EQUILBRIUM
TIERS 1 & 2
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Describe what is meant by a reversible reaction
Define forward reaction
Define reverse reaction
Describe a system that has reached dynamic equilibrium
Define equilibrium position
Define equilibrium expression
Define equilibrium constant
Describe a closed system
Reversible Reactions
Theoretically, every reaction can proceed in two directions, forward and
reverse.
Essentially all chemical reactions are considered to be reversible under suitable
conditions.
A chemical reaction in which the products can react to re-form the reactants is
called a reversible reaction.
A reversible chemical reaction is in chemical equilibrium when the rate of its forward
reaction (reactants to products) equals the rate of its reverse reaction( products
back to reactants) and the concentrations of its products and reactants remain
unchanged.
A state of dynamic equilibrium has been reached when the amounts of products
and reactants remain constant.
Both reactions continue, but there is no net change in the composition of the
system.
Reversible Reactions, continued
The chemical equation for the reaction at equilibrium is written using double
arrows to indicate the overall reversibility of the reaction.
FORWARD REACTION


2HgO(
s
)
2Hg(
(g )


2
2HgO (s)
2Hg (l)l ) ++OO
(g)
2
REVERSE REACTION

 2Hg(
2Hg (l)
+ O2 (g)
2HgOl ) + O2 (g )
2HgO(
s ) 

The Equilibrium Expression
GIVEN THE FOLLOWING REACTION
THE EQUILIBRIUM EXPRESSION IS:

 xC  yD
nA  mB 

[C]x [D]y
K
[A]n [B]m
THE EQUILIBRIUM EXPRESSION IS DEFINED AS THE RATIO OF THE
CONCENTRATION OF THE PRODUCTS TO THE REACTANTS WHERE EACH
CONCENTRATION IS RAISED TO THE POWER EQUAL TO ITS COEFFICIENT
After equilibrium is reached, the individual concentrations of A, B, C, and D
undergo no further change if conditions remain the same.
A ratio of their concentrations should also remain constant.
The equilibrium constant is designated by the letter K.
The Equilibrium Expression
The constant K is independent of the initial concentrations.
This means that there can be a infinite number of equilibrium positions but only
one equilibrium constant at a particular temperature
Equilibrium position refers to the physical change in the equilibrium if it is
disturbed. The concentrations may change but the ratio between products and
reactants remain constant.
K is dependent on the temperature of the system.
The Equilibrium Constant
The numerical value of K for a particular equilibrium system is obtained
experimentally.
The equilibrium constant, K, is the ratio of the mathematical product of
the concentrations of substances formed at equilibrium to the
mathematical product of the concentrations of reacting substances.
Each concentration is raised to a power equal to the coefficient of that
substance in the chemical equation.
FOR A REACTION TO REACH EQUILIBRIUM, IT MUST TAKE PLACE IN A
CLOSED SYSTEM
A CLOSED SYSTEM IS A SYSTEM THAT PREVENTS THE EXCHANGE OF
MATTER WITH THE SURROUNDING
EXAMPLES WOULD BE: STOPPED FLASK, SEALED JAR ETC.
THE END