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.
Run the investigation
- 1Observe the baseline color balance for the N2O4 ⇌ 2NO2 system.
- 2Increase the temperature and record which side becomes more prominent.
- 3Decrease the temperature and record how the color balance changes.
- 4Use those observations to infer which direction of the reaction is endothermic.
Booting the lab…
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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