Le Chatelier’s Principal

Le Chatelier’s Principal

Le Chatelier’s principal states: if a system at equilibrium is stressed by changing a factor (reactant/product concentration, heat, volume or pressure) the system will respond by shifting to oppose the imposed change.

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*Keep it Simple, Stupid* Systems do not like change!! They do whatever they can to stay the way they are. It’s like Lenz’s Law in physics. Systems do what they can to stay the same.

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To determine which side the equilibrium will shift, draw the reactants and products.

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If a chemical that is presently on the product side is added to the system, it’s concentration is increasing so we draw an arrow pointing up above the products. Now the reaction can be drawn like a slide.

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The product side is up so the equilibrium “slides” to the reactants. The equilibrium has shifted to the left, favouring;reactants as to consume excess product.

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Temperature change is the same as adding or removing energy from a system. There are two types of reactions. Endothermic (energy is on reactant side OR enthalpy change is positive) and Exothermic (energy is on the product side OR enthalpy change is negative).

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The energy added/subtracted to a system IS NOT just to one side of the reaction – it is to the whole reaction. What determines the equilibrium shift is by looking at which side initially has more/less energy.

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Ex: If the reaction is endothermic and energy is taken away, the reactant side will “drop” and the equilibrium will shift left as to replenish the lost heat.

Make example with NO2

According to Boyle’s Law: Pressure and Volume of a gas are INVERSELY proportional (as one increases, the other decreases.)

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In a system: Increase Pressure = Decrease Volume
;;;;;;;;;;;;;;;;;; Decrease Pressure = Increase Volume

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More moles of gas means more pressure. Therefore, if the system experiences a DECREASE in pressure (INCREASE in volume), it will shift to the side that has more moles.

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Example: In the nitrogen dioxide reaction, if the volume in the container is increased which way will the equilibrium shift?

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Volume increase = Pressure decrease. The system wants to counteract the pressure decrease by increasing the pressure. The system will shift to the side with the most amount of moles. The reactant side has 2 moles, the product side has 1 mole. Thus, the reaction will shift left, favouring the reactants.

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Make example

Adding or removing a chemical in the system changes the concentration in the system.

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Concentration in the system can also change by:
-Adding a chemical that will precipitate with an ion already in reaction
-Adding a chemical that has an ion in it that is also already in the system

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An increase in the concentration will cause the equilibrium to shift so that the system can consume the extra amount of chemical.

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A decrease in the concentration will cause the equilibrium to shift so that the system can replenish the extra amount of chemical.

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“The equilibrium will shift to the _____, favouring the ______, so that the excess/removed ______, can be consumed/replenished by producing more ________.”

Catalyst http://iannonechem.com/Science/Chemistry/Iannone/equilibrium/equilibriumworkbookanswers.htm#Work6

The addition of a catalyst DOES NOT cause the equilibrium to shift nor does it change the equilibrium constant (Keq)

(Get picture from key booklet of text book)

When looking at equilibrium Concentration vs. Time graphs, the most effective way of approaching them is:

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1.) Look at options given in multiple choice answers (Temperature increase, ____ added, volume decreased, ect…)

2.) Look at the given reaction and determine what each choice given would do to the equilibrium system.

3.) Look at graph and draw a straight vertical line where a change has occurred.

4.) At one line, look what the chemicals do AFTER the stress has occurred. If it is curving up, it increased as a result. If is is curving down ,it decreased as a result.

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