ib chem past paper

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The equilibrium law
DYNAMIC EQUILIBRIUM forward A +8

reaction

""r=====~'~ reverse reaction

Most chemical reactions do not go to completion. Once some products are formed the reverse reaction can take place to reform the reactants. In a closed system the concentrations

of all the

reactants and products will eventually become constant. Such a system is said to be in a state of dynamic equilibrium.
The forward and reverse reactions continue to occur, but at equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.
(ii)

(i)

Dynamic
2.0

2.0
7
E 1.8
-o
~ 1.6

N,O,(gl

=

E 1.8
-o
~ 1.6

2N02(gl

.~ 1.2

=

2N,O,(gl

~ 1.0

~1.4

25 v 0.6

back into liquid.

0.4

0.2

Equilibrium

rate of vaporization

0.6

0.4

molecules

in the

liquid will escape from the surface to become vapour and the slower moving molecules in the vapour will condense

80.8

0.8

also occurs when physical changes

the water vapour. The faster moving

~1.4 o .~ 1.2

~ 1.0

2NO,(gl

equilibrium

take place. In a closed flask, containing some water, equilibrium will be reached between the liquid water and

will be established

when the

equals the rate of condensation.

0.2

NO,

time

time

CLOSED SYSTEM

Graph (i) shows the decomposition of
N20.\" Graph (ii) shows the reverse reaction starting with NOr Once equilibrium is reached (sh-own by the dotted line), the composition of the mixture remains constant and is independent of the starting materials.

A closed system is one in which

the macroscopic properties remain constant. If the system is open some of the products from the reaction could escape and equilibrium

Examples

THE EQUILIBRIUM CONSTANT
Consider the following

general reversible

reaction

in which

w

moles of A react with x moles of B to form y moles of C and z moles of D. wA + x8
At