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SCH 4U. Le Chatelier’s Principle. What happens when we disturb chemical systems at equilibrium?. Demo: Cobalt Complex Equilibrium. [Co(H 2 O) 6 ]Cl 2 + heat [ CoCl 2 (H 2 O) 2 ] + 4 H 2 O pink blue. Analogy. Le Chatelier’s Principle:. - PowerPoint PPT Presentation
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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!