8.1 DYNAMIC EQUILIBRIUM

Chapter 8

Chemical Equilibrium

F2F: 6 HOURS NF2F: 6 HOURS

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CHEMICAL EQUILIBRIUM

8.1 Dynamic Equilibrium
8.2 Equilibrium constant, Kc and Kp

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Learning Outcomes

8.1 Dynamic equilibrium

At the end of the lesson, you should be able to:
a) Explain a reversible reaction
b) State the characteristics of a dynamic equilibrium

system and law of mass action or law of chemical
equilibrium
c) Interpret the curve of concentration reactants and
products against time for a reversible reaction

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NON-REVERSIBLE REACTION

Few chemical reactions proceed in only one direction:

A+B C+D

Example: CH3Cl + HCl
CH4 + Cl2 UV

• Reactions proceed in only one direction.

• The reaction proceeds toward the formation of product.

• Reactions in which limiting reagents are used up are
said to “go to completion

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REVERSIBLE REACTION

But, most chemical reactions are reversible, at least
to some extent.

AB

reactant product

• Forward reaction : reaction proceeds from left

to right.

• Backward reaction : reaction proceeds from

right to left.

• Reversible reaction : reactions which take place

in both forward and reverse
directions.

= indicates reversible reaction 5

REVERSIBLE REACTION

A B

reactant product

• In a reversible reaction, initially the reaction
proceeds toward the formation of products

• As soon as some product molecules formed, the
reverse process begins to take place.

• Reactant molecules are formed from the
decomposition of product molecules.

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REVERSIBLE REACTION

Consider the following reversible reaction:

AB

The graph of the concentration of A and B against time:

[B]  [A] decrease with time, rate of
forward raction decrease
[A]
t1  [B] increase with time, rate of
reversed reaction increase

 After time, t1, [A] and [B]
remains unchanged

 The system is in the state of

equilibrium

Note : [ ] = concentration 7

[B]  The equilibrium is a
[A] dynamic equilibrium
t1
 Means that after t1, the
reaction did not stop

 But, the rate of forward
reaction = rate of reverse
reaction

 There is no net change in

[A] and [B] 8

The rate of forward and
reversed recations are now
equal (dynamic equilibrium)

Forward
reaction

Reversed
reaction

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Time [reactants] [products]
(h) (mol L-1) (mol L-1)
0
2 0.40 0
4 0.22 0.22
6 0.08 0.28
8 0.06 0.30
10 0.06 0.30
0.06 0.30

Initially, at time=0, only reactants are present in the reaction vessel
• [reactants] is at their maximum value (0.4 molL-1)
• [products] = 0 molL-1

After time 0, reactants begin to react to produce products
• The [reactants] decreases with time.
• the [products] increases with time.

After 4 hours:
• [reactants] remains steady at 0.06 mol L-1.
• [products] remains steady at 0.30 mol L-1.
• Equilibrium is reached after 4 hours.

At equilibrium 10
rate of the forward reaction = rate of the reverse reaction

DYNAMIC EQUILIBRIUM

 Reactant and product concentrations stop
changing because the forward and reverse
rates have become equal.
ratefwd = raterev

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CHARACTERISTICS OF A SYSTEM IN
EQUILIBRIUM

i. Reactant and product concentrations are constant
over time.

ii. Forward reaction rate = Reverse reaction rate
iii. Occurs only in a closed system

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N2O4(g) 2NO2(g)
At equilibrium,

Rate of forward reaction: N2O4(g)  2NO2(g)
= Rate of reverse reaction: 2NO2(g)  N2O4(g)

 [N2O4(g)] and [NO2(g)] remain constant

But!

[N2O4(g)] ≠ [NO2(g)]

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LAW OF MASS ACTION

Also known as EQUILIBRIUM LAW.

At a given temperature, a chemical system
reaches a state in which particular ratio of
reactant and product concentrations has
a constant value.

For a particular system and temperature, the
same equilibrium state is attained regardless
of how the reaction is run.

Cato Maximilian Guldberg (1839 – 1902). Norwegian chemist and Mathematician.

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Peter Waage (1833 – 1900). Norwegian chemist.

LAW OF MASS ACTION

Consider this general reaction equation:

aA + bB cC + dD

At equilibrium: [ C ]c [ D ]d = K
[ A ]a [ B ]b

Where:

a, b, c and d: stoichiometric coefficients for the
reacting species A, B, C and D

K = Equilibrium constant, [ ] = concentration15

aA + bB cC + dD [C]c [D]d
REACTANTS PRODUCTS K=

[A]a [B]b

By convention,
substances on the left  “reactants”
substances on the right  “products”

cC + dD aA + bB [A]a [B]b
REACTANTS PRODUCTS K=

[C]c [D]d

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Remember the following terms !!

1.Reversible reaction : Chemical reaction
which take place in both forward & reverse
directions.

2.Dynamic equilibrium : is established
when the rate of the forward reaction
equals the rate of the reverse reaction.

3.Law of Mass Action : explains the
relationship between the concentrations of
the reactants and products present at
equilibrium in any reaction.

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