Download Chapter 19, part II Notes

Chapter 19, part II Notes
Equilibrium Reactions
LeChatelier’s Principle
Equilibrium Constants
Reactions so far…
•Up to now, we have
assumed that reactions go
one way and stop when
all reactants have become
products.
•This is true sometimes,
but not always. There
are also…
Reversible Reactions
•In a reversible reaction,
the reaction occurs
simultaneously in both
directions.
•Products are being
formed and are also being
used to form reactants.
2SO2 + O2 D 2SO3
Equilibrium
• So how much product is
formed? It depends on the
reaction.
• Eventually, the reaction will
reach chemical equilibrium,
where forward and reverse
reactions take place at the
same rate.
Equilibrium
• This does NOT mean that the
amounts of product and
reactant are the same.
• The equilibrium position is
the relative percent of
products and reactants in a
system at equilibrium.
LeChatelier’s Principle
• Henri LeChatelier studied
shifts in the equilibrium of
reactions and came up with
his principle. He said:
• If a stress is applied to a
system in dynamic
equilibrium, the system
changes to relieve the stress.
LeChatelier’s Principle
• Let’s look at how several
factors will shift the
equilibrium point.
Concentration
• Increasing the concentration
of a product or reactant in
equilibrium will shift the
equilibrium point away from
that substance.
• Decreasing the concentration
would have the opposite
effect.
H2CO3 D H2O + CO2
• Which way will equilibrium
shift if adding:
– CO2
– H2CO3
– H2O
Which way will equilibrium
shift if removing:
– CO2
– H2CO3
Temperature
• Adding heat to a reaction at
equilibrium will shift the
equilibrium point towards
the side that absorbs heat.
• Removing heat will shift the
equilibrium point towards
the side that produces heat.
2SO2 + O2 D 2SO3 + heat
• Which way will equilibrium
shift if heat is added?
• Which way will it shift if
heat is removed?
Pressure
• Adding pressure to a system
at equilibrium will shift the
equilibrium point towards
the side that has the least
moles of gas.
• Decreasing pressure shifts
equilibrium towards the
side with the most moles of
gas.
N2 + 3H2 D 2NH3
• Assuming all the products
and reactants are gases,
which way does equilibrium
shift when pressure is
added?
• Which way does it shift
when pressure is relieved?
Equilibrium Constants
• Generally, the equilibrium
point is expressed as a
numerical value, called the
equilibrium constant, for
any reversible reaction at a
given temperature.
Given the equation of a general reaction to be:
coefficients
aA + bB D cC + dD
The equilibrium constant would be:
Keq =
c
d
[C] [D]
a
b
[A] [B]
The brackets around the substances
means it is their concentration (molarity).
Equilibrium constant
• If the equilibrium constant
(Keq) is bigger than one, it
means the products are
favored at equilibrium.
• A Keq less than one means
the formation of reactants is
favored at equilibrium.
N2O4 D 2NO2
• In the above reaction, at the
equilibrium point,
dinitrogen tetroxide has a
concentration of 0.0045M
and nitrogen dioxide has a
concentration of 0.030M.
What is the equilibrium
constant for the reaction?
N2 + 3H2 D 2NH3
•Calculate Keq for the
reaction if at equilibrium in
1.0L there is 0.15mol H2,
0.25mol N2 and 0.10mol
NH3.
2HI D H2 + I2
•Keq for the above reaction is
0.020; if the concentration
of hydrogen and iodine are
both 0.50M, what is the
concentration of HI?