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K at Temperature T2
4.9870
K at Temperature T2 vs K at Temperature T1
Formule
## Van't Hoff Equation The van't Hoff equation predicts how the equilibrium constant changes with temperature. ### Formula **ln(K2/K1) = (delta_H/R) × (1/T1 - 1/T2)** where R = 8.314 J/(mol·K) is the gas constant and delta_H is the reaction enthalpy. - Exothermic reactions (delta_H < 0): K decreases as T increases - Endothermic reactions (delta_H > 0): K increases as T increases This is the thermodynamic basis for Le Chatelier's principle regarding temperature.
Exemple Résolu
An exothermic reaction with K=100 at 298 K, delta_H = -50 kJ/mol. Find K at 350 K.
- 01ln(K2/K1) = (-50000/8.314) × (1/298 - 1/350)
- 021/298 - 1/350 = 0.003356 - 0.002857 = 0.000499
- 03ln(K2/K1) = -6013.4 × 0.000499 = -3.001
- 04K2/K1 = exp(-3.001) = 0.0498
- 05K2 = 100 × 0.0498 = 4.98
Questions Fréquentes
Why does K decrease for exothermic reactions when heated?
For exothermic reactions, heat is a product. Raising temperature shifts equilibrium toward reactants (Le Chatelier), decreasing K. The van't Hoff equation quantifies this shift.
Is delta_H assumed constant over the temperature range?
Yes, the basic van't Hoff equation assumes delta_H is constant. For large temperature ranges, delta_H may change due to differences in heat capacities, and the extended van't Hoff equation with Kirchhoff's law should be used.
Can I use this for Kp and Kc?
Yes. The van't Hoff equation applies to any equilibrium constant form. Kp and Kc are related by Kp = Kc × (RT)^(delta_n), where delta_n is the change in moles of gas.
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