Why I Stopped Teaching Le Chatelier’s Principle and Q Separately
I may be the last chemistry teacher on Earth to realize this about chemical equilibrium.
The reaction quotient (Q) and Le Chatelier’s Principle are just two different sides of the same coin. And in fact, Keq, Q, ICE boxes and Le Chatelier’s Principle are all on the same coin too. This is the first time that I have taught them essentially as a single concept. And I have to admit, student understanding about all of it has improved significantly. Here was my progression through the unit that connects Q, Le Chatelier shift, ICE boxes and ants. Yes ants.
Teach Keq expressions
After a simple demonstration that you can read about here, I headed straight into Keq expressions. They get drilled into their heads that it is products over reactants skip solids and liquids. Over and over.
This year I decided to introduce the question of “is it currently in equilibrium” to the class earlier. I would give them some data and have them find out if it was in equilibrium. They easily understood that if the calculated value was not equal to Keq, then the system was out of balance.
At that point they didn’t even know that they were calculating the reaction quotient (Q), but they understood the importance of the numbers not matching.
Introduce the reaction quotient (Q Test)
After they had been using the idea of the reaction quotient, I formalized the name and expression. Since a system may or may not be in equilibrium when tested by the reaction quotient, I changed the name from simply reaction quotient. We call it the Q Test. I have found that adding this colloquialism helps them understand what the point of it is.
They are testing to see if the system is in equilibrium.
They are also determining if the numerator or the denominator of the Q expression is too large. Too big of a Q value means the top of the fraction is too big. Too small of a Q value means the bottom of the fraction is too big. This is very simple but also very central to their understanding of Le Chatelier’s Principle.
Apply changes in Q to ICE box calculations
The kids then connect the sign of the Change line in the ICE box to the change that must happen to Q in order to reach equilibrium.
Pro Tip: Before students put any number or variable in the C row of the ICE box, they must put in the sign of all changes. Having them put +, -, - across the middle row helps them visualize what is happening before muddying their thinking with actual math.
If Q is too big, then the top of the fraction is too big, then the products are too big and must be subtracted. Repeating that concept over and over gets the students locked in to changes that must occur to regain equilibrium.
Teach Le Chatelier’s Principle with ants
It is a silly little story, but it works every year.
There is a tiny teeter totter with ants on both sides that is in equilibrium. Then a flycatcher swoops down and eats one of the ants. Is the teeter totter in equilibrium anymore?
Obviously, no.
Could it ever reach equilibrium again? Yes, but the ants will have to move. If the bird ate the ant from the right side of the teeter totter, then the right side is up in the air, and the ants will have to SHIFT to the right.
If the bird ate from the left side, the left side would be up in the air, and the ants would have to SHIFT left.
This simple picture helps the students qualitatively determine responses to perturbations in the system. (I threw that term in because 30 years ago, my professor Dr. Powers loved to use it.) And the best part of teaching Le Chatelier’s Principle with ants on a teeter totter is that I can literally watch the students thinking.
I train them to use their forearm as a teeter totter and actually show which way it will tilt. Then they can see which way the ants need to shift to regain equilibrium.
Create a synthesis of concepts between Keq, Q, ICE and Le Chatelier’s Principle
I didn’t intentionally do it, but I have taught each of these like nearly independent ideas in the past. But when I pushed them all into one continuous story this year, the students were able to make the connections much stronger. Keq is the balance point. Sometimes systems aren’t balanced and we have to use the Q test to find out. The Q test tells us the signs needed on the changes to regain equilibrium. The shift according to Le Chatelier is the qualitative response that demonstrates the change. It’s one idea.
And one idea in AP chemistry takes some stress off of their minds.
Plus, they get to watch everyone making tiny ant teeter totters out of their arms. That makes us all smile.
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