Transcript Equilibrium - Independent School District 196

Equilibrium
I love chemistry!!!
What is Equilibrium?
• A dynamic condition in which 2 opposing
changes occur at equal rates in a closed
system
Ex. A phase change can be a physical condition
that can be described to be in equilibrium
Solid + ∆H
Liquid
Other examples of Equilibrium
• Vapor pressure
Patm
Pgas
• Saturated solutions
NaCl(s)
Na+(aq) + Cl-(aq)
LeChatlier’s Principle
• A system will remain at equilibrium until
something occurs to change this condition
• When a system at equilibrium is disturbed by
an application of stress, it attains a new
equilibrium position that minimizes that stress
What are the possible stressors?
•
•
•
•
•
Change in temperature
Change in pressure
Change in volume
Addition or removal of reactants
Addition or removal of products
Understanding how to minimize the
stress
• Equilibrium is like a teeter totter
• When stress is placed on it, the system must
adjust to rebalance itself
Using color to predict shifting
∆H + Co(H2O)6+2(aq) + 4Cl-(aq)
CoCl4-2(aq) + 6H2O(l)
Pale pink
Deep blue
At equilibrium, the color of a beaker containing this system
would be violet (light purple)
What would you see if the following possible stressors were
Introduced according to LeChatlier?
•Addition of Co(H2O)6+2 •Removal of Cl-(aq)
•Removal of H2O(l)
•Addition of CoCl4-2(aq)
•Addition of ∆H
•Removal of ∆H
•Increase Pressure
•Decrease Pressure
Sample Problem #1
2NO2(g)
N2O4(g) + ∆H
Very dark brown
1.
2.
Very light brown
3.
1.
2.
3.
*At equilibrium, the system will display a medium brown color as shown in flask
#2
2NO2(g)
Very dark brown
N2O4(g) +∆H
Very light brown
Where would the shifting take place when the following possible stressors are added?
What would you see experimentally?
• Removal of heat
• Increase in pressure
• Addition of NO2(g)
• Decrease in pressure
• Removal of N2O4(g)
• Addition of CO2(g)
• Addition of heat
• Increase in volume of container
• Decrease in volume of container
1.
2.
3.
1.
#1 represents a shift to the right (lighter brown color)
#2 represents the original equilibrium situation
#3 represents a shift to the left (darker brown color)
2.
3.
Things to remember about equilibrium
shifting
• Pure solids and liquids will not have an effect
on equilibrium
• Only gases will be affected by a change in
pressure or volume
How do you know which side of the
equilibrium expression is favored?
• Sometimes at equilibrium, there is a higher
concentration of reactants or products
• To determine this by:
• 1. Looking at the equilibrium constant
• 2. Looking at the reversible arrows shown
in the equation
HBr(aq) + H2O(l)
H3O+(aq) + Br-(aq)
The forward reaction shown above is favored, therefore there is a higher concentration
of products than of reactants at equilibrium
H2CO3(aq) + H2O(l)
H3O+(aq) + HCO3-(aq)
The reverse reaction shown above is favored, therefore there
is a higher concentration of reactants than of products at
equilibrium
• In some cases, both the forward and reverse
reactions occur nearly at the same rate before
chemical equilibrium is established. Neither reaction
is favored
• Do not just assume that if you see an equilibrium
arrow (
) that neither side of the system is
favored. Use the equilibrium constant to determine
for sure!
The equilibrium expression (Keq)
• After equilibrium has been achieved, the
concentrations of products and reactants
remain constant
• The ratio of the concentrations should also
remain constant
***The only stressor that can affect these
concentration ratios is temperature
***Keq is temperature dependant
How do you find Keq?
• Keq is the constant that represents the ratio of
the mathematical product to the
mathematical reactant
• The general equation for an equilibrium
expression is as follows:
nA + mB
xC + yD
*n, m, x, y = coefficients from balanced chemical equation
*A, B, C, D = molarities of reactants and products
nA + mB
Keq =
xC + yD
[C]x [D]y
[A]n [B]m
*Keq will only change if temperature changes
*Only gases and aqueous solution concentrations will be included in Keq
*Pure solids and liquids are omitted because their concentrations cannot change
Sample Problem
• Consider the following equilibrium equation at
425°C:
H2(g) + I2(g)
2HI(g)
1.
Write the Keq expression for this reaction
2.
Determine Keq mathematically when given the following concentrations: [H2] =
.015M, [I2] = .015M, [HI] = .11M
3.
Using the known Keq from #2 and given the following concentrations for H2 and I2,
determine the concentration of HI: [H2] = .018M, [I2] = .018M
How to use Keq to determine if the
reactants, products or neither are
favored?
• If Keq > 1, the forward reaction (products)
are favored
• If Keq < 1, the reverse reaction (reactants)
are favored
• If K = 1, neither reaction is favored (the
concentrations of both products and
reactants should be equal)
• For the previous sample problem, which of
the reactions is favored?
H2(g) + I2(g)
2HI(g)