Common misconceptions
Look at the conversation between two students about Le Chatelier's Principle and then answer the questions that follow.
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What two misconceptions does student A reveal about the nature of Le Chatelier's Principle?
Student A believes that Le Chatelier's Principle is a cause of behaviour rather than a description of it.
He also thinks (incorrectly) that Le Chatelier's Principle can be applied in all situations.
Student A hasn't recognised that the Equilibrium Law can be used to do something that Le Chatelier's Principle cannot achieve, What is this?
The Equilibrium Law can be used to calculate the quantitative changes that occur when the concentration of a component of the equilibrium mixture is changed. Le Chatelier's Principle can only be used to predict qualitative changes.
Look at the following ideas that a student might have about the effect of adding additional reactant to an equilibrium mixture.
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What misconception does the student have about change of Kc?
The student doesn't appreciate that Kc is a constant at constant temperature so it's unchanged by adding additional reagent.
What misconception does the student have about using up added reagent?
Only some, not all, of the added reagent is used up in re-establishing equilibrium.
The idea that the rate of the forward reaction increases until equilibrium is re-established is untrue. What happens to the rates of the forward and reverse reactions when additional reactant is added to an equilibrium mixture?
The forward reaction increases when the reagent is added but then decreases as some of it is used up. The reverse reaction increases as more products are formed. Equilibrium is re-established when the rates become equal.
What misconception does the student display about the effect of adding a reagent on the reverse reaction?
The student may well be thinking of a static rather than a dynamic equilibrium. The forward and reverse reactions occur all the time. Adding a reagent produces more products. The increase in product concentration increases the rate of the reverse reaction.