UNIT15:FACTORS THAT AFFECT CHEMICAL EQUILIBRIUM
UNIT15: FACTORS THAT AFFECT CHEMICAL EQUILIBRIUM
Key unit competency: Deduce how concentration, pressure, catalyst and
temperature affect the chemical processes in industry.Learning Objectives
By the end of this unit, students should be able to:
• Distinguish between complete and reversible reactions.
• Explain dynamic equilibrium.
• State the characteristics of dynamic equilibrium.
• Explain the factors that affect the position of the equilibrium in a reversible
reaction.
• Apply Le Châtelier’s principle to explain the effects of changes in the
temperature, concentration and pressure on a system in equilibrium.
• Compare and contrast theoretical and actual optimal conditions in the
industrial processes.
• Relate the effect of concentration, temperature, pressure and catalyst to the
amount of products in the manufacturing industries.
• Recognize the importance of Le Châtelier’s principle in Haber and Contactprocesses.
The above figure shows that when two teams pull on a rope with equal force. The
resulting force is equal in magnitude and equal to zero and the rope does not move,
the system is said to be in equilibrium. Students in figure (a) represent a system
in equilibrium. The equal and opposite forces on both ends of the seesaw are
balancing. If, instead one force is greater in magnitude than the other, the system
is not in equilibrium [ figure (b)]In chemistry, a chemical reaction is a process where old bonds are broken and new
bonds are formed. For a chemical reaction to take place, two or more substances
called reactants are interacted. In general, when reactants collide with sufficient
energy and in a proper orientation, the products are formed. Many chemical
reactions proceed to a certain extent and stop. In some cases, reactants combine
to form products and the products also start combining to give back the reactants.
When such opposing processes take place at equal rates, no reaction appears and itis said that a state of equilibrium has reached.
15.1. Difference between complete and incomplete reactions
(irreversible versus irreversible reactions)
Activity 15.1
1. Write any two equations of your choice to show a reaction that undergo completion.
2. Write any two equations of your choice to show a reaction that does not go
completion
A chemical reaction can proceed in either non-reversible (irreversible or
complete) or reversible reaction.
During chemical processes, many chemical reactions do not undergo completion
but instead they attain a state of chemical equilibrium. Chemical reaction can
proceed in either non-reversible (irreversible or complete) or reversible reaction.
A non-reversible reaction is a reaction which proceeds in only one direction, inother words, the reactants are completely transformed into products.
15.2. Concept of equilibrium (dynamic equilibrium) and its
characteristics
Activity 15.2
1. Explain the terms used in equilibrium reactions.
(a)Reversible reaction (b) equilibrium state (c) dynamic equilibrium (d) position of
equilibrium.
2. Suggest and explain the characteristics of dynamic equilibrium and how it can
be attained.
3. Learners should do a tug-of-war game outsidetheclassroom and comment on
the game.
4. In a given Hotel, clients enter others leave. At a certain moment if the number
of leavers and arrivals is equal, the number of the clients in the Hotel doesn’t
change.
i. Has the movements of clients coming in and out stopped?
ii. How can you qualify that status?iii. How can you compare this with chemical equilibrium?
15.2.1. Concept of equilibrium reactions
When a chemical reaction takes place in a container which prevents the entry or
escape of any of the substances involved in the reaction, the quantities of these
components change as some are being consumed and others are being formed at
the same time.
Chemical equilibrium is the state at which the rate of forward reaction becomes
equal to the rate of backward reaction.At the initial state, the rate of forward reaction is greater than the rate of backward
reaction. However as the products are formed, the concentration of reactantsdecreases and the concentration of products increases.
Figure 15.5 variation of concentration of A and B with time for reversible reaction
Consider the reaction A B; the figure15.4 (b) indicates how the concentration
of A decreases while that of B increases for the reaction. The dotted vertical line
indicates the time when the concentrations of A and B are no longer changing.
If the reversible reaction is carried out in a closed system, the reaction is said to be in
the equilibrium state when the forward and backward reaction occur simultaneously
at the same rate and the concentrations of reactants and products do not change
with time (Figure 15.4 b).
At this point, the rates of forward and reverse reactions are the same and the system
is said to have reached a state of dynamic equilibrium.
A dynamic equilibrium is a process where the forward and reverse reactions
proceed at the same rate;at that moment the concentrations of reactants andproducts remain constant (do not change).
However, in dynamic equilibrium, even if the concentrations of reactants andproducts do not change, it does not mean that the reaction has stopped. Rather, the
reaction is proceeding in a way that it keeps the concentrations unchanged (the net
change is zero).
There are two types of chemical equilibrium: homogeneous and heterogeneous
equilibria.
