Multiple choiceQuestion 1 · Recall
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By the end you can state Le Chatelier's principle, say which way the position of equilibrium moves when a concentration or a pressure is changed, and justify it by counting gas molecules.
At equilibrium a reaction is balanced, but the balance is not fixed. Change the conditions and the system responds by opposing your change.
That single idea is Le Chatelier's principle, and it turns equilibrium from a fact you memorise into something you can reason with.
Le Chatelier's principle
If a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change.
"Minimise" is the key word. The system does not cancel the change completely — that is usually impossible — it just reduces its effect as far as it can.
The position of equilibrium describes the balance of amounts in the equilibrium mixture.
A shift is not a change in rate. At the new equilibrium the two rates are equal again, only at different concentrations from before.
Suppose you add more of a reactant. The forward reaction now has more to work with, so it speeds up and makes extra product — which uses up part of what you added.
The result: the position of equilibrium moves to the opposite side from the substance you added.
Removing a substance works the same way in reverse: the reaction that replaces it is favoured.
Worked example: Worked example: the blood-red complex
Iron(III) ions react with thiocyanate ions to give a deep blood-red complex:
The mixture is pale orange at equilibrium. Predict the colour change when a few drops of iron(III) chloride solution are added.
Pressure only matters for equilibria that involve gases, because only gases can be squeezed.
Raising the pressure pushes the system towards whichever side has fewer gas molecules, since that reduces the pressure again.
Count the gas molecules using the coefficients in the equation — ignore solids and liquids entirely.
Worked example: Worked example: squeezing nitrogen dioxide
Brown nitrogen dioxide dimerises to colourless dinitrogen tetroxide:
A sealed gas syringe at equilibrium is brown. The plunger is pushed in, halving the volume at constant temperature. Predict what happens to the colour, and explain why the colour does not simply double in intensity.
A shift can only undo part of a change. Squeezing a gas always increases its concentration, even though the shift uses some of it up.
Think of a spring: it resists being compressed, but it still ends up shorter than it started.
Increasing the pressure always shifts the equilibrium to the right.
It shifts towards the side with fewer gas molecules, which may be the left. If both sides have the same number, there is no shift at all.
Adding a catalyst shifts the equilibrium to the products.
A catalyst speeds up both directions equally, so it only helps the system reach equilibrium sooner. It does not change the position.
Changing the pressure changes the value of .
depends only on temperature. A pressure change alters the concentrations, not the constant.
Adding more reactant increases the amount of product without limit.
It shifts the position right, but the new equilibrium still contains all three substances; the extra reactant is only partly used up.
A shift right means the reaction is now faster.
The rates change briefly, but a shift is about the final amounts at the new equilibrium, not about speed.