1 Chemical Equilibrium Chapter 13 AP CHEMISTRY. 2 Chemical Equilibrium The state where the concentrations of all reactants and products remain constant.

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Chemical Equilibrium

Chapter 13

AP CHEMISTRY

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Chemical Equilibrium

The state where the concentrations of all reactants and products remain constant with time.On the molecular level, there is frantic activity. Equilibrium is not static, but is a highly dynamic situation.

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The Concept of Equilibrium

Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

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The Concept of Equilibrium As the substance warms it begins to

decompose: N2O4(g) 2NO2(g)

A mixture of N2O4 (initially present) and NO2 (initially formed) appears brown.

When enough NO2 is formed, it can react to form N2O4:

2NO2(g) N2O4(g). At equilibrium, as much N2O4 reacts to form NO2

as NO2 reacts to re-form N2O4

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The Concept of Equilibrium As a system approaches

equilibrium, both the forward and reverse reactions are occurring.

At equilibrium, the forward and reverse reactions are proceeding at the same rate.

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A System at Equilibrium

Once equilibrium is achieved, the amount of each reactant and product remains constant.

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Depicting Equilibrium

Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow.

N2O4 (g) 2 NO2 (g)

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Notes on Equilibrium Expressions (EE)

For a given equation: jA + kB ↔ lC + mDK does not include any pure solids or liquidsThe expression shows products divided by reactantsLike the rate constant, k, the units of K depend on the experiment being performed For the reverse reaction K’ = 1/K (reactants and products switch)Sometimes you will see K written as Kc

Law of mass action

K = [C]l[D]m

[A]j[B]k

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Equilibrium Expression Write the equilibrium expression for: 4NH3(g) + 7O2(g) 4NO2(g) + 6H2O(g)

Complete sample problems #1-4. See sample problems 13.1 and 13.2 in your textbook for more worked examples.

Complete sample problems # 1-4.

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Significance of Equilibrium Expression The inherent tendency for a reaction is occur

is indicated by the magnitude of K. A K value much larger than 1 means that at

equilibrium the reaction system will consist of mainly product – equilibrium lies to the right

A very small K means that the system at equilibrium will consist mainly of reactants – equilibrium position is far to the left

The size of K and the time required to reach equilibrium are NOT directly related.

Complete sample problem #5.

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Notes on Equilibrium Expressions (EE)

For a reaction multiplied by an integer, “n”, Knew = (Korig)n

See sample exercise 13.2(c) on page 614.For a given reaction, K is dependent only on temperature

K = [C]l[D]m

[A]j[B]k

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Heterogeneous Equilibria

. . . are equilibria that involve more than one phase.

CaCO3(s) CaO(s) + CO2(g)

K = [CO2]The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.

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Practice Problem: Consider the reaction represented by the equation: Fe3+(aq) + SCN-(aq) FeSCN2+(aq) In trial #1, you start with 6.00 M Fe3+(aq) and 10.0 M SCN-(aq), and

at equilibrium the concentration of FeSCN2+(aq) is 4.00 M. What is the value of the equilibrium constant for this reaction?

Fe3+(aq) + SCN-(aq) FeSCN2+(aq)

Initial Change Equilibrium

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Practice Problem

Fe3+(aq) + SCN-(aq) FeSCN2+(aq) Equilibrium 2.00 6.00 4.00

0.33 K ]0.2][0.6[

]00.4[K

Complete sample problems 6 & 7 for more practice using ICE charts.

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Equilibrium Constant in Terms of Pressure Equilibria involving gases can be described in terms

of either pressure or concentrations. The relationship can be seen using the Ideal Gas

Law, PV=nRT. Rearranging this equation gives n/V represents concentration in M.

CRT RTV

n P

CRT Por RT

P C

Complete sample problem # 8 now.

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K vs. Kp

For any reaction:Kp = K(RT)n

n = sum of coefficients of gaseous products minus sum of coefficients of gaseous reactants.

Complete Sample Problem #9.

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Reaction Quotient

H2(g) + F2(g) 2HF(g)

Q HF

H F2 2

02

0 0

After the equilibrium constant (K) is known, we can use it to determine if a reaction is at equilibrium.

The reaction quotient, Q, has the same form as the equilibrium constant expression EXCEPT initial concentrations are used instead of equilibrium concentrations.

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Predicting the Direction of a Reaction Using Reaction Quotient

If Q > K then the reverse reaction must occur to reach equilibrium (i.e., products are consumed, reactants are formed, the numerator in the equilibrium constant expression decreases and Q decreases until it equals K). If Q < K then the forward reaction must occur to reach equilibrium.If Q = K then the reaction is at equilibrium.

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Practice Problem Part 2:

Using the previous reaction: Fe3+(aq) + SCN-(aq) FeSCN2+(aq) and

the K value we determined: K = 0.33 determine if the following concentrations are at equilibrium:

Initial:10.0 M Fe3+(aq), 8.00 M SCN-(aq), and 2.00 M FeSCN2-

Complete sample problems 10 & 11.

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Solving Equilibrium Problems 1. Balance the equation. 2. Write the equilibrium expression. 3. List the initial concentrations. 4. Calculate Q and determine the shift to

equilibrium.

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Solving Equilibrium Problems(continued) 5. Define the change needed to reach

equilibrium. 6. Substitute equilibrium concentrations

into equilibrium expression and solve. 7. Check calculated concentrations by

calculating K.

Complete more complex sample problems #12-15.

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Le Châtelier’s Principle

. . . if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.

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C

Consider the production of ammonia

As the pressure increases, the amount of ammonia present at equilibrium increases.

As the temperature decreases, the amount of ammonia at equilibrium increases.

Can this be predicted? Le Châtelier’s Principle: if a system at equilibrium is

disturbed, the system will move in such a way as to counteract the disturbance.

N2(g) + 3H2(g) 2NH3(g)

Le Châtelier’s Principle

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C

Le Châtelier’s Principle

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Effects of Changes on the System1. Concentration: The system will shift away

from the added component.

2. Temperature: K will change depending upon the temperature (treat the energy change as a reactant).

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Increase of Pressure to an Equilibrium.

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Effects of Changes on the System (continued)

3.Pressure:

a. Addition of inert gas does not affect the equilibrium position.

b. Decreasing the volume shifts the equilibrium toward the side with fewer moles.

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Change in Reactant or Product Concentrations Consider the Haber process

If H2 is added while the system is at equilibrium, the system must respond to counteract the added H2 (by Le Châtelier).

That is, the system must consume the H2 and produce products until a new equilibrium is established.

Therefore, [H2] and [N2] will decrease and [NH3] increases.

Le Châtelier’s Principle

N2(g) + 3H2(g) 2NH3(g)

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Change in Reactant or Product ConcentrationsLe Châtelier’s Principle

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The Haber ProcessLe Châtelier’s Principle

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•N2 and H2 are pumped into a chamber.

•The pre-heated gases are passed through a heating coil to the catalyst bed.•The catalyst bed is kept at 460 - 550 C under high pressure.

•The product gas stream (containing N2, H2 and NH3) is passed over a cooler to a refrigeration unit.

•In the refrigeration unit, ammonia liquefies but not N2 or H2.

•The unreacted nitrogen and hydrogen are recycled with the new N2 and H2 feed gas.

•The equilibrium amount of ammonia is optimized

The Haber Process for producing NH3

Le Châtelier’s Principle