Province of the

EASTERN CAPE
EDUCATION

PHYSICAL SCIENCES LESSON MATERIAL

TERM 2 -2020
CHEMICAL EQUILBRIUM
SUBTOPIC- Chemical equilibrium and factors affecting
chemical equilibrium.

DURATION: 8hrs

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 CHEMICAL EQUILBRIUM
1. Chemical equilibrium and factors affecting chemical equilibrium

Key Concepts

-Open system
-Closed system
-Reversible reaction
-Dynamic equilibrium

Outcomes

Explain what is meant by:

 Open system
 Closed system
 A reversible reaction
 Chemical equilibrium
 List the factors that influence the position of an equilibrium

Open and Closed system

 The reactants and products together with the container in which the
reaction occurs, are known as the system
 Open and closed systems: An open system environment, while a closed
system is isolated from its surroundings.

A reversible reaction
It is a reaction is reversible when products can be converted back to reactants

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Chemical equilibrium
It is a dynamic equilibrium when the rate of the forward reaction equals the rate
of the reverse reaction.
The factors that influence the position of an
equilibrium: NB: These
factors apply to
A change in pressure (gases only). all reversible
A change in concentration of a reactant or a product. chemical
A change in temperature. reactions

▪ Temperature- increasing or decreasing the temperature of the equilibrium

mixture.
▪ Concentration- adding substance to or removing a substance from the
system at constant volume (this only applies to solutions (aq) and to gases
(g)
The reaction that counteracting the change will be favoured.
▪ Pressure –increasing or decreasing the pressure on an equilibrium mixture
(this ONLY applies to gases
If pressure if pressure is increased, the reaction that proceeda towards the
least number of moles of gas will be favoured.
If pressure is decreased, the reaction that proceeds towards the most number
of
moles of gas is favoured.

NB: Catalyst do not affect the equilibrium position.

The change in equilibrium The effect on equilibrium

condition

Increases in concentration of The rate of the reaction towards the

reactants formation of products is increased.

Decrease in concentration of The rate of the reaction towards

reactants products is increased.

Increase in temperature Both forward and reverse reaction

rate increase, however, the
equilibrium shifts towards the
endothermic side of the reaction

Decrease in temperature Reaction rate slows down

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 The shift in equilibrium is towards
the exothermic side

Change in pressure( for gases) An increase in pressure will shift the

equilibrium towards the reaction
where there are fewer moles of gas
and vice versa

2. EQUILIBRIUMS CONSTANT

Key Concepts

Equilibrium constant

Outcomes

▪ List the factors that influence the value of the equilibrium constant, Kc.

▪ Write down an expression for the equilibrium constant having been

given the
equation for the reaction.

▪ Perform calculations based on Kc values.

▪ Explain the significance of high and low values of the equilibrium

constant.

Equilibrium constant expression

Equilbrium constant can be described as an expression of the concentration of the

products over the concentration of the reactants for a reaction that is in equilbrium.

kc 
 product
reac tan t 
Where :
▪ Kc is an equilibrium constant (no unit)
▪ [Substance] is concentration of (aq) or (g) reactant or product in mol.dm-3
▪ mol is number of moles of each compound in the balanced reaction
equation.

Low Kc value (Kc <1) indicates that a higher concentration of reactants than
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 products are present (the equilibrium lies to the left).

In a closed system in equilibrium:

The value of Kc does NOT change:
▪ If pressure in the system changes
▪ If the concentration of a reactant or product in the system changes
▪ If a catalyst is added to the system
The value of Kc changes if the temperature changes.

The effect of a change in temperature on Kc

Forward reaction favoured Reverse reaction favoured

More products form More reactants form

[Products] increases [products] decreases

[Products] decreases and [Reactants] increases and

Kc increases Kc decreases

Worksheet 1
1. Explain the difference between open and closed system. (2)

2. Explain what is meant by the term reversible reaction (2)

3. Explain is what is meant when a system is in dynamic equilibrium. (2)

4. How do we present a system that is in dynamic equilibrium. (2)

5. Name three factors that are able to change the equilibrium of a system in (3)
dynamic equilibrium.

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Worksheet 2

0 2 4 6 8 10 12

Time (minutes)

2.1 Write down the balanced equation which is represented by the broken line. (2)

2.2 After 8 minutes the pressure is decreased.

2.2.1 State Le Chatelier’s Principle. (2)

2.2.2 Apply Le Chatelier’s principle to the reaction in order to (3)

EXPLAIN the
changes shown on the graph between 8 and 10 minutes.

