Quick Notes Enthalpy of Solution

  • When an ionic compound dissolves in water, an enthalpy change will occur and is called enthalpy of solution
    • Energy is required to break apart the solid lattice into free ions and energy is released when these ions are 'hydrated' with water molecules.
      • If the energy required to break apart the lattice is greater than the energy released by hydration, enthalpy of solution is endothermic (+ΔH).
      • If the energy required to break apart the lattice is smaller than the energy released by hydration, enthalpy of solution is exothermic (-ΔH).
  • Enthalpy of solution is the enthalpy change that occurs when 1 moles worth of an ionic lattice dissolves completely in water (to form an infinitely dilute solution).
  • Enthalpy of hydration is the enthalpy change that occurs when 1 moles worth of gaseous ions dissolve completely in water (to form an infinitely dilute solution).
    • Enthalpy of hydration is always an exothermic process (-ΔH)
  • Lattice dissociation enthalpy is the enthalpy change that occurs when 1 moles worth of an ionic compound is converted into its constituent, gaseous ions.
    • Lattice dissociation enthalpy is always an endothermic process (+ΔH)
    • Lattice dissociation enthalpy is the opposite of lattice enthalpy (lattice enthalpy has same value, but is exothermic (-ΔH))
  • Can use born-harber cycles with enthalpies of solution to find lattice enthalpies indirectly.

Full Notes Enthalpy of Solution

When an ionic substance dissolves in water, an energy change will occur. For some compounds, the process is exothermic (energy is released) and for others, it is endothermic (energy is absorbed).

We can explain these energy changes by considering the dissolving as a two stage process.

Firstly, the forces holding the structure together are broken to free the ions in the solid before new forces are formed between the ions and water molecules.

To break apart the solid structure being dissolved, energy is required (positive enthalpy change).

breaking apart ionic lattice endothermic process enthalpy of hydration

The forming of new forces between ions and water molecules releases energy (negative enthalpy change). This is referred to as ‘hydration’.

ions dissolving in water enthalpy of hydration

When a solid actually dissolves, the above two steps happen, essentially, at the same time and it is not possible to separate them into two stages.

Instead, we simply measure the overall enthalpy change that occurs and this is called ‘enthalpy of solution’.

Standard enthalpy of solution refers to the enthalpy change that occurs when 1 mole of solute is dissolved completely in water, to form an infinitely dilute solution. Measured at standard conditions.

Note – infinitely dilute means that the addition of any more water molecules would have no impact on the enthalpy change that is occurring.

Using Enthalpies of Solution

Using Hess’ Law (see Hess’ Cycles), we can construct a simple Born-Harber cycle for the dissolving of an ionic compound by using two possible ‘routes’. The Born-Harber cycle can be used to find the lattice enthalpy of the compound*.

born-harber cycle for enthalpy of hydration of sodium chloride

Route 1:

Dissolve the ionic compound in water (enthalpy of solution).

NaCl(s) → Na+(aq) + Cl-(aq)

dissolving sodium chloride enthalpy of solution born-harber cycle

Route 2:

By breaking apart an ionic lattice (lattice dissociation – see Lattice Enthalpies), and releasing free, gaseous ions. These gaseous ions can then be mixed with water and the enthalpy change that occurs measured – ‘enthalpy of hydration’.

lattice enthalpy and enthalpy of hydration born-harber cycle sodium chloride

NaCl(s) → Na+(g) + Cl-(g)

Na+(g) + Cl-(g) → Na+(aq) + Cl-(aq)

The ‘start’ and ‘end’ of both route 1 and 2 are the same.

enthalpy of solution and lattice dissociation enthalpy and enthalpy of hydration

Meaning...

enthalpy of hydration and lattice dissociation enthalpy and enthalpy of hydration

Meaning...

lattice enthalpy using enthalpy of solution and hydration

*see below

If more energy is required to break apart the lattice than is released when the ions are hydrated, the enthalpy of solution will be endothermic.

If less energy is required to break apart the lattice than is released when the ions are hydrated, the enthalpy of solution will be exothermic.

*Lattice enthalpies are defined as the enthalpy change that occurs when one moles worth of ionic lattice is formed from its constituent gaseous ions. When using enthalpies of solution, we are really referring to the lattice dissociation enthalpy – this is the enthalpy change that occurs when one moles worth of an ionic lattice is converted into its constituent, gaseous ions.

Lattice dissociation enthalpies and lattice enthalpies for a particular compound have the same value, they just have opposite enthalpy changes. Lattice enthalpies are always exothermic (negative enthalpy change, -ΔH) and Lattice dissociation enthalpies are always endothermic (positive enthalpy change, +ΔH).