In a homogeneous equilibrium, all the reactants and the products are in the samephase.
Checking up 15.2
1. Briefly explain the characteristics of reactions at equilibrium
2. Compare the homogeneous and heterogeneous reactions using specific
examples.
3. By giving an example, describe the term dynamic equilibrium.
4. When does a reaction attain equilibrium state?
5. Using a graph and specific examples, explain what happens during a reactionbefore, at and after the equilibrium has been attained.
Activity 15.3(a)1. Around 1908-1909 a young German research chemist, Fritz Haber, had discovered that nitrogen and hydrogen would form an equilibrium mixture containing ammonia.
(a) Write a balanced equation for the formation of ammonia.
(b)Haber’s experiment yielded an equilibrium mixture containing only 8% by volume of ammonia. What conditions of temperature and pressure does Le Châtelier’s principle predict for maximum yield of ammonia at equilibrium?
(c) Why do you think Haber employed the catalyst accompanied with promoters
and heat exchanger in his equipment?
2. How is Le Châtelier’s principle used to explain the conditions that affect the
equilibrium reactions?
Many industrial processes involve reversible reactions. It is important to understand
how the variation of conditions can affect the composition of a chemical equilibrium.
Some reactions to take place involve some conditions. For example, the rate of a
chemical reaction depends on factors that affect the reaction.
Different factors which can affect the chemical equilibrium include:
1. Temperature
2. Pressure
3. Concentration of reactants and products
The effect of the above-mentioned factors on chemical equilibrium can be explained
by the Le Châtelier’s Principle.
Le Châtelier’s Principle
According to Le Châtelier’s Principle, when the temperature, pressure or concentration
of a reaction in equilibrium is changed, the reaction shifts in the direction where the
effect of these changes is reduced.
15.3.1. Effect of Temperature on equilibrium
Activity 15.3(b)
1. Explain the following terms
(a)Endothermic (b)Exothermic
(c) Suggest how temperature affects the position of equilibrium.
When dealing with temperature, we distinguish exothermic and endothermic
reactions. A change in the temperature of a system already in equilibrium could
either shift the equilibrium to the right (favoring the forward reaction) or to the left
(favoring the backward reaction). This depends on whether the forward reaction is
exothermic or endothermic. Heat can be considered a reactant in an endothermic
reaction and a product in an exothermic reaction. For a reversible reaction, when
the forward reaction is exothermic, the enthalpy change is negative (ΔH < 0), then
the backward reaction is endothermic and the enthalpy change is positive (ΔH > 0).
For exothermic forward reactions, an increase in temperature will cause the system
to counter balance it by favouring the reaction that consumes heat, hence the
backward reaction will be favoured or promoted. On the contrary, if the temperature
is decreased, the system reacts to produce more heat by favouring the forwardreaction.
15.3.2. The effect of change in concentration on equilibrium
Activity 15.3(c):
Experiment to investigate the effect of changing concentration on
equilibrium
Equipment/materials
According to Le Châtelier’s principle, an increase in pressure favours the reaction in
the direction where the volume of reactants is reduced, or less molecules of gas are
formed, and a decrease in pressure favours the reaction in the direction where thevolume of reactants is increased, or more molecules of gas are formed.
To obtain much ammonia in the equilibrium mixture, a high pressure of 200
atmospheres is needed.The effect of pressure can be summarized by the graph indicated below
High Pressure gives a good yield of ammonia as indicated from the graph above, at
400 atmosphers the yield of ammonia is 70%
Higher pressure increases the rate of reaction
However, the higher the pressure used, the higher the cost of the equipment needed
to withstand the pressure.
The higher the pressure the higher the electrical energy costs for pumps to produce
the pressure.
A moderately high pressure of between 150 – 300 atmospheres is used.
15.3.4. The effect of a catalyst on equilibrium
Activity 15.3.4
1. What is an enzyme?
2. What is a catalyst? Name the catalyst used in the Haber process and contact
process
3. What would happen if the enzymes involved in the digestion of food were not
present?
4. Most of the metabolic processes in the body are controlled by enzymes. What
would happen to these metabolic processes if the enzymes were missing?
The function of a catalyst is to speed up the reaction by lowering the activation
energy. The catalyst lowers the activation energy of the forward reaction and reverse
reaction to the same extent. Adding a catalyst doesn’t affect the relative rates of the
two reactions and therefore the catalyst has no effect on the equilibrium system.
But the catalyst helps the system to reach the equilibrium more quickly. The catalyst
does not appear in the overall equation of the reaction.
Practical and financial aspects: In industry, all the above factors must be considered,
taking in account not only the theoritical advantages but also their costs and risks.That is why for example the manufacture of ammonia is based on a compromise of
15.4. End unit assessment
