Enthalpy of solution
The enthalpy of solution or heat of solution is the change in enthalpy during the dissolving of a substance in a solvent . If one disregards volume changes, i.e. mechanical work against the air pressure , the enthalpy is equal to the energy .
The enthalpy of solution can:
- be negative, d. H. the solution becomes warmer: the dissolution process is exothermic, energy is released (e.g. when sodium hydroxide is dissolved in water ). In this case, the solubility decreases when heated;
- be close to zero, d. H. the temperature remains the same (e.g. sodium chloride in water). In this case the solubility is almost independent of the temperature;
- be positive, d. H. the solution cools down: The dissolution process is endothermic, energy is absorbed (e.g. ammonium nitrate in water). In this case, the solubility becomes greater when heated.
The enthalpy of solution is composed of (here using the example of a salt):
- the lattice energy of the substance to be dissolved
- the binding energy of the solvent (e.g. hydrogen bonds between water molecules)
- the solvation energy , d. H. the energy released by the attachment of solvent particles to the particles of the solute
Example of a teaching experiment
The following chemicals are dissolved in three test tubes filled with water at the same temperature :
The following temperature changes can be observed in the test tubes:
- The temperature in the first test tube drops, the dissolution process is endothermic.
- The temperature in the second test tube remains roughly the same, exothermic and endothermic components are roughly the same (in fact, the process is slightly endothermic).
- The temperature in the third test tube rises and the dissolution process is exothermic.
If you take a closer look at the endothermic and exothermic reaction in this experiment, you can see that in this case the endothermic reaction means the destruction of the connections between the anions (negatively charged) and the cations (positively charged). In the first step, the connections are destroyed, that is, the attracting particles are separated from one another. From an energetic point of view, this process is endothermic because it works against the forces of attraction of the particles.
The second of the two sub-processes is hydration . The polar water molecules ( dipoles ) attach to the "still free" anions and cations. From an energetic point of view, this process is exothermic , because the particles attract each other voluntarily due to their charge. The heat of solution is made up of all these processes and reactions.
- Charles E. Mortimer, Ulrich Müller: Chemistry the basic knowledge of chemistry; 126 tables . Georg Thieme Verlag, 2007, ISBN 978-3-13-484309-5 , p. 205 ff . ( limited preview in Google Book search).
- G. Jander, H. Spandau: Short textbook of inorganic and general chemistry . Springer-Verlag, 2013, ISBN 978-3-642-71367-5 , pp. 109 ( limited preview in Google Book search).
- Claus Czeslik, Heiko Seemann, Roland Winter: Basic knowledge of physical chemistry . Springer-Verlag, 2010, ISBN 978-3-8348-0937-7 , pp. 233 ( limited preview in Google Book search).
- Jan Hoinkis: Chemistry for Engineers . John Wiley & Sons, 2015, ISBN 978-3-527-68456-4 , pp. 66 ( limited preview in Google Book search).