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Concentration

Example

When a reactant or product is added to a system at equilibrium, the system shifts away from the added component.

  • moves in a direction that uses up the excess component

But if a reactant or product is removed, the system shifts toward the removed component.

Example

What would happen if we add more

Fe3+ to the system?

How does this affect each of the reaction participants?

Sometimes we remove some of the substances by adding another substance that reacts with a participant in a side reaction.

What happens if we remove some SCN-?

If you add reactants (or increase their concentration).

  • The forward reaction will speed up.
  • More product will form.
  • Equilibrium “Shifts to the right”

Reactants Products

If you add products (or increase their concentration)

The opposite will occur

Equilibrium “Shifts to the left”

Reactants Products

****Note: Le Chatelier's Principle does not apply to pure solids or pure liquids***

Changing Pressure

& Volume

Changes in volume are going to have the opposite affect of changes in pressure

Example:

Increase volume (therefore decrease pressure) results in a shift to the right

Example

For each of the following predict whether the given change will cause the equilibrium to shift in favour of the products or the reactants, or neither.

If both sides of the equation have equal number of moles, then change in pressure will have no effect on the balance of equilibrium

Example: Predict the effect on equilibrium when the pressure is increased for the following reaction:

Only applies to gases in equilibrium in a closed system

With gases, equal volumes contain equal numbers of particles (no matter what gases we are comparing)

  • because 1 mole of any gas occupies 22.4L at STP

As the pressure increases, the reaction with shift in the direction of the least gases (the number of moles)

At high pressure

  • 3 moles vs. 2 - go to side with fewer moles

At low pressure

Definition

We can think of heat as a "reactant" for endothermic reactions

Or a "product" for exothermic reactions

Changing Temperature

For this example: an increase in temperature will favor the forward reaction since it is endothermic.

By adding more heat, equilibrium will shift to use up the extra heat, creating more NO2.

3 Types of Stress

1. Concentration

2. Pressure and Volume

3. Temperature

Le Châtelier’s Principle states that if a system in equilibrium is subject to an external stress, the equilibrium will shift to minimize the effects of that stress.

Essentially, if equilibrium is disrupted, the system will do what it takes to get back to equilibrium

These factors come from last unit: factors that affect reaction rates

We are missing:

Nature of reactants

We can not change the nature of the reactants

Catalyst

Is not a stress that shifts the equilibrium because it increases the rate of both the forward and the reverse reactions

Review of Unit 1 concepts:

Is this a endothermic or exothermic process?

How would you incorporate the heat into the equation?

External Stress

  • Is a factor that causes the rate of either the forward or reverse reaction to change
  • This throws the system out of balance.

HAND IN LAB REPORTS!!!

Le Chatelier's Principle

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