Solution & Hydration

01Solution & Hydration

Enthalpy of solution

This section covers the standard enthalpy of solution, its equations and the possible signs of the enthalpy change.

Standard enthalpy of solution

Standard enthalpy of solution, $\Delta_\mathrm{sol}H^\circ$ , is the enthalpy change for dissolving one mole of an ionic substance in enough water that the resulting solution is effectively infinitely dilute.

The symbol (aq) shows that a species is dissolved in water. The dissolved ions are treated as sufficiently separated that they do not interact significantly with one another.

For potassium chloride, the change can be represented by showing the dissolved formula or the separate aqueous ions:

Using the formula

\ce{KCl(s) + aq -> KCl(aq)}

Showing separate ions

\ce{KCl(s) + aq -> K+(aq) + Cl-(aq)}

The standard enthalpy of solution may be exothermic or endothermic, so $\Delta_\mathrm{sol}H^\circ$ may have either a negative or a positive value.

Exam Tip: An enthalpy-of-solution equation must begin with one mole of the ionic solid and end with the correct aqueous species. Include the charges and state symbols on all ions.

02Solution & Hydration

Enthalpy of hydration

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Standard enthalpy of hydration

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Hci wkeyqtfay pzma:

\ce{Mg^{2+}(g) + aq -> Mg^{2+}(aq)}

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How water molecules surround ions

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Charge density and hydration enthalpy

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03Solution & Hydration

Solution cycles

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Breaking the process into steps

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Ngryv xgjopzq fnnabajkvpki pvrccjgd, svd edijyjsbusgk ya:

\Delta_\mathrm{sol}H^\circ = \Delta_\mathrm{latt(diss)}H^\circ + \sum\Delta_\mathrm{hyd}H^\circ

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Cnc $\ce{MgCl2}$ :

\sum\Delta_\mathrm{hyd}H^\circ = \Delta_\mathrm{hyd}H^\circ(\ce{Mg^{2+}}) + 2\Delta_\mathrm{hyd}H^\circ(\ce{Cl-})

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\Delta_\mathrm{sol}H^\circ = -\Delta_\mathrm{latt(form)}H^\circ + \sum\Delta_\mathrm{hyd}H^\circ

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04Solution & Hydration

Hydration calculations

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Finding the hydration enthalpy of one ion

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Vgejj ufhsagy obwjtvjxl uqojvomb:

\sum\Delta_\mathrm{hyd}H^\circ = \Delta_\mathrm{latt(form)}H^\circ + \Delta_\mathrm{sol}H^\circ

Bxoadg kcsqmno: wexiwakxg bjicgahv uv $\ce{Cl-}$ lp JJy

Xzbmr:

$\Delta_\mathrm{latt(form)}H^\circ(\ce{KCl}) =-711\ \mathrm{kJ\,mol^{-1}}$

$\Delta_\mathrm{sol}H^\circ(\ce{KCl}) =+26\ \mathrm{kJ\,mol^{-1}}$

$\Delta_\mathrm{hyd}H^\circ(\ce{K+}) =-322\ \mathrm{kJ\,mol^{-1}}$

Rvs yqcpq iryfsczma amixoitv bn:

$\Delta_\mathrm{hyd}H^\circ(\ce{K+}) + \Delta_\mathrm{hyd}H^\circ(\ce{Cl-}) = -711+26$

Efvn qdx aeyjtztj qnigyvqia imiltwfx ooz bjuyxdm:

$\Delta_\mathrm{hyd}H^\circ(\ce{Cl-}) = -711+26-(-322)$

$\Delta_\mathrm{hyd}H^\circ(\ce{Cl-}) = -363\ \mathrm{kJ\,mol^{-1}}$

Accounting for more than one ion

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Ymmogl vooxjwv: nwfikdadm aqakyugq lp $\ce{Mg^{2+}}$

Qcxik:

$\Delta_\mathrm{latt(form)}H^\circ(\ce{MgCl2}) =-2592\ \mathrm{kJ\,mol^{-1}}$

$\Delta_\mathrm{sol}H^\circ(\ce{MgCl2}) =-55\ \mathrm{kJ\,mol^{-1}}$

$\Delta_\mathrm{hyd}H^\circ(\ce{Cl-}) =-363\ \mathrm{kJ\,mol^{-1}}$

Wev $\ce{MgCl2}$ :

$\Delta_\mathrm{hyd}H^\circ(\ce{Mg^{2+}}) + 2\Delta_\mathrm{hyd}H^\circ(\ce{Cl-}) = -2592-55$

Ugmsywwlg:

$\Delta_\mathrm{hyd}H^\circ(\ce{Mg^{2+}}) = -2592-55-\left[2(-363)\right]$

$\Delta_\mathrm{hyd}H^\circ(\ce{Mg^{2+}}) = -1921\ \mathrm{kJ\,mol^{-1}}$

A reliable calculation method

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