Chemical EquilibriumChapter 18

Chemical EquilibriumSection 18-1

Pp. 589 - 591

Equilibrium is…Equilibrium is not static

Opposing processes occur at the same time and at the same rate

Rate of forward reaction = Rate of reverse reaction

For example 1: water at 0.0°CAt 0.0, some liquid H2O is freezing and some ice (solid)

H2O is melting. Their rates are equal, so equilibrium

For example 2: adding sugar molecules into waterAt equilibrium, the rate of sugar molecules in solution

crystallizing equals the rate of sugar crystals dissolving

Reversible ReactionsEvery reaction can proceed:

Reactants Products OR Reactants Products

Def. – chem rxn in which products can react to re-form reactants

2HCl H2 (g) + Cl2 (g)

Here most HCl is decomposing into hydrogen gas & chlorine gas

However, some H2 and Cl2 are synthesizing into HClAt equilibrium, these rates are equal

ConclusionA reversible chemical reaction is in

chemical equilibrium when rate of forward reaction = rate of reverse reaction

The concentrations of the reactants and products are static

[Reactants] DOES NOT = [Products] !!!

Usually see double arrows to indicate reversibility:

Equilibrium lies to the right?

Some reactions “favor” the formation of products

At equilibrium: higher concentrations of Products than of Reactants

[Products] > [Reactants]Sometimes the forward arrow will be longer than

the reverse arrow, to indicate “product favored”

Some reactions “favor” the formation of reactants

At equilibrium: [Products] < [Reactants]Sometimes the reverse arrow will be longer than the

forward arrow, to indicate “reactant favored”

Equilibrium ConstantGiven the general reaction:

nA + mB xC + yD Initially, there is only A & B but no C or D, so

the forward reaction rate is at its maximum.

Over time, C + D accumulate, so forward rate slows & reverse rxn rate increases

Eventually the two rates become equal to each other Equilibrium !!!

Equilibrium ConstantAfter equilibrium is reached, the individual

concentrations of A, B, C, and D undergo no further change if conditions remain the same.

A ratio of their concentrations should also remain constant.

The equilibrium constant is designated by the letter K.

K =

[C]x[D]y

[A]n[B]m

The constant K is independent of the initial concentrations.

K is dependent on the temperature of the system.

The Equilibrium Constant

The numerical value of K for a particular equilibrium system is obtained experimentally.

If the value of K is small, the reactants are favored.

A large value of K indicates that the products are favored.

Only the concentrations of substances that can actually change are included in K.

Pure solids and liquids are omitted because their concentrations cannot change.

Determining Keq for Reaction at Chemical Equilibrium

Shifting EquilibriumSection 18.2

Pp. 598 - 601

Le Chatelier’s Principle A system is happily at equilibrium, then a change in

concentration, pressure, or temperature occurs

What will happen to the equilibrium?

Def. – if a system in equilibrium is stressed, then the equilibrium is shifted in a way to relieve that stress

3 main Stressors: pressure, concentration, and temperature

Pressure ChangeN2(g) + 3H2(g) 2 NH3(g)

What does the (g) stand for?How many moles of gas on the left? On the right?

So if there was a pressure INCREASE:Causes an increase in gas concentrationWhich side will experience the biggest change in

gas pressure (concentration)?How would the system relieve the stress?Shifts toward side with fewer moles!!Pressure increase = shift to side with fewer

moles

Pressure Change (Page 2)

What is pressure was DECREASED?Shifted to side with more moles of gas

INVERSE relationship between pressure change and side with number of moles

INCREASE pressure = shift to side with FEWER moles

DECREASE pressure = shift to side with MORE moles

Pressure change & Keq

Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant.

Increasing pressure by adding a gas that is not a reactant or a product cannot affect the equilibrium position of the reaction system.

Concentration Change If one side INCREASES concentration, then the

system will shift the reaction to the opposite side

If one side DECREASES concentration, then the system will shift the reaction to its side

If we increase the H2 concentration, what will happen?Will N2 increase or decrease? What about

NH3?

If we decrease the N2 concentration, what will happen?Will H2 increase or decrease? What about

NH3?

Concentration & KeqChanges in concentration have no effect on the

value of the equilibrium constant.

Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression.

Temperature ChangeReactions are either exothermic or endothermic

If endothermic, then energy is a REACTANT

If exothermic, then energy is a PRODUCT

So if a reaction is ENDOTHERMIC, then increasing temperature shifts equilibrium to products

Increase TEMP Shift to PRODUCTSDecrease TEMP Shift to REACTANTS

Temperature Change If a reaction is exothermic, then heat is a

PRODUCT

In an EXOTHERMIC rxn, when temp is INCREASED, then the rxn shifts to REACTANTS

In an EXOTHERMIC rxn, when temp is DECREASED, then the rxn shifts to PRODUCTS

The value of the equilibrium constant (Keq) for a given system is affected by the temperature.

Haber Process

Artificial production of ammonia

N2(g) + 3H2(g) 2 NH3(g)+92 kJ

In this reaction, we want to produce as maximum amount of NH3 (Ammonia)

There are how many moles of reactants? Products?

So how do we create the maximum product concentration by changing the pressure?

Haber Process (Page 2) N2(g) + 3H2(g) 2

NH3(g) +92 kJ

Temperature Is the reaction endothermic or exothermic? Is heat a reactant or product? So what should be done to maximize products?

ConcentrationHow would me maximize products by altering

concentrations?

Last ExampleBicarbonate Buffer in mammalian blood

H2CO3 H+ + HCO3-

What happens when pH decreases? What happens to the [H+]? So what happens to the equilibrium?

What about the [H2CO3] & [HCO3-]

What happens when pH increases? What happens to the [H+]? So what happens to the equilibrium?

What about the [H2CO3] & [HCO3-]