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Syllabus
Syllabus
The relative amounts of reactants and products at equilibrium depends on the conditions of the reaction.
Any change of condition made to an equilibrium will cause the equilibrium to move to oppose the changes.
The effects of changing conditions is predicted by Le Chatelier's Principle.
Changing the conditions of an equilibrium causes the system to no longer be in equilibrium.
If the concentration of reactants is increased, more product will form until equilibrium is reached again.
If the concentration of products is decreased, more product will form until equilibrium is reached again.
If the temperature is increased the endothermic reaction is favoured until until equilibrium is reached again.
If the temperature is decreased the exothermic reaction is favoured until equilibrium is reached again.
If pressure is increased the equilibrium will shift to favour the side with fewer moles of gas.
If pressure is decreased the equilibrium will shift to favour the side with more moles of gas.
Students should be able to make qualitative predictions about the effect of changes made to an equilibrium.
What does this mean?
What does this mean?
A system is in dynamic equilibrium when the forward and backward rate are equal.
This doesn't mean that the concentrations of reactants and products are equal.
It's possible that 90% (or more) of the atoms are still bonded together as reactants and only 10% of the atoms have bonded together as the products.
When one of the conditions of the equilibrium (such as the temperature or pressure) is changed, both rates will change until a new equilibrium is formed with a different percentage of reactants and products.
Whatever change is made the equilibrium will move to cancel out the change.
This is called Le Chatelier's Principle.
The effect of changing Temperature
The effect of changing Temperature
In this equilibrium, the forward reaction is exothermic
We know this because ΔH is negative.
So, if the temperature is raised the equilibrium will move left (endothermic) to cancel out this change by making it colder.
The effect of changing Pressure
The effect of changing Pressure
In this example, there is no gas on the left of this equilibrium, but two moles of gas on the right.
So, moving to the right would increase the pressure (the more gas, the higher the pressure).
If we raise the pressure this will force the equilibrium to the left to reduce the pressure (by removing gas)
In this example, we can see that there is gas on both sides.
But there are more moles of gas on the left.
So moving left would increase the pressure.
If we raise the pressure this will force the equilibrium to the right to reduce the pressure (by removing gas)
In this example, there are two moles of gas on both sides.
So, there is no difference in pressure.
If we raise the pressure there will be no effect.
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Le Chatalier's Principle
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