Weak acids

In chemistry, weak acids are a subgroup of acids . They are only partially ionized in aqueous solutions . So they are weak electrolytes . The acid strength always refers to the reaction of the acid with water.

Reactivity

When a pure acid is added to water, an acidic solution is formed. This reaction, in which the acid gives up its proton and transfers it to the base, is called protolysis . Weak acids are always only partially protolyzed / ionized in aqueous solution. The reaction equilibrium is on the side of the educts . The following is a common example of an acid HA that protolyzes in water:

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

In the case of a weak acid, this equilibrium is, as already mentioned above, on the left side, the educt side. A positively charged oxonium ion and a negatively charged anion are formed . The anion is the corresponding base to the acid. These pairs are called corresponding acid-base pairs .
The reactivity of the weak acids essentially depends on the corresponding base. In addition, it also depends on the concentration of the oxonium ions.

pK _S and pK _B values

One way to find out how strong an acid is, goes beyond the proto chemical voltage series in which the pK _S - and pK _B values of many 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 degree to which an acid is present protolyzed at the equilibrium reaction with water. The following applies: the higher the value, the weaker the acid / base. Agents Strong acids have a pK _S value of greater than 4.75, weak acids have a pK _S value of greater than 8 and very weak acids a greater than 14. Weak acids are always strong bases.

The following table lists pK _S - and pK _B values of some very strong to very weak acids and their conjugate bases at 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 strong	−10	HClO 4	ClO 4 -	24	very weak
	−10	HI	I -	24
	−6	HCl	Cl -	20th
	−3	H 2 SO 4	HSO 4 -	17th
	−1.32	ENT 3	NO 3 -	15.32
strong	0.00	H 3 O +	H 2 O	14.00	weak
	1.92	HSO 4 -	SO 4 2−	12.08
	2.13	H 3 PO 4	H 2 PO 4 -	11.87
	2.22	[Fe (H 2 O) 6 ] 3+	[Fe (OH) (H 2 O) 5 ] 2+	11.78
	3.14	HF	F -	10.86
	3.75	HCOOH	HCOO -	10.25
medium strength	4.75	CH 3 COOH	CH 3 COO -	9.25	medium strength
	4.85	[Al (H 2 O) 6 ] 3+	[Al (OH) (H 2 O) 5 ] 2+	9.15
	6.52	H 2 CO 3	HCO 3 -	7.48
	6.92	H 2 S	HS -	7.08
	7.20	H 2 PO 4 -	HPO 4 2−	6.80
weak	9.25	NH 4 +	NH 3	4.75	strong
	9.40	HCN	CN -	4.60
	10.40	HCO 3 -	CO 3 2−	3.60
	12.36	HPO 4 2−	PO 4 3−	1.64
	13.00	HS -	S 2−	1.00
	14.00	H 2 O	OH -	0.00
very weak	15.90	CH 3 CH 2 -OH	CH 3 -CH 2 -O -	−1.90	very strong
	23	NH 3	NH 2 -	−9
	48	CH 4	CH 3 -	−34

Weak inorganic acids

The list of weak acids is relatively long compared to the list of strong acids, so only a few examples are given here. The inorganic acids include hydrofluoric acid (HF). In addition, the already deprotonated anions of moderately strong polyprotonic acids are often classified as weak. These include, for example, hydrogen phosphate (HPO ₄ ²⁻ ) and hydrogen carbonate (HCO ₃ ^- ). Furthermore, hydrogen cyanide (HCN) and the ammonium ion (NH ₄ ⁺ ) are weak acids.

Weak organic acids

With organic acids , a distinction is made between different groups of substances . They are classified according to their functional groups . The best known group are the carboxylic acids . These are rather medium in strength. Really weak acids are phenols, alcohols, thiols and CH- and NH-acidic compