Transcript Le Chatelier’s Principle

Le Chatelier’s Principle
?
• What controls the position of chemical
equilibrium?
• We can change certain aspects of an
equilibrium reaction to force the
equilibrium to shift to one side of the
reaction (reactant or product side) This is
called Le Chatelier’s Principle.
LeChatelier’s Principle
• Le Chatelier’s Principle: A change (stress)
imposed on a system at equilibrium shifts
in a direction that tends to reduce the
effect of that change.
• ???
What does it mean?
• When a reaction is at equilibrium, we can
add a stress (change) to the system. The
reaction shifts its equilibrium position to
reduce that stress.
• What can we do to cause the equilibrium
stress?
Stresses
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Concentration
Pressure (for gases)
Volume (for gases)
Temperature
Effect of Change in
Concentration
• Look again at the following reaction:
– N2(g) + 3H2(g)↔2NH3(g)
• Let’s say we have the following concentrations at
equilibrium: [N2]=0.399 M, [H2]=1.197 M,
[NH3]=0.203 M
– What is the equilibrium constant?
What is going to happen if we all of a
sudden add 1.00 mol/L of N2?
• What is going to happen if we all of a
sudden add 1.00 mol/L of N2?
• Remember, the system was at equilibrium
right before the extra N2 was added. Also,
when you increase concentration, what
happens to the rate of the reaction?
• The rate of the forward reaction has
increased, so what is going to happen to
the concentration of [NH3]?
• The system wants to reestablish
equilibrium, but the equilibrium has shifted
now to the right, or more products.
• Take a look at the new equilibrium
concentrations: [N2]=1.348 M, [H2]=1.044 M,
[NH3]=0.304 M
• What is the equilibrium constant?
• The equilibrium constant does not change when
reactant or product is added or removed.
TO SUMMARIZE
• When a reactant or product is added to a
system at equilibrium, the system shifts
away from the side with the added
component.
• When a reactant or product is removed
from a system at equilibrium, the system
shifts toward the side with the removed
component.
As4O6(s) + 6C(s) ↔As4(g) + 6CO(g)
• Predict the shift in the equilibrium position
for this reaction in response to the
following changes:
• A)
Addition of CO
• B)
Addition of C(s)
• C)
Removal of As4(g)
Effect of a Change in Volume
• For equilibrium containing a gas only
• When the volume of a gas decreases, what
happens to the pressure?
• This is a sudden stress on equilibrium.
What can the reaction do to reduce this
stress?
Consider the following reaction:
CaCO3(s) ↔CaO(s) + CO2(g)
• -If the volume containing the above
reaction is suddenly deceased, the pressure
of CO2 suddenly increases. To reduce this
stress, the reaction will shift away from the
gas, or to the left. Again, no change in the
equilibrium constant.
To SUMMARIZE
• When the volume of a gaseous reaction
system at equilibrium is decreased (thus
increasing the pressure), the system shifts
in the direction that gives the smaller
number of gas particles.
• Reverse is true…increasing volume shifts to
the side with more gas molecules
• Predict the shift in equilibrium position that
occurs in each of the following reactions
when the volume is reduced:
• A)
P4(s) + 6Cl2(g) ↔4PCl3(l)
• B)
PCl3(g) + Cl2(g)↔ PCl5(g)
• C)
PCl3(g) + 3NH3(g) ↔P(NH2)3(g) + 3 HCl(g)
Effect of a Change in
Temperature
• **When the temperature of reaction changes, the
equilibrium constant changes.
• Reactions can either absorb heat or release heat.
• A reaction that releases heat (heat is a product) is
called an exothermic reaction.
• A reaction that absorbs heat (heat is a reactant) is
called an endothermic reaction.
• What you do is you put energy into the
chemical equation. For example, the
synthesis of ammonia is an exothermic
reaction, so we would write the reaction
with energy included like:
• N2(g) + 3H2(g)↔2NH3(g) + Energy
• The decomposition of calcium carbonate is
an endothermic reaction, written like this:
• CaCO3(s) + Energy ↔CaO(s) + CO2(g)
• Once you have energy written as a reactant
or product, treat it like any other substance.
In Summary
• Treat energy as a reactant or product. If
energy is added (increasing temperature),
the equilibrium will shift away from the
position of energy.
• Reverse is true
Examples
• For each of the following reactions, predict
how the equilibrium will shift if the
temperature is increased.
– A)N2(g) + O2(g)↔2NO(g)
– B)2SO2(g) + O2(g) ↔2SO3(g)
– C)C2H2(g) + 2Br2↔C2H2Br4(g)