Enthalpy change of solution

The enthalpy of solution or solution heat or cold solution is the enthalpy change when dissolving a substance in a solvent. The enthalpy is - apart from volume changes, ie mechanical work against the air pressure - equal to the energy.

The enthalpy of solution can:

• Be negative, ie the solution is warmer: The dissolution process is exothermic, energy is released (for example, when solving sodium hydroxide in water). In this case, the solubility decreases on heating;
• Be substantially zero, i.e., the temperature remains constant (eg, sodium chloride in water). In this case, the solubility is substantially independent of the temperature;
• Be positive, indicating that the solution cools down: The dissolution process is endothermic, energy is absorbed (eg ammonium nitrate in water). In this case, the solubility on heating increases.

The enthalpy of solution is made (example of a salt) composed of:

• The lattice energy of the solute
• Of the binding energy of the solvent
• The solvation energy, ie the energy of the particles of the solute is released during the addition of solvent particles

The solubility of a substance in a solvent is determined in addition to the enthalpy of solution by the Lösungsentropie.

Example of a teaching experiment

Three test tubes filled with equal tempered water following chemicals to be solved:

The following temperature changes are observed in the test tubes:

If the endothermic and the exothermic reaction in this experiment we look in more detail, it can be seen that in this case the endothermic reaction is the destruction of the connections between the anions ( negatively charged) and cations ( positively charged). In the first step, that is, the compounds are destroyed, i.e., separated from each other, the mutually attracting particles. Energetically this process is endothermic since you are working against the forces of attraction of the particles.

The second of the two processes is the hydration. Here, the polar water molecules ( dipoles ) accumulate on the " still free " anions and cations. Energetically this process is exothermic, because the particles attract each voluntarily due to their charge. For all these processes and reactions, the heat of solution sets together.