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Chemical Equilibrium. A Balancing Act. Equilibrium. two opposing processes occurring at the same rate. a system at equilibrium is in balance. During a game, players enter and leave. Always the same number of players on field. H 2 O(l) H 2 O(g). Photochromic sunglasses.

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Chemical

Equilibrium

A Balancing Act


Equilibrium

two opposing processes occurring at the same rate

a system at equilibrium is in balance


During a game, players enter and leave.

Always the same number of players on field.


H2O(l) H2O(g)


Photochromic

sunglasses

AgCl + light Ag + Cl

(transparent) (dark)


Reversible reaction

a chemical reaction that can occur in both the forward and the reverse directions

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


Chemical equilibrium

a state in which the forward and reverse reactions balance each other because they take place at equal rates

dynamic state; no net change


Law of chemical equilibrium

At a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.


Equilibrium constant expression

ratio of molar concentrations of products to reactants; each raised to a power equal to coefficient in balanced equation


aA + bB cC + dD


Equilibrium constant(Keq)

numerical value of the ratio of product to reactant concentrations

constant only at a specific temperature


Value of equilibrium constant (Keq) shows

the extent to which reactants are converted into products.


Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium


Homogeneous equilibrium

all reactants and products in same phase


Write an equilibrium constant expression for:

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

[NH3]2

Keq =

___________

[N2] [H2]3


Heterogeneous equilibrium

reactants and products present in more than one physical state

H2O(l) H2O(g)


Heterogeneous equilibrium

Since concentrations of pure liquids and solids remain constant, these substances are omitted from the equilibrium constant expression.


Write an equilibrium constant expression for:

H2O(l) H2O(g)

Keq =

[H2O(g)]


Equilibrium constant expression Keq = . . .

Products over reactants raised to power of coefficient; leave out pure solids and liquids.


Equilibrium position

Equilibrium concentrations can vary from trial to trial.


H2(g) +I2(g) 2HI(g)


Each set of equilibrium concentrations represent an equilibrium position.

A system has only one value for Keq at a specific temperature, however there are unlimited number of equilibrium positions.


A system at equilibrium must:

  • take place in closed

  • system

  • temperature remain

  • constant

  • all reactants and

  • products are present

  • (both reactions can occur)



Calculate the value of Keq if [N2] = 0.20 mol/L,

[O2] = 0.15 mol/L, and

[NO] = 0.0035 mol/L.

Keq = 4.1 x 10-4

What does the value of Keq tell you about the equilibrium?



Le Chatelier’s Principle the balance?

If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.


Le Chatelier’s Principle the balance?

Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.


Changes in Concentration the balance?

Add reactant

1. Forward rxn speeds up.

2. Over time, forward

rxn slows down and

reverse rxn speeds up.


3. At equilibrium, the balance?

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

  • Equilibrium has

  • shifted right, value

  • of Keq unchanged.


Changes in Concentration the balance?

Remove reactant

1. Reverse rxn speeds up.

2. Over time, reverse

rxn slows down and

forward rxn speeds up.


3. At equilibrium, the balance?

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

  • Equilibrium has

  • shifted left; value of

  • Keq unchanged.


Changes in Pressure (Volume) the balance?

  • For a gas, decreasing

  • volume of container

  • increases pressure;

  • particles have less

  • space, collide more

  • frequently.


2. System will respond the balance?

by trying to relieve

stress (decrease

pressure.)

3. Shifts to the side with

fewer moles of gas.


4. Forward rxn speeds up. the balance?

5. Over time, forward

rxn slows down and

reverse rxn speeds up.


6. At equilibrium, the balance?

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

7. Equilibrium has

shifted right, value

of Keq unchanged.


Changes in Pressure (Volume) the balance?

Note:

If volume is decreased concentrations of all gaseous substances increases.

Side with fewer moles of gas increases more.


Changes in Temperature the balance?

Review:

If DH is negative

rxn is exothermic

heat written as product

If DH is positive

rxn is endothermic

heat written as reactant


Changes in Temperature the balance?

N2O4(g) 2NO2(g)

DH = 55.3 kJ

55.3 kJ + N2O4(g) 2NO2(g)

light dark


55.3 kJ + N the balance?2O4(g) 2NO2(g)


Le Chatelier’s Principle the balance?

Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.

Chemists can change conditions to make rxn more productive.


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