Weak bases
In chemistry, weak bases are a subgroup of bases . They are only partially ionized in aqueous solutions . So they are weak electrolytes . The base strength always refers to the aqueous solutions of the base, not to the pure substances.
Reactivity
When a pure base is added to water, an alkaline solution is formed. This reaction, in which the base picks up a proton from the acid, is called protolysis . Weak bases are always only partially protolyzed / ionized in aqueous solution. The reaction equilibrium is on the side of the educts . The following is a general example of a base B that protolyzes in water:
In the case of a weak base, as already mentioned above, this equilibrium is on the left-hand side, the educt side. A negatively charged hydroxide ion and a positively charged cation are formed . The cation is the corresponding acid to the base. These pairs are called corresponding acid-base pairs .
The reactivity of the weak bases depends essentially on the acid formed. In addition, the reactivity also depends on the concentration of the hydroxide ions.
pK s and pK b values
One way to find out how strong a base is is to use the protochemical series in which the pK b and pK s values of some acid-base pairs are listed. The pK b values denote the base strength and the pK s values denote the acid strength. The pK s and pK b values indicate the extent to which a base is ionized in the equilibrium reaction with water. The following applies: the higher the value, the weaker the acid / base. Weak bases have a pK b value greater than 8. Medium-strength bases have a pK b value greater than 4.75 and very weak bases have a pK b value greater than 14. Weak bases are always strong acids. The following table lists the pK s and pK b values of some very strong to very weak bases and their corresponding acids under standard conditions. Medium-strength acids and bases are highlighted in light gray, while strong to very strong acids and bases are highlighted in dark gray. They are only listed for comparison:
Acid starch | pK s | Acid + H 2 O H 3 O + + base | pK b | Base strength | |
---|---|---|---|---|---|
very weak | 48 | CH 4 | CH 3 - | −34 | very strong |
23 | NH 3 | NH 2 - | −9 | ||
15.90 | CH 3 CH 2 -OH | CH 3 -CH 2 -O - | −1.90 | ||
weak | 14.00 | H 2 O | OH - | 0.00 | strong |
13.00 | HS - | S 2− | 1.00 | ||
12.36 | HPO 4 2− | PO 4 3− | 1.64 | ||
10.40 | HCO 3 - | CO 3 2− | 3.60 | ||
9.40 | HCN | CN - | 4.60 | ||
9.25 | NH 4 + | NH 3 | 4.75 | ||
medium strength | 7.20 | H 2 PO 4 - | HPO 4 2− | 6.80 | medium strength |
6.92 | H 2 S | HS - | 7.08 | ||
6.52 | H 2 CO 3 | HCO 3 - | 7.48 | ||
4.85 | [Al (H 2 O) 6 ] 3+ | [Al (OH) (H 2 O) 5 ] 2+ | 9.15 | ||
4.75 | CH 3 COOH | CH 3 COO - | 9.25 | ||
strong | 3.75 | HCOOH | HCOO - | 10.25 | weak |
3.14 | HF | F - | 10.86 | ||
2.22 | [Fe (H 2 O) 6 ] 3+ | [Fe (OH) (H 2 O) 5 ] 2+ | 11.78 | ||
2.13 | H 3 PO 4 | H 2 PO 4 - | 11.87 | ||
1.92 | HSO 4 - | SO 4 2− | 12.08 | ||
0.00 | H 3 O + | H 2 O | 14.00 | ||
very strong | −1.32 | ENT 3 | NO 3 - | 15.32 | very weak |
−3 | H 2 SO 4 | HSO 4 - | 17th | ||
−6 | HCl | Cl - | 20th | ||
−10 | HI | I - | 24 | ||
−10 | HClO 4 | ClO 4 - | 24 |
Weak inorganic bases
The weak base list is longer compared to the strong base list. The inorganic bases are mainly halides dissolved in water , for example chlorides (Cl - ) and fluorides (F - ), as well as some oxygen compounds such as sulfates (SO 4 2− ) and nitrates (NO 3 - ).
Weak organic bases
In the organic bases , there are many weak bases. Actually, all known organic bases, with a few exceptions, are weak bases. The best known examples of weak bases are the nucleic bases adenine , cytosine , thymine and guanine .
Individual evidence
- ^ A b c Charles E. Mortimer, Ulrich Müller: Chemistry . Thieme, Stuttgart 2010, ISBN 978-3-13-484310-1 , p. 310 f .
- ↑ a b T. L. Brown; HE LeMay; BE Bursten: Chemistry, study compact . Pearson Verlag, Munich 2011, ISBN 978-3-86894-122-7 .
- ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 245.
- ↑ Gerhart Jander , Karl Friedrich year, Gerhard Schulze, Jürgen Simon (eds.): Measure analysis. Theory and practice of titrations with chemical and physical indications. 16th edition. Walter de Gruyter, Berlin a. a. 2003, ISBN 3-11-017098-1 , p. 81.
- ↑ PW Atkins, TL Overton, JP Rourke, MT Weller, FA Armstrong: Shriver & Atkins' inorganic chemistry. 5th edition. Oxford University Press, Oxford New York 2010, ISBN 978-0-19-923617-6 , p. 115.
- ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 , p. 241.
- ↑ Jerry March : Advanced Organic Chemistry. Reactions, Mechanisms, and Structure. 3. Edition. Wiley, New York NY et al. a. 1985, ISBN 0-471-88841-9 , p. 222.
- ↑ List of some pKb values .