Le Chatelier’s PrincipleSCH 4U

What happens when we disturb chemical systems at equilibrium?

Demo: Cobalt Complex Equilibrium

[Co(H2O)6]Cl2 + heat [ CoCl2(H2O)2] + 4 H2O pink blue

Analogy...

Le Chatelier’s Principle:

When a chemical system at equilibrium is disturbed by a change in a property, the systems adjusts in a way that opposes the change

...eventually establishing a new

equilibrium!

Three Factors Affecting Equilibrium:

1.Concentration Change

2.Temperature Change

3.Total Pressure Change

1. Concentration Changes

aA + bB cC + dD

Adding a reactant= more available to react= favours forward rxn (“equil shifts right”)

With time, product concentration(s) increase

Eventually new equilibrium established...

... with different reactant/product concentrations from initial equilibrium

NOTE:Initial and final K values are

the same!

aA + bB cC + dD

Removing (decreasing conc of) a product

also favours forward rxn (shifts equil right)

Remember: system shifts to try to counteract the disturbance!

e.g. 2 CO2(g) 2 CO(g) + O2(g)

aA + bB cC + dD

What about...

....decreasing a reactant concentration?

....increasing a product concentration?

Common Ion EffectAdd an ion to solution that already

contains that ion......equilibrium shifts away from added ion

Industrial Example

3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g)

desired productremovewhat

about removing water?

Biological Example

Hb(aq) + O2 HbO2(aq)

Increases as blood goes into lungs

Decreases as blood circulates through body

2. Temperature Changes

Endothermic:Exothermic:

reactants + E productsreactants products + E

If we heat/ cool the system, equilibrium shifts to minimize the change...

If we cool the system...

If we heat up the system...

Equilibrium shifts in direction that produces heat (favours exothermic)

Equilibrium shifts in direction that absorbs heat (favours endothermic)

Temperature Change & KNOTE: The value of K changes with temp. (only constant at specific temperature)

Predict how K will change if......we heat up an endothermic (fwd) rxn?

...we cool down an endothermic rxn?

And what about exothermic (fwd) reaction?

→ Equil. shifts right, K increases

→ Equil. shifts left, K decreases

Temperature Change Example

Remember this?

N2O4(g) + energy 2 NO2(g)

3. Pressure (Volume) ChangeRecall Boyle’s Law:

Remember, pressure is caused by gas molecules striking the walls of a container, so you change pressure by changing the number of collisions (change volume)

ONLY affects GASES!

Increasing total pressure of system shifts equilibrium to side of balanced equation showing fewest number of gas molecules

If P (↑), and concentrations change (i.e. increase)

then V (↓)

2 SO3(g) ↔ 2 SO2(g) + O2(g)

Note

If both sides of the equation have the same number of molecules of gas, any change in pressure or volume will have no effect on equilibrium

i.e. H2(g) + I2(g) 2 HI(g)

So what doesn’t affect equilibrium?

Addition of a catalyst A catalyst speeds up the rate of a

reaction Lower the activation energy, which

increases the rate of reaction, of both the forward and reverse reactions

Catalysts do NOT affect position of equilibrium, only time taken

So what doesn’t affect equilibrium?

Adding Inert Gases Inert gases do NOT react with

other gases (i.e. NOT part of the equilibrium system)

Presence of inert gas changes the probability of successful collisions for both the reactants and products equally, resulting in NO shift in equilibrium

Le Chatelier’s Summary

Homework

Pg 457 #1 – 4

Pg 459 #2 – 4

Read pgs 461 – 462 on the Haber Process

Haha, just kidding!