Pharmaceutical Chemistry
Heat of Formation Calculator
Estimates the enthalpy change of solution (ΔH) from the solubility of a substance at two temperatures, using the van't Hoff equation.
ΔH of solution
+52903.1J/mol
Endothermic · Large endothermic process
Solubility data
Solubility measured at the first temperature.
Room temperature is 298 K (25 °C).
Solubility measured at the second temperature, in the same unit as S₁.
Both temperatures share one unit — changing either switches both.
Working
Temperatures converted to kelvin, then substituted into the equation.
ΔH = 2.303 × 8.314 × (log(0.2) − log(0.1)) × (298.00 × 308.00) / (308.00 − 298.00)
= +52903.1 J/mol
Common ΔH values
Enthalpy of solution in water.
| Substance | Type | ΔH (kJ/mol) |
|---|---|---|
| NaOH in water | Exothermic | -44.5 kJ/mol |
| NH₄NO₃ in water | Endothermic | +25.7 kJ/mol |
| KCl in water | Endothermic | +17.2 kJ/mol |
| H₂SO₄ in water | Exothermic | -95.3 kJ/mol |
| NaCl in water | Endothermic | +3.9 kJ/mol |
ΔH = 2.303 × R × (log S₂ − log S₁) × (T₁ × T₂) / (T₂ − T₁)
ΔH — enthalpy of solution (J/mol). R — gas constant = 8.314 J/(mol·K). S₁, S₂ — solubilities at temperatures T₁ and T₂. T₁, T₂ — absolute temperatures in kelvin (°C inputs have 273.15 added).
Note: positive ΔH indicates an endothermic process, negative an exothermic one. The 2.303 converts natural logarithms to base-10 logarithms.
References
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
Common questions
Any, as long as S₁ and S₂ use the same one. The equation uses the ratio S₂/S₁ (through the difference of their logarithms), so mol/L, g/L or g/100 mL all give the same ΔH.
The equation comes from thermodynamics, where temperature is absolute. Using °C directly would give a wrong product T₁ × T₂. Enter °C if you prefer — the calculator adds 273.15 before substituting.
Divide by 1000. A result of +52903.1 J/mol is +52.9 kJ/mol, which is how the reference table lists its values.
ΔH comes from how solubility changes between them. If T₁ equals T₂ the denominator is zero and there is nothing to compare, so the calculator waits for two different values.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy