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