Enthalpies and Dissolving

Enthalpy is a state function that measures the heat content in a chemical system at a constant pressure.
The change in enthalpy is represented by ΔH.
Exothermic reactions have a negative ΔH, as heat is released, while endothermic reactions have a positive ΔH as heat is absorbed.
Enthalpy is typically reported in Joules or kilojoules per mole of a sample.

Dissolving involves breaking and forming definite bonds, and these processes modify the system’s enthalpy.
The enthalpy of solution, ΔHsoln, is the energy change associated with the process of dissolving.
It’s the sum of the enthalpy of hydration (energy given out when ions are surrounded by water molecules) and the lattice enthalpy (energy needed to break the lattice).
The dissolution process could be either exothermic or endothermic depending upon the net effect.

The enthalpy of solution plays a significant role in determining how much of a solute can dissolve in seawater.
Endothermic dissolving (positive ΔHsoln) occurs when the energy required to break the ionic bonds in the solute exceeds the energy released during hydration.
Exothermic dissolving (negative ΔHsoln) happens when the energy released during hydration exceeds the energy required to break the solute’s ionic bonds.
Examples of substances that dissolve endothermically in water include ammonium nitrate and potassium chloride, while substances that dissolve exothermically include sodium hydroxide and sodium sulphate.
Sea water, due to its salinity, has distinct interactions in regards to enthalpy changes during the dissolving process compared to fresh water. An understanding of these processes helps us in predicting how ocean currents behave.