Solution tension

The solution tension or the solution pressure describes the tendency of substances to go into solution. The solution pressure becomes noticeable as an osmotic pressure and can thus be determined experimentally.

Walther Nernst introduced the term "electrolytic solution tension" based on this in 1889; Wilhelm Ostwald called this quantity "electrolytic solution pressure". The electrolytic solution pressure characterizes the tendency of an element to form ions and to be dissolved. Accordingly, the pressure to dissolve the base metal zinc in contact with water leads to the fact that zinc ions are dissolved in the water.

application

The less noble a metal is, i. H. the smaller or more negative its value in the electrochemical series , the greater its solution pressure . The more noble a metal is, i. H. the greater or more positive its value in the electrochemical series , the lower its solution pressure . If you put a metal in water , metal ions go into solution due to the dissolution tension and the metal becomes negatively charged. The solution tension that is characteristic of each metal depends on the energy gained during the formation of a metal ion, i.e. on the difference between the hydration energy and the lattice energy . The greater the difference between the hydration energy and the lattice energy, the greater the solution pressure. The smaller the difference between the hydration energy and the lattice energy, the smaller the solution pressure. In addition, the solution tension of a metal depends on how many of its ions are already present in the solution.

If you put strips of two different metals in water, you can measure the different solution tension with a very high-resistance voltmeter as the potential difference . A corresponding arrangement of different metals that can serve as a DC voltage source is called a galvanic cell . In particular when several such cells are connected in series, one speaks of a battery , e.g. B. the voltaic column .

values

The solution tension is the concentration of cations for which the electrode potential becomes zero. Since the activity of the solid metal can be taken as one, the Nernst equation is ${\displaystyle C}$

${\displaystyle E=E^{\circ }+{\frac {RT}{z_{e}F}}\ln {C}=0}$.

From this it follows for the relationship between solution tension and standard potential : ${\displaystyle C}$ ${\displaystyle E^{\circ }}$

${\displaystyle \ln C=-{\frac {z_{e}FE^{\circ }}{RT}}}$.

Under standard conditions (25 ° C and 1 atm) the following applies with the value of the electrode slope at 25 ° C (see):

${\displaystyle \log C=-{\frac {z_{e}E^{\circ }}{0{,}05916\mathrm {V} }}}$.

Equilibrium reaction	Standard potential ${\displaystyle {E^{\circ }}}$	${\displaystyle {\log {C}}}$	Solution tension ${\displaystyle {C}}$	comment
Zn Zn 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	−0.7622 V	25.8	5.85 x 10 25 mol / l	The calculated values ​​for are not meaningful concentrations here, since they exceed the solubility of the corresponding salts in water and even the appropriate amount of salt in one liter. Severe corrosion of the metal can occur. ${\displaystyle {C}}$
Fe Fe 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	−0.44 V	14.9	7.50 x 10 14 mol / l
Cd Cd 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	−0.4021 V	13.6	3.92 x 10 13 mol / l
Co Co 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	−0.283 V	9.6	3.69 x 10 09 mol / l
Pb Pb 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	−0.1263 V	4.3	1.86 x 10 04 mol / l
Cu Cu 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	0.345V	−11.7	2.17 · 10 −12 mol / l	The only concentration that can actually be achieved. Accordingly, it is expected that copper will dissolve slightly.
Hg Hg 2+ + 2 e - ${\displaystyle \rightleftharpoons }$	0.861 V	−29.1	7.81 · 10 −30 mol / l	The calculated very small value of is not a meaningful Hg 2+ concentration as it does not even correspond to one Hg 2+ ion in one cubic meter of water. The metal is so precious that it does not dissolve. ${\displaystyle {C}}$

The solution tension is an equilibrium concentration. The only realistic numerical value for a concentration is that for copper. The calculated solution tensions of the base metals and given in the table exceed the solubility of the salts. The calculated concentrations are therefore not sensible, also because the assumed standard potential no longer applies.

Like the voltage series, the term solution pressure describes the tendency to dissolve and react quantitatively, but not by specifying the potential, but by specifying the concentration. As can be seen from the table and the minus sign in the equations, a low or high negative standard potential corresponds to large values ​​of the solution pressure. In contrast to the osmotic pressure, the solution pressure does not lead to an effect that can be measured as pressure.

See also

• Electromotive force

Web links

• Declaration on Hamm's chemistry page (PDF file; 15 kB)

Individual evidence

1. ^ ^{A b} Max Le Blanc: Textbook of Electrochemistry . Publishing house by Oskar Leiner, Leipzig 1896, p. 117 ( Textbook of Electrochemistry. Online at openlibrary.org [accessed September 27, 2014] pages 116 and 117 ).
2. ^ ^{A b} Gustav Kortüm: Textbook of Electrochemistry . Verlag Chemie GmbH, Weinheim / Bergstr. 1952, p. 238-239 .
3. ↑ Max Le Blanc: Textbook of Electrochemistry . Publishing house by Oskar Leiner, Leipzig 1896, p. 118 ( Textbook of Electrochemistry. Online at openlibrary.org [accessed September 28, 2014] pages 118 and 119 ).