Intermediate Topic 327 of 1102 States of Matter & Solutions
Dissolution Enthalpy
Heat change when a solute dissolves, determined by lattice and solvation energies.
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In-depth explanation of Dissolution Enthalpy
The enthalpy of solution is the heat change when a solute dissolves in a solvent at constant pressure. It can be considered as the sum of two steps: the lattice enthalpy, which is the energy needed to separate the ions or molecules of the solute, and the hydration enthalpy, which is the energy released when solvent molecules surround the separated particles. The lattice enthalpy term is positive, while the hydration term is negative. If the sum is negative the dissolution is exothermic and the solution warms, while a positive sum gives an endothermic dissolution and cooling. Entropy increase often drives dissolution even when the process is endothermic.
Going deeper: Dissolution Enthalpy
Key ideas: Hess cycle: ΔHsoln = lattice enthalpy (positive) + hydration enthalpy (negative), with lattice enthalpy defined as breaking solid into gaseous ions. Example: NaCl has +788 and -784, so ΔHsoln = +4 kJ/mol, slightly endothermic. In a calorimeter: q = m c ΔT, then ΔH = -q / n for the solute. Spontaneity needs delta G negative, so entropy matters.
Common mistakes and connections: Common errors are sign mistakes in the calorimeter and using the mass of solution for moles of solute. Another is saying endothermic dissolution cannot occur spontaneously; entropy gain drives it. Larger lattice enthalpy often means lower solubility but not always. Links to Born-Haber cycles, hydration enthalpy, solubility versus temperature and calorimetry.
Visualization of Dissolution Enthalpy
Enthalpy of solution is lattice energy absorbed plus hydration energy released; the net sign decides whether the solution cools or warms. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ammonium nitrate dissolves endothermically in water, which is why it is used in instant cold packs.
Frequently asked questions about Dissolution Enthalpy
What is Dissolution Enthalpy?
Heat change when a solute dissolves, determined by lattice and solvation energies.
Is Dissolution Enthalpy a beginner, intermediate or advanced topic?
Dissolution Enthalpy is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Dissolution Enthalpy?
Study these earlier topics in States of Matter & Solutions first: Hardy-Schulze Rule, Solid Solutions, Alloys.
What comes after Dissolution Enthalpy?
The next topics in States of Matter & Solutions are Solvation and Hydration.
Can I practise Dissolution Enthalpy with quizzes?
Yes. The interactive lesson includes a self-check question on Dissolution Enthalpy, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Dissolution Enthalpy fits in States of Matter & Solutions
Dissolution Enthalpy is topic 108 of 109 in States of Matter & Solutions.