# Protolysis

The proteolysis (or proteolytic reaction ) is a chemical reaction in which a proton (H + ion) between two reactants is transmitted . The term protolysis is often wrongly used to describe the splitting off of protons. Protolysis is the decisive process according to the important Brønsted acid-base theory . Then an acid transfers a proton (H + ) to a reaction partner. The compound called acid acts as a proton donor ( proton donor ), the base (often water ) takes up the protons and is therefore called the proton acceptor . A chemical equilibrium is established between the reaction partners .

## Protolytic reactions

If the gas hydrogen chloride (HCl) is introduced into water, hydrochloric acid is formed with protolysis . In this equilibrium reaction , the HCl molecule and the H 3 O + ion are proton donors, i.e. acids according to Brønsted . H 2 O and Cl - act as proton acceptors, so they are bases according to Brønsted .

${\ displaystyle \ mathrm {H_ {2} O \ + \ HCl \ \ rightleftharpoons \ H_ {3} O ^ {+} \ + \ Cl ^ {-}}}$.

If, for example, pure acetic acid (H 3 C – COOH) is added to water, H 3 O + and the acetate anion (H 3 C – COO - ) are formed with protolysis . Here, CH 3 COOH and H 3 O + are proton donors, while H 3 C – COO - and H 2 O are proton acceptors.

${\ displaystyle \ mathrm {H_ {3} C {-} COOH \ + \ H_ {2} O \ \ rightleftharpoons \ H_ {3} C {-} COO ^ {-} \ + \ H_ {3} O ^ { +}}}$

Protolysis of the biprotonic compound sulfuric acid in water:

${\ displaystyle \ mathrm {H_ {2} SO_ {4} \ + \ 2 \ H_ {2} O \ \ rightleftharpoons \ H_ {3} O ^ {+} \ + \ H_ {2} O \ + \ HSO_ { 4} ^ {-} \ rightleftharpoons \ 2 \ H_ {3} O ^ {+} \ + \ SO_ {4} ^ {2-}}}$

In this reaction equation, the molecules H 2 SO 4 and the ion H 3 O + are proton donors, i.e. acids according to Brønsted . H 2 O and SO 4 2− act as proton acceptors, so they are bases according to Brønsted . HSO 4 - , which, depending on the direction of the reaction , can react as a proton acceptor or proton donor, plays a special role . Substances with such properties are called ampholytes .

If the gas ammonia (NH 3 ) is introduced into water, ammonium ions (NH 4 + ) and hydroxide ions (OH - ) are formed. Proton donors here are NH 4 + and H 2 O, while OH - and NH 3 are proton acceptors.

${\ displaystyle \ mathrm {NH_ {3} \ + \ H_ {2} O \; \ rightleftharpoons \ NH_ {4} ^ {+} \ + \ OH ^ {-}}}$

## Autoprotolysis

Pure water is subject to so-called autoprotolysis (also autodissociation ). This creates oxonium ions (H 3 O + ) and hydroxide ions (OH - ). H 2 O can react both as a proton donor (as an acid) or as a proton acceptor (as a base). One therefore speaks of an ampholyte here too .

${\ displaystyle \ mathrm {2 \ H_ {2} O \ \ rightleftharpoons \ H_ {3} O ^ {+} \ + \ OH ^ {-}}}$

The balance is very much on the water side. The ion product for this reaction at 298 K (25 ° C) is about 10 −14  mol 2  l −2 . The autoprotolysis of water is the reason why chemically pure water also has at least a low electrical conductivity . One application of autoprotolysis for electrical charge separation is found in the Kelvin generator when it is operated with chemically pure water.

The autoprotolysis (and thus the pH value) is strongly temperature dependent. The ion products of water are (in mol 2 l −2 ):

temperature K w / (10 −14 M 2 ) pK w neutral pH
0 ° C 0.11 14.94 7.47
10 ° C 0.29 14.53 7.27
20 ° C 0.68 14.17 7.09
25 ° C 1.01 14.00 7.00
30 ° C 1.47 13.83 6.92
40 ° C 2.92 13.53 6.77
50 ° C 5.47 13.26 6.63
60 ° C 9.6 13.02 6.51
70 ° C 16 12.80 6.40
80 ° C 25th 12.60 6.30
90 ° C 37 12.43 6.22
100 ° C 54 12.27 6.14

### Model of the autoprotolysis of water

If the autoprotolysis of water is considered in the following form:

${\ displaystyle \ mathrm {\ H_ {2} O \ \ rightleftharpoons \ H ^ {+} \ + \ OH ^ {-}}}$

For the forward reaction, i.e. the dissociation, formally a reaction of 0th order results. For the reverse reaction, a second order reaction follows formally.

### Autoprotolysis in non-aqueous solutions

In addition to water, other sufficiently polar solvents can also serve as reactants in Brønsted acid-base reactions, for example methanol or ethanol . A good example is the autoprotolysis of liquid ammonia . The ions ammonium and amide are formed.

${\ displaystyle \ mathrm {2 \ NH_ {3} \ \ rightleftharpoons \ NH_ {4} ^ {+} \ + \ NH_ {2} ^ {-}}}$

Ion product = 10 −32

Analogous reactions are also known in concentrated sulfuric acid:

${\ displaystyle \ mathrm {2 \ H_ {2} SO_ {4} \ \ rightleftharpoons \ H_ {3} SO_ {4} ^ {+} \ + \ HSO_ {4} ^ {-}}}$

Ion product = 10 −4

Likewise of hydrogen fluoride:

${\ displaystyle \ mathrm {3 \ HF \ \ rightleftharpoons \ H_ {2} F ^ {+} \ + \ HF_ {2} ^ {-}}}$

Ion product = 10 −10.7 (0 ° C)

(More examples under Ampholyte # Examples of Ampholytes .)