Questions (MCQs)
1. The rate of a reaction.
(a) Increases (b) Decreases
(c) Remains the same (d) May increase or decrease.
2. The activation energy for a reaction can be:
(a) Increased by increasing temperature
(b) Increased by decreasing temperature
(c) Decreased by increasing concentration of reactants
(d) None of these
3. Reactions with high activation energy are usually:
(a) Fast (b) Slow
(c) Exothermic (d) Reversible
4. In a reversible reaction catalyst lowers the activation energy of the:
(a) Forward reaction
(b) Reverse reaction
(c) Forward as well as reverse reaction
(d) Forward reaction but increases for the reverse reaction
5. Which of the following is NOT a factor affecting the rate of reaction according to collision theory?
a) Number of particles per unit volume
b) Activation energy
c) Presence of a catalyst
d) Molar mass of reactants
6. How does a catalyst increase the rate of reaction?
a) By decreasing the number of particles per unit volume
b) By increasing the activation energy
c) By providing an alternate pathway with higher activation energy
d) By providing an alternate pathway with lower activation energy
7. Which physical parameter is NOT typically affected by the rate of reaction?
a) Change in mass b) Temperature
c) formation of gas d) Color of the reactants
8. Which factor can affect the rate of reaction involving gases?
a) Change in solubility b) Change in pressure
c) Change in volume d) Change in viscosity
9. Increasing the surface area of solids generally:
a) Decreases the rate of reaction
b) Increases the rate of reaction
c) Has no effect on the rate of reaction
d) Makes the reaction irreversible
10. How does temperature affect the rate of reaction according to collision theory?
a) Higher temperature decreases the frequency of collisions
b) Higher temperature increases the activation energy
c) Higher temperature decreases the kinetic energy of particles
d) Higher temperature increases the frequency of collisions and kinetic energy of particles
2. Short Questions
i. Draw energy diagrams that represent the activation energy and show the effect of a catalyst.
ii. What is the effect of a catalyst on the following?
a) The rate of reaction
b) The energy of activation
iii. Why powdered Zn reacts faster with acid than a piece of Zn? Give reason.
iv. List physical parameters which are affected by reaction rates.
v. Explain collision theory and its key components.
vi. Discuss the effect of changing the concentration of a reactant on the rate of reaction.
vii. Describe how graphs are used to interpret the rate of reaction.
viii. Why is activation energy important in determining the rate of reaction?
ix. If you increase the temperature of a reaction, how does this affect the kinetic energy of the particles?
x. Why do only a fraction of collisions lead to a reaction according to collision theory?
xi. Evaluate how increasing activation energy affects the rate of reaction.
xii. Suggest a way to identify if a catalyst has been effective in a reaction. Assess the impact of catalysts on
industrial chemical processes.
xiii. Evaluate the impact of adding an enzyme to a biochemical reaction.
xiv. Analyze how temperature control during transportation can affect the quality of the product.
3. Long Questions
i. Examine the graph depicting
a. the concentration of reactants over time.
b. the effect of temperature on the rate of reaction.
c. the effect of the catalyst on reaction rate.
ii. An increase in surface area increases the reaction rate. Evaluate this statement in the light of collision
theory to support your answer.
iii. Interpret the role of a catalyst in a chemical reaction.
iv. Discuss the impact of temperature on the rate of reaction.
v. Justify the role of chemical kinetics in the food industry.
vi. Propose a hypothesis for how temperature and surface area affect the reaction rate. Design a set of
experiments to test your hypothesis, detailing the methods and measurements.
vii. You are investigating a new catalyst for a reaction that produces a valuable pharmaceutical product.
viii. Explain the role of the catalyst in the reaction mechanism, including its effect on activation energy.
ix. Design an experiment to compare the reaction rates with and without the catalyst.
x. A chemical reaction between substances A and B follows collision theory. You are tasked with maximizing
the reaction rate. Describe how you would modify the number of particles per unit volume, temperature,
and activation energy to achieve this goal.
xi. Compare and contrast the effects of temperature and concentration on the rate of reaction.
xii. Discuss the significance of activation energy in chemical reactions and its relationship to reaction rates.
THINK TANK
1. Analyze a scenario where a catalyst is not effective in increasing the rate of reaction.
2. Evaluate the role of chemical kinetics in optimizing food production processes in the food industry.
3. Predict the effect of doubling the surface area of a solid reactant on the rate of reaction, providing
reasoning based on collision theory.
4. Explain why the formation of gas is often associated with an increase in the rate of reaction.