CHEMICAL EQUILIBRIUM 2010 (M. Caiafa)
CHEMICAL EQUILIBRIUM: occurs in a reversible reaction, when
the FORWARD reaction rate equals the REVERSE reaction
rate.
1) When equilibrium is established, the amounts of reactants and
products present are termed the equilibrium concentrations.
2) There will be un-reacted reactants at equilibrium.
3) Many reactions produce little product before equilibrium is
reached.
4) The equilibrium constant ( K ) indicates the extent to which the
reaction produces product. K values greater than 1 favor high
product equilibrium concentrations. K values less than one
indicate the reactants are favors to be the predominant species at
equilibrium.
5) Equilibrium reactions will have a double arrow  . NOTE :
equilibrium is a reversible chemical change in a chemical
reaction.
4. Equilibrium also occurs in physical changes such as phase
changes. This is called phase equilibrium, the equilibrium of
two phases at the same time at the same temperature.
Le Chatelier’s Principle
States that a system (reaction) at equilibrium will resist any
external change (stress) that is applied. Stresses include any
change in Temperature, Pressure or changing the
concentrations of any species in the reaction.
Le Chatelier’s Algorithms:
1) If you add to one side of the arrow, the equilibrium shifts
(runs) to the opposite side of the arrow.
2) If you remove something on one side of the arrow, the
equilibrium will run to the same side.
4. EXAMPLES: THE EFFECT OF TEMPERATURE ON
EQUILIBRIUM REACTIONS.
RULE: Heating an exothermic reaction drives it in reverse,
cooling it drives it in forward.
*** What happens if you increase the temperature of the following
EXOTHERMIC (heat of reaction is a negative value ) reaction?
Heat is on the RIGHT of the reaction, you
added to the RIGHT, therefore the
reaction shifts LEFT, runs in reverse.
The heat is consumed. What you add, the
reaction tries to get “rid” of .
Write heat on the PRODUCT side
of an exothermic reaction, apply
the algorithm as if heat were a
chemical species.
A + 2B   C + D + HEAT
+X
+2X
-X
-X
REACTION RUNS IN REVERSE, CONSUMES ADDED HEAT,
shift away form an addition
RULE: cooling an endothermic reaction drives it in reverse, warming
it drives it forward.
*** What happens if you increase the temperature of the following
ENDOTHERMIC (heat of reaction is positive value) reaction?
Heat is on the LEFT of the reaction,
you added to the LEFT, therefore the
reaction shifts RIGHT, runs in forward
direction.
The heat is consumed. What you add,
the reaction tries to get “rid” of .
HEAT
For endothermic reactions write
heat on the REACTANT side of the
reaction, apply the algorithm as if
it were a chemical species.
+ A + 2B   C + D
-X
-2X
+X
THIS REACTION RUNS FORWARD.
Shift away from an addition.
+X
RULE: cooling an endothermic reaction drives it in reverse, warming
it drives it forward.
*** What happens if you DECREASE the temperature of the following
ENDOTHERMIC (heat of reaction is a positive value) reaction?
Heat is on the LEFT of the reaction, you
REMOVED from the LEFT, therefore the
reaction shifts LEFT, runs in reverse
direction. The heat is consumed. What you
remove, the reaction tries to replace.
HEAT
For endothermic reactions write
heat on the REACTANT side of the
reaction, apply the algorithm as if
it were a chemical species.
+ A + 2B   C + D
+X
+2X
-X
-X
REACTION SHIFS LEFT, TO A REMOVAL
(SUBTRACTION).
RULE: cooling an Exothermic reaction drives it in FORWARD,
warming it drives it in REVERSE.
*** What happens if you DECREASE the temperature of the following
EXOTHERMIC (heat of reaction in negative value) reaction?
Heat is on the right of the reaction, you
REMOVED from the RIGHT, therefore
the reaction shifts LEFT, runs in forward
direction. The heat is consumed. What you
remove, the reaction tries to replace.
For Exothermic reactions write
heat on the PRODUCT side of the
reaction, apply the algorithm as if
it were a chemical species.
H2O(l)   H2O(s) +
-X
HEAT
+X
THIS REACTION RUNS IN FORWARD.
SHIFT TO A REMOVAL (COOLING REMOVES HEAT).
RULE: IF A SPECIES IS REMOVED from ONE SIDE OF THE
REACTION, THE EQUILIBRIUM, WILL RUN TO THE SAME SIDE
OF THE ARROW; THE SYSTEM WILL REPLACE WHAT WAS
REMOVED.
Example, for the reaction below, what will happen if the concentration of
A is REDUCED?
A IS REMOVED FROM
THE LEFT,
THERFORE THE
REACTION SHIFTS TO
THE LEFT
A + 3B   2C + D
+X
+3X
-2X
-X
REACTION RUNS IN REVERSE, REPLACES REMOVED A.
SHIFTS TO A REMOVAL (DECREASED CONCENTRATION)
RULE: IF A SPECIES IS ADDED TO ONE SIDE OF THE
REACTION, THE EQUILIBRIUM, WILL RUN TO THE OPPOSITE
SIDE OF THE ARROW; THE SYSTEM WILL CONSUME WHAT
WAS ADDED.
Example, for the reaction below, what will happen if the concentration of
A is increased?
A IS ADDED TO THE
LEFT, THERFORE THE
REACTION SHIFTS TO
THE RIGHT
A + 3B   2C + D
-X
-3X
+2X
+X
THIS REACTION RUNS IN FORWARD.
SHIFTS AWAY FROM AN ADDITION (INCREASED
CONCENTRATION