Chem 30: equilibrium

Conditions required to reach equilibrium? (2)

1) closed system (no matter enters or leaves, but energy can go in or out

2) reactants and products present

Equilibrium:

Constant macroscopic properties

Equilibrium: solids (rare)

-Constant mass

Equilibrium: liquids *common example

- pure

- constant mass or volume

- esterification (organic reaction)

Fluid Equilibrium questions mass

-Always constant mass if at equilibrium or not*

-does not tell you if it's at equilibrium due to conservation of mass

Reasons why reactions don't go to completion:

- rates of collision theory

- think of mr. Hopes night club analogy

Rates (collision theory): in order to effectively react reactants:

1) must be collisions

A) concentrations

B) anything to alter the # of collisions

2) must collide often

3) effectiveness of collision (activation energy)

Equilibrium

RXN rate forward = RXN rate reversed

- reactions proceed in both directions at equilibrium at the same rate once equilibrium has been reached

Pressure is

Particles hitting the walls of the container

Law of chemical equilibrium

- in a chemical system at equilibrium, there is a constant ratio between the (concentration) of the products and the concentrations of the reactants

aA+bB <-> cC + dD

Kc = ?

Kc= ([C]^c [D]^d)

/ ( [A]^a [B]^b)

K=?

Products / Reactants

For the equilibrium expression don't include? Except?

Don't include H2O(l) , solids and liquids in the equation.

EXECPT in esterification : alcohol + carboxlic acid -> ester + water

It is possible to describe an equilibrium in several ways

- the equilibrium expression

- the equilibrium constant and it's magnitude

- the position of equilibrium

- the % reaction

- the equilibrium

Kc

- A genetic concentration equilibrium constant

Kb

A generic pressure equilibrium constant

Ka

An acid equilibrium constant / dissociation constant/ ionization constant

Kb

An base equilibrium constant/ dissociation constant/ ionization constant

Ksp

a solubility product- this describes the solubility of low solubility ionic salts

Kw

- the water equilibrium constant / auto dissociation constant/ ion products

Equilibrium: solutions

- constant pH

- constant colour

- constant precipitate (hours)

Equilibrium: gases

- constant colour

- constant pressure

-constant volume

Concentrations of reactants and products are

Not usually equal

What number should you get if: neither products or reactants are favourited

K= 1

What number should you get if: reactants are favoured

K < 1

What number should you get if: products are favoured

K > 1

The Keq tells us

When a chemical reaction had reached equilibrium.

The equilibrium constant is a comparison between

the equilibrium concentrations of the products and the equilibrium concentrations of the reactants.

K expression=?

Q expression= ?

K expression= at EQ already

Q expression= at some point during the reaction

When initiating a chemical reaction, we mixed various concentrations of reactants and products. It can be useful to know the direction in which the system will shift. To determine this we use?

The reaction quotient, Q.

The reaction quotient is derived by using the initial concentration do the products.

To determine in which direction a system will shift to reach equilibrium, we can compare the values of K vs Q. There are three possible cases. 1) Q=K

Q=K, the system is in equilibrium and no shift will occur

To determine in which direction a system will shift to reach equilibrium, we can compare the values of K vs Q. There are three possible cases. 2) Q>K

Q>K, the ratio of initial concentration of products to reactants is to large. This means there are more products, than reactants in the reaction mixture. The equilibrium will shift the reaction to less the amount of products. *The reverse reaction will be favoured

To determine in which direction a system will shift to reach equilibrium, we can compare the values of K vs Q. There are three possible cases.

3) Q

Q

Chatellier's principle (qualitative)

- if a stress is placed on a system in equilibrium, the system will shift to maximize (counteract) the stress placed on it.

7 stresses

1. Adding or removing products and reactants

2. Adding of removing heat (endothermic and exothermic)

3. Volume change (relieve or add pressure)

4. Adding a catalyst or inert gas

5. Neutralization

6. Precipitation

7. Low ionization acids

Endothermic reaction

- heat on reactant side

- surroundings cool

- 🔼h = +

Exothermic reaction

- heat on product side

- warm the surrounding

🔼H = -

Heat will

Permanently change the k value

The only stress that permanently changes the k value is

Heat

When dealing with pressure/volume stresses, it's good to recall?

Boyle's law - pressure is inversely proportional to the volume of the container

- or: the concentration of a gas in a container is directly proportional to there pressure of the gas.

When dealing with pressure/ volume you go from?

Pressure to volume:

increase pressure: decrease volume

Decrease pressure increase volume

Memorize what two things about pressure/volume stresses

1) more volume for more moles of gas

2) less volume for less moles of gas

To predict whether a change in pressure will affect a system equilibrium, you must consider?

The total amount in moles of gas reactants and the total amount of gas products.

Increased volume decreases the pressure what response is there by the system

System wants to get back to equilibrium and previous pressure so the reaction favours whatever reaction that increases # of miles therefore increasing pressure.

Effects of adding an inert gass or catalyst generally

Causes no shift

Effect of adding an inert gas

Does increased the partial pressure of the gases. But effects a gas equally.

An inert gas? Examples?

An inert gas doesn't react.

Eg: Nobel gases except xenon

Catalyst

- speeds up a reaction by providing an alternative reaction pathway, which decreases activity ion energy of a successful collision.

Effects on equilibrium of adding at catalyst

Increases the rate forward and reverse by the same amount

Secondary reaction stresses (3)

1) neutralization

2) precipitate

3) low ionization acid

Stress neutralization

- species list

It's the reaction between an acid and a base that is causing the shift.

When every they have something written above the equilibrium arrow this something is most likely

A catalyst

Stress: precipitation

-species list

-Addition of a substance that causes a precipitation to from from one of the other substances (usually causes a decrease stress in one of the factors)

Stress: low ionization acid

- Weak acid formation

-species list

- will from a weak acid (so will most likely slightly decrease a concentration of a molecules)

All reactants and products lines become level when?

Equilibrium had been achieved

La Chatellier's principle- Graphs: concentration, it not reactant or product is added then?

i) only 1 line increases vertically at the stress point

ii) Concentration adjust accordingly after the stress point

For La Chatellier's principle- Graphs: becareful of

Mole ratios

- especially with concentrations

La Chatellier's principle- Graphs: concentration: reactant or product is removed

i) only one line decreases vertically at stress point

How does the k value change with concentrations forLa Chatellier's principle- Graphs

Trick question! It doesn't

La Chatellier's principle- Graphs: Stress with no concentration change (inert gas/catalyst)

- Lines stay flat

La Chatellier's principle- Graphs: pressure/volume: volume increased

Concentrations go down

-* all gaseous lines go down

La Chatellier's principle- Graphs: pressure increases

- concentrations go up- all gaseous lines go up

La Chatellier's principle- Graphs: pressure/ volume keep in kind?

C= (n/V)

La Chatellier's principle- Graphs: heat?

- some line go up gradually

- some lines go down gradually

La Chatellier's principle is used to

Qualitatively predict an increase or decrease in the concentration of any substance when a change is introduced to a system equilibrium

Ice tables what does each part stand for?

I= initial concentration

C= change (in constant)

E = equilibrium concentration

f= final

Which parts of the ice table have variables

E and C

(Equilibrium concentration) (change in constant)

=E?

I +C = E

F=? difference?

E

F does not have a variable, E does

Rule of 1000?

You can make a number zero of

([initial]/ K)> 1000