2.3 Write down an expression for the equilibrium constant (Kc ) for this reaction. (2)

2.4 Initially 5mol of XA3 ( )g was sealed in a 2dm3 flask. At equilibrium the (6)
reaction mixture contained exactly 1,5mol of A g2 ( ) at 300ºC. Calculate
the value of the equilibrium constant (Kc ) at this temperature.

2.3 7 moles of nitrogen gas (N2 ) and 2 moles of oxygen gas (O2 ) are placed in
an empty container of volume 2dm3 . The container is sealed and the
following equilibrium is established:

The Kc value for this reaction at 250C is 4 ,8 x 10-31 .

2.3.1 What information does this value of Kc indicate with regards (2)
to the amount of NO(g) in the equilibrium mixture at 250C?
The container is heated and the system reaches a new

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 equilibrium at 2500ºC. At this temperature it is found that
there are 0,4 moles of NO(g) present.

2.3.2 How much N2 reacted? (1)

2.3.3 How much O2 is there at equilibrium? (1)

2.3.4 Determine the concentration of NO at equilibrium. (3)

2.3.5 Determine the Kc value at this temperature. (4)

2.3.6 Making use of Le Chatelier's principle, explain why the (4)

forward reaction is endothermic.
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Worksheet 3

1. A certain amount of NO2 gas is sealed in a gas syringe at 25ºC. When

equilibrium is reached, the volume occupied by the reaction mixture in the
gas syringe is 80cm3 . The balanced chemical equation for the reaction
taking place is:

1.1 State two conditions necessary for chemical equilibrium to (2)

occur.

1.2 At equilibrium the concentration of the NO2 (g ) is 0 2, mol dm-3

. The equilibrium
constant for the reaction is 171 at 250C.

1.3 Calculate the number of moles of N2O4 at equilibrium. (3)

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 1.4 Calculate the initial number of moles of NO2 ( )g placed in the (3)
gas syringe.
1.5 Write an expression for the equilibrium constant, Kc , of this (2)
reaction.

1.6 The diagram below shows the reaction mixture in the gas syringe after
equilibrium is established as seen at time t1 .

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80 cm

The pressure is now increased by decreasing the volume of the gas

syringe at constant temperature as illustrated in the diagram belo.

Immediately after increasing the pressure, before the new equilibrium is

established, the colour of the reaction mixture in the gas syringe appears
darker than before.
1.7 Give a reason for this observation. (1)

After a while, at time t2 , a new equilibrium is established as illustrated

below. The colour of the reaction mixture in the gas syringe now appears
lighter than the initial colour.

1.8 Use Le Chatelier’s principle to explain the colour change observed in the gas
syringe.

1.8.1 Colour of the reaction mixture (3)

1.8.2 Value of the equilibrium constant, Kc (3)

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Worksheet 4

1 A chemical engineer studies the reaction of nitrogen and oxygen in a laboratory.

The reaction reaches equilibrium in a closed container at a certain temperature, T,
according to the following balanced equation:

Initially, 2 mol of nitrogen and 2 mol of oxygen are mixed in a 5dm 3 sealed
container.
The equilibrium constant (Kc ) for the reaction at this temperature is 1,2 x 10-4 .

1.1 Is the yield of NO(g) at temperature T HIGH or LOW? Give a reason (2)
for the
answer.
1.2 4.2 Calculate the equilibrium concentration of NO(g) at this (8)
temperature.

1.3 How will each of the following changes affect the YIELD of NO(g)?
Write down only INCREASES, DECREASES or REMAINS THE SAME.

1.3.1 The volume of the reaction vessel is decreased at constant (1)

temperature.

1.3.2 An inert gas such as argon is added to the mixture. (1)

1.4 It is found that Kc of the reaction increases with an increase in temperature. (3)
Is this reaction exothermic or endothermic? Explain the answer.
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