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Here is where equilibrium comes in……… B.Some reactions don’t go to completion because they are reversible - the products collide and react to produce the original reactants A B Does NOT make 100% of the products, because products regenerate reactants
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Chapter 16Chapter 16Chemical EquilibriumChemical Equilibrium
Before we look at chemical equilibrium, let’s look at the past
A. Many reactions in chemistry go to 100% completion1. Example: burning methane.
CH4 + O2 CO2 + H2O
100% of the products are made
Here is where equilibrium comes in………
B. Some reactions don’t go to completion because they are reversible - the products collide and react to produce the original reactants
A
B
Does NOT make 100% of the
products, because products
regenerate reactants
I. Chemical Equilibrium:1. The RATEAt equilibrium, both the forward and reverse reactions are occurring at the same rate, equal.
2. ConcentrationAt equilibrium the amount of each reactant andproduct remains constantconstant.
II. How to express chemical equilibrium Keq: Equilibrium Constant
a. aA + bB cC + dD
b. keq= [products]coefficient
[reactants] coefficient
[ ]=Concentration,
Molarity
c. keq= [C]c [D]d
[A]a [B]b
III. Homogeneous and Heterogeneous Equilibria
Homogeneous: All states of matter the sameHeterogeneous: Different states of matter in the chemical reaction
Only use the gases (g) when writing the Keq equation!!!
Problems on overhead.
IV. Calculating Keq
On Overhead
What Does the Value of Keq Mean?
If Keq >> 1, the reaction is product-favored; product predominates at equilibrium.
If Keq << 1, the reaction is reactant-favored; reactant predominates at equilibrium.
Stop here
Warm Up
1. What is chemical equilibrium?
2. What’s the significance of Keq?
3. Write the keq expression for:Br2(g) + 2NO(g) 2NOBr(g)
4. Calculate Keq for the reaction in #3[.214]=Br2
[.0677]=NOBr[0.428]=NO5. What does the reaction mainly have at
equilibrium?
a. rates of forward/ reverse reactions are equal
b. the Concentrations of reactants and products are constant.
The value tells you at Equilibrium, what you mostly have……reactants OR products OR equal amounts of both
Keq= [NOBr]2
[Br2][NO]2
Keq= [.0677]2
[.214][.428]2
Keq=0.117 <1
At equilibrium, the system has more reactants
The Reaction Quotient (Q)1. Q gives the same ratio the
equilibrium expression gives, but for a system that may or may not be at
equilibrium.
2. Indicates if a system is at equilibrium and if not, which position
does it need to shift to reach equilibrium.
K is @ Equilibrium
Q is @ Any Time (eq. or not)
If Q = K,
the system is at equilibrium.
If Q > K,there is too much product and the equilibrium
shifts to the left.
If Q < K,there is too much reactant, and the equilibrium
shifts to the right.
Remember the
Q uic K
way of figuring the shift out.
0.035 moles of SO2, 0.500 moles of SO2Cl2, and 0.080 moles of Cl2 are combined in an evacuated 5.00 L flask and heated to 100oC. What is Q before the reaction begins? Which direction will the reaction proceed in order to establish equilibrium?
1. Write the expression to find the reaction quotient, Q 2. Determine the concentrations of each gas.3. Calculate Q by substituting the concentrations.4. Determine how the reaction proceeds.
SO2Cl2(g) SO2(g) + Cl2(g) Keq = 0.078 at 100oC
0.500 mole SO2Cl2/5.00 L = 0.100 M SO2Cl2
0.035 mole SO2/5.00 L = 0.0070 M SO2
0.080 mole Cl2/5.00 L = 0.016 M Cl2
0.078 (K) > 0.0011 (Q)Since K >Q, the reaction will proceed in the forward direction in order to increase the concentrations of both SO2 and Cl2 and decrease that
of SO2Cl2 until Q = K.
Q = 0.0011
1. Proposed by a Chemistry Professor, Henry Chatelier2. Came up with this (LeChatelier’s) principle in 1884
3. If a change in condition occurs in a system, the system will change to offset the change.
What will happen to a system that is already at equilibrium if I change
conditions??
Possible changes (stressors)
Temperature
Pressure
Concentration
1. Change in Concentration
2NO2(g)
a.More NO2 is added:
b.More N2O4 is added:
c.N2O4 is taking away:
d.NO2 is taken away:
Shift Right
Shift Right
Shift Left
Shift Left
N2O4(g)Example:
N2O4
2. Change in Pressure:When pressure is applied the shift goes from more moles of gas to less moles of gas.
2NO2(g) N2O4(g)
NO2
NO2
a. What will happen if the pressure is increased??Shift to the Right
Applies only to gases
N2O4
N2O4N2O4
N2O4
P
Pressure Animation
b. What will happen if the pressure is decreased?
2NO2(g) N2O4(g)
Pressure will shift to the left.
Bottom line:
The shift will always be toward the deficiency and away from the excess.
P
3. Change in: Temperature
1. Heat + NH4Cl(s)NH3(g) + HCl(g)
2. H2O2(l) H2O(l) + O2(g) + 190KJ
3. Heat + H2O(s) H2O(l)
4. H2(g) + Cl2(g) 2 HCl(g) + Heat
IF HEAT IS INCREASED
IF HEAT IS DECREASED
Equilibrium video
http://www.sciencegeek.net/Chemistry/taters/LeChatelier.htm
Warm up 3/__/13