7.1 b)

EQUILIBRIUM LAW
THE EQUILIBRIUM CONSTANT EXPRESSION, K c e.g. Consider the following equilibrium system:
H
2(g)
+ I
2(g)
2HI
(g)
*Equilibrium concentrations (mol dm -3 ) @ 440

C
Trial
1
2
3
4
[H
2
]
0.0222
0.0350
0.0150
0.0442
[I
2
]
0.0222
0.0450
0.0135
0.0442
[HI]
0.156
0.280
0.100
0.311
[HI]
2
/ [H
2
][I
2
]
K = _________________
*EQUILIBRIUM CONSTANT EXPRESSION: aA + bB cC + dD
K c
=
Where:

A, B, C, D are chemical entities in gas or aqueous phases.
 a, b, c, d = ______________ in balanced chemical equation

K is the ________________ constant
*Variances in equilibrium __________________ or _______________ do not affect the value of K c
!
*The magnitude of the equilibrium constant IS altered by changes in_______________________ONLY!!
PRACTICE: Write the Equilibrium Constant Expressions for the following systems:
(i) N
2(g)
+ 3H
2(g)
2NH
3(g)
K c
=
(ii) 2NO
K
HINT: Equilibrium Constant of the REVERSE Reaction (K′)
K′ =
1
K
(i.e.
K′
= the _________________ of K) c
=
(g)
+ O
2(g)
2NO
2(g)
1

EXTENSION: HETEROGENEOUS EQUILIBRIUM
Def’n 
equilibrium in which reactants and products are in ___________ than one phase
*The concentration of reactants and products that are pure ____________ or ____________ are constant and
____ _______ appear in the equilibrium expression !
Example:
*1 mol of NaHCO
3(s) occupies a volume of 38.9 cm
3
*2 mol of NaHCO
3(s)
occupies a volume of 77.8 cm
3
 The ratio of moles to volume (i.e. the molar concentration ) remains ____________. e.g. Write the equ’m law expression for the hydrolysis of H
2
O in a closed container.
2H
2
O
(l)
2H
2(g)
+ O
2(g)
K
C
=
[ H
2 ( g )
]
2
[ O
2 ( g )
]
… becomes 
[ H
2
O
( l )
]
*The [H
2
O
(l)
] is incorporated into the value of the equilibrium constant, K
C
.
*HOW TO INTERPRET THE MAGNITUDE OF THE EQUILIBRIUM CONSTANT, K c
:
As chemists, we are interested in the products of our reactions, so in terms of K c
, we are interested in increasing the value of K c
, and thus increasing the extent of the reaction. Increasing the magnitude of K c equates to an increase in the percent reaction. actual product
%Rxn = theoretica l yield product yield
x 100 (%)
Summary:
K c
Value % Reaction
K >> 1 >> 99 %
K > 1
K ≈ 1
K < 1
K << 1
> 50%
≈ 50%
< 50%
<< 1%
Interpretation
_____________________ strongly favoured – reaction essentially goes to
_______________________ . Another way of saying this is that the reaction is ________________________ .
Equilibrium position favours ______________________.
Reactant and product concentrations are ___________________.
Equilibrium position favours ____________________.
___________________ strongly favoured – virtually ______ reaction.
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