Use heat stress to predict an equilibrium shift
Three steps, the way the exam actually works: work through the lab, write down your own observations, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own evidence will cost you the point — exactly as it would with a real reader.
Predict before you look
- Le Châtelier’s principle says a system shifts to relieve a stress. If added heat is the stress, which side of the reaction relieves it by consuming heat?
- In N₂O₄ ⇌ 2NO₂, which species is the darker brown one? A color change is your only readout here, so this matters.
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you found
This is your reading of the source, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Baseline appearance | |
|---|---|
| Appearance after heating | |
| Appearance after cooling | |
| Side favored by heating |
Answer the free response
The equilibrium N2O4(g) ⇌ 2NO2(g) shifts visibly with temperature. (a) Using your observations, state whether the forward reaction is endothermic or exothermic and justify the claim with Le Châtelier’s principle. (b) Explain why adding heat is treated as adding a reactant to one side of the equilibrium expression conceptually, even though heat does not appear in K. (c) Predict the effect of cooling the system and explain why the same equilibrium constant cannot apply at both temperatures. (d) A student says the color change proves the reaction “speeds up toward products” rather than that the equilibrium position changed. Evaluate that statement carefully.
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