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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

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Chemical equilibrium

Chemical

Equilibrium

A Balancing Act


Chemical equilibrium

Equilibrium

two opposing processes occurring at the same rate

a system at equilibrium is in balance


Chemical equilibrium

During a game, players enter and leave.

Always the same number of players on field.


Chemical equilibrium

H2O(l) H2O(g)


Chemical equilibrium

Photochromic

sunglasses

AgCl + light Ag + Cl

(transparent) (dark)


Chemical equilibrium

Reversible reaction

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

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


Chemical equilibrium

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


Chemical equilibrium

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.


Chemical equilibrium

Equilibrium constant expression

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


Chemical equilibrium

aA + bB cC + dD


Chemical equilibrium

Equilibrium constant(Keq)

numerical value of the ratio of product to reactant concentrations

constant only at a specific temperature


Chemical equilibrium

Value of equilibrium constant (Keq) shows

the extent to which reactants are converted into products.


Chemical equilibrium

Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium


Chemical equilibrium

Homogeneous equilibrium

all reactants and products in same phase


Chemical equilibrium

Write an equilibrium constant expression for:

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

[NH3]2

Keq =

___________

[N2] [H2]3


Chemical equilibrium

Heterogeneous equilibrium

reactants and products present in more than one physical state

H2O(l) H2O(g)


Chemical equilibrium

Heterogeneous equilibrium

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


Chemical equilibrium

Write an equilibrium constant expression for:

H2O(l) H2O(g)

Keq =

[H2O(g)]


Chemical equilibrium

Equilibrium constant expression Keq = . . .

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


Chemical equilibrium

Equilibrium position

Equilibrium concentrations can vary from trial to trial.


Chemical equilibrium

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


Chemical equilibrium

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.


Chemical equilibrium

A system at equilibrium must:

  • take place in closed

  • system

  • temperature remain

  • constant

  • all reactants and

  • products are present

  • (both reactions can occur)


Chemical equilibrium

Write the equilibrium constant expression for

N2(g) + O2(g) 2NO(g)


Chemical equilibrium

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?


Chemical equilibrium

What happens when a system is at equilibrium and you upset the balance?


Chemical equilibrium

Le Chatelier’s Principle

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


Chemical equilibrium

Le Chatelier’s Principle

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


Chemical equilibrium

Changes in Concentration

Add reactant

1. Forward rxn speeds up.

2. Over time, forward

rxn slows down and

reverse rxn speeds up.


Chemical equilibrium

3. At equilibrium,

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

  • Equilibrium has

  • shifted right, value

  • of Keq unchanged.


Chemical equilibrium

Changes in Concentration

Remove reactant

1. Reverse rxn speeds up.

2. Over time, reverse

rxn slows down and

forward rxn speeds up.


Chemical equilibrium

3. At equilibrium,

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

  • Equilibrium has

  • shifted left; value of

  • Keq unchanged.


Chemical equilibrium

Changes in Pressure (Volume)

  • For a gas, decreasing

  • volume of container

  • increases pressure;

  • particles have less

  • space, collide more

  • frequently.


Chemical equilibrium

2. System will respond

by trying to relieve

stress (decrease

pressure.)

3. Shifts to the side with

fewer moles of gas.


Chemical equilibrium

4. Forward rxn speeds up.

5. Over time, forward

rxn slows down and

reverse rxn speeds up.


Chemical equilibrium

6. At equilibrium,

forward and reverse

rxns occur at same

rate, new equilibrium

position established.

7. Equilibrium has

shifted right, value

of Keq unchanged.


Chemical equilibrium

Changes in Pressure (Volume)

Note:

If volume is decreased concentrations of all gaseous substances increases.

Side with fewer moles of gas increases more.


Chemical equilibrium

Changes in Temperature

Review:

If DH is negative

rxn is exothermic

heat written as product

If DH is positive

rxn is endothermic

heat written as reactant


Chemical equilibrium

Changes in Temperature

N2O4(g) 2NO2(g)

DH = 55.3 kJ

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

light dark


Chemical equilibrium

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


Chemical equilibrium

Le Chatelier’s Principle

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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