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Types of Equilibrium and

Le Chateliers Principle

Mr. Shields Regents Chemistry U13 L02

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Equilibrium reactions can be categorized as falling into 1 of 3types:

1)Phase Equilibrium

2)Solution Equilibrium

3)Chemical Equilibrium

Chemical Equilibrium reactions can be further categorized as either

1) Homogeneous or 2) Heterogeneous

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Types of EquilibriumTypes of Equilibrium

A homogeneous equilibrium is one in which the states of Matter for all reactants and products are the same

Example: H2 (g) + I2 (g) 2HI (g)

A heterogeneous equilibrium is one in which the states ofMatter vary between reactants and products

Example: C (s) + O2 (g) CO2 (g)

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Phase EquilibriumPhase EquilibriumPhase equilibrium exists between two phases of matter(S, L, G) usually in a closed vessel. They are easily recognizable:

1) H2O(s) ↔ H2O(l) ice – water equilibrium (def. of MP)

2) H2O (l) ↔ H2O(g) Water – Vapor equilibrium

3) I2 (s) ↔ I2 (g) Iodine sublimation

1 above occurs at 0 deg

2 & 3 above must beSealed systems! Why?

DYNAMIC EQUILIBRIUM

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Solution EquilibriumSolution EquilibriumBesides The dynamic equilibrium of a liquid vapor (orGas) and a liquid, solution equilibriums are created …

- When additional solid is added to an already saturated solution

For a solid in a saturatedsolution at equilibrium:

The rate at which solidsre-crystallize from sol’n &the rate at which Crystalsdissolve must be equal

(Saturated sol’n)

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Solution EquilibriumSolution Equilibrium

In our example using NaCl we would write the equilibrium as:

NaCL (s) + H20 ↔ Na+ (aq) + Cl- (aq)

If the solid is molecular (for example sugar) then the Following equation would define the equilibrium condition:

C12H23O11 (s) + H20 ↔ C12H23O11 (aq)

Notice there are No IONS formed for molecular compounds.

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Solution EquilibriumSolution Equilibrium

For GASES dissolved in Liquids in a SEALED system

The rate at which the gas leaves the liquid and the Rate at which the gas re-dissolves must be equal tobe in equilibrium.

O2 (aq) ↔ O2 (g) Why a sealed system?

But this equilibrium can be affected by shifts in T and/or P

- if T increases the equilibrium shifts to the right and- If P increases the equilibrium shifts to the left

WHY ?

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Le Chatelier’s PrincipleLe Chatelier’s Principle

This principle describes how a system atequilibrium reacts to changes (stresses) insystem conditions.

Le Chatelier’s principle:

If a change is imposed on a system at equilibrium, then thesystem will shift the direction of the reaction to reducethe change in conditions.

In other words, When a stress is imposedon a system the system will respond tocounteract the stress

Henri Le Chatlier1850-1936)

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Le Chatelier’s PrincipleLe Chatelier’s PrincipleFor example: consider the rxn … A + B ↔ C + D + Heat

If the above reaction proceeds to the right, we produce moreC, D and heat.

If the reaction proceeds to the left we consume heat, and Produce more A and B

If we consume heat what does Le Chatelier’s principle predict?

Le Chatelier’s Principle says if we remove heat the system willTry to return to equilibrium conditions by producing heat to Replace the heat consumed. The Rxn will be Driven to the right!

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Factors affecting EquilibriumFactors affecting Equilibrium

When considering Le Chatelier’s principle there are SeveralFactors that will affect equilibrium:

1) Adding or Removing HEAT

2) Increasing or decreasing PRESSURE

3) Changing the CONCENTRATION …

By adding, decreasing OR totally removing one or one or more of the reactants/products

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Head to Tail MethodHead to Tail Method

We can use Le Chatelier’s principle to predict how ChemicalReactions will respond to external forces.

It sounds like this might be difficult to do but there’s anEasy way to determine how an equilibrium will react to stress.

It’s called the “Head to Tail” method and its used todetermine the Direction of the shift as a stress is applied To a system in equilibrium.

There are 3 simple rules …

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3

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Head to Tail MethodHead to Tail Method

Draw the 1st ARROW next to the component that is changing. Draw it upwards if it is increasing and downwardIf it is decreasing

Lets say we increase the heat in a system in which thereIs an equilibrium chemical reaction occurring

A + B ↔ C + D + Heat

Draw a second arrow from the 1st arrow going acrossthe TOP of “↔”. Each arrow must go from “Head to Tail”

A + B ↔ C + D + Heat

A + B ↔ C + D + Heat

Rule 1:

Rule 2:

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Head to Tail MethodHead to Tail Method

The direction of the second arrow indicates the Direction the equilibrium reaction shifts when the stress isApplied.

A + B ↔ C + D + Heat

Rxn Shifts in this direction

Of course we could also have predicted this using LeChatelier’s principle. If we add stress to the system byincreasing heat the system responds by trying to oppose theStress. Reaction moves to the left because that directionConsumes heat.

Rule 3:

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Problem:

In the following reaction the concentration of C is decreased. Which way will the equilibrium reaction shift?

A + B ↔ C + D + Heat

Ans:

A + B ↔ C + D + Heat

Reaction shifts towards the right in Response to the stress (decreasing concentration of C)to try to increase the concentration of C back to it’sEquilibrium value

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We’ve discussed how to determine the affects of Changing 2 of the three factors that affect EquilibriumNamely using le Chatlier’s principle:

1)Changing temperature2)Changing concentration

The effect of the last factor, PRESSURE, we’ll leaveFor discussion in the next lesson.