Enthalpies of solution and hydration
Introduced| English | Meaning |
|---|---|
| enthalpy of solution/enˈθælpi ɒv səˈluːʃn/ | enthalpy of solution |
| enthalpy of hydration/enˈθælpi ɒv haɪˈdreɪʃn/ | enthalpy of hydration |
| exothermic/eɡzəˈðɜːmɪk/ | Describing a process that transfers heat to its surroundings. |
| lattice energy/ˈlætɪs ˈenədʒi/ | The enthalpy change associated with forming an ionic solid from gaseous ions, or the reverse process under a stated sign convention. |
Dissolving, energetically
- Dissolving an ionic solid involves two enthalpy steps.
- Hydration releases energy; pulling the lattice apart costs it.
- A cycle links them to the enthalpy of solution 溶解焓.
Enthalpy of solution lab
deltaHsol = lattice + hydration terms
Change hydration strength and see solution enthalpy shift.
The enthalpy change when one mole of gaseous ions dissolves in water is the enthalpy of ______.
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More negative for smaller, more highly charged ions.
Enthalpy of solution can be found from lattice energy and hydration enthalpies using Hess's law.
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A Hess cycle links them.
The two changes
- enthalpy of hydration 水合焓 $\Delta H_{\text{hyd}}$: change when one mole of gaseous ions is surrounded by water (exothermic 放热).
- enthalpy of solution $\Delta H_{\text{sol}}$: change when one mole of solute fully dissolves.
Dissolving energy cycle links lattice energy 晶格能, hydration and solution
The enthalpy change of hydration is:
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Water molecules are attracted to the ions, releasing energy, so hydration is exothermic.
The dissolving cycle
To dissolve, you first pull the lattice apart, then hydrate the ions:
- Like lattice energy, $\Delta H_{\text{hyd}}$ is more exothermic for ions of higher charge and smaller radius.
The enthalpy of solution is given by:
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You reverse the lattice energy (to separate the ions) then add the hydration enthalpy.
The enthalpy of hydration is more exothermic for ions with:
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Higher charge and smaller radius pull the water molecules in more strongly, releasing more energy.
Linking the enthalpies
- Enthalpy of solution = (−lattice energy) + enthalpies of hydration of the ions.
- A Hess cycle links the three quantities.
You've got it
- hydration $\Delta H_{\text{hyd}}$ (exothermic): gaseous ions surrounded by water
- solution $\Delta H_{\text{sol}}$: one mole of solute dissolves
- the cycle: $\Delta H_{\text{sol}} = -\Delta H_{\text{latt}} + \Delta H_{\text{hyd}}$
- $\Delta H_{\text{hyd}}$ is more exothermic for higher charge and smaller radius