After midterm · Session 5 of 6 · 180 minutes
30 September 2026
Why can composition change while the equilibrium constant stays the same?
Write your prediction before opening the lab. By the end, support an answer with one controlled comparison and an independent check.
| Activity | Minutes |
|---|---|
| Opening question and essential concepts | 30 |
| One hand calculation | 20 |
| One lab demonstration | 20 |
| Break | 10 |
| Student pair investigation | 70 |
| Discussion and exit question | 30 |
The 70-minute investigation is student work with instructor support.
\ln K=-\frac{\Delta_rG^\circ}{RT},\qquad Q=\prod_i a_i^{\nu_i}
K is tied to T and the fixed reaction/standard-state convention. Q describes the supplied state.
Q=K is an equilibrium condition, not an identity for every measured composition.
Independent inputs can disagree; the lab preserves the disagreement.
Lab 12 synthetic A₂ ⇌ 2A at 350 K uses ΔrG°=−4.000 kJ/mol.
\ln K=\frac{4000}{R(350)}=1.37454,\qquad K\approx3.95326
With ΔrH°=50.000 kJ/mol and ΔrS°=54000/350 J mol⁻¹ K⁻¹, the H−TS route gives the same value.
Keep the reaction basis and standard states fixed. Convert kJ to J before calculating ln K, then check ln Q−ln K for the calculated equilibrium state.
Use the synthetic Lab 12 case at its reference temperature. Compare the supplied Gibbs-energy and H−TS routes, then change only pressure and inspect K and extent.
Keep one saved baseline for the pair investigation.
In Lab 12, compare the supplied Gibbs-energy and H−TS routes at the reference temperature. Change pressure at fixed T and compare K with equilibrium extent. Check one balance.
Use 20 minutes to compare results and discuss unexpected behavior.
Why can composition change while the equilibrium constant stays the same?
Use the final 10 minutes to explain your answer individually and name one assumption that limits it.
In Lab 12, compare the supplied Gibbs-energy and H−TS routes at the reference temperature. Change pressure at fixed T and compare K with equilibrium extent. Check one balance.
Assessment 5 and independent checkpoint
Retain one study JSON, one comparison and a short explanation. Continue from your classroom case. The hand checkpoint is part of this time budget.
Choose one extension: vary temperature with and without a heat-capacity correction, reverse the reaction, or scale its coefficients consistently.
Session appendix · Module reference
Learning path · Offline package
Optional work is additional. Complete the required investigation first.