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:

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

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 ${\ displaystyle \ \ rightleftharpoons \}$		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 ₄²⁻	PO ₄³⁻	1.64
	10.40	HCO ₃^-	CO ₃²⁻	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 ₄²⁻	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 ₄²⁻	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 ₄²⁻ ) 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 .

[Mortimer-1] A ^b ^c Charles E. Mortimer, Ulrich Müller: Chemistry . Thieme, Stuttgart 2010, ISBN 978-3-13-484310-1 , p. 310 f .

[Pearson-2] T. L. Brown; HE LeMay; BE Bursten: Chemistry, study compact . Pearson Verlag, Munich 2011, ISBN 978-3-86894-122-7 .

[Hollemann-3] 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.

[4] 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.

[5] 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.

[6] 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.

[March-7] 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.

[pkb-8] List of some pKb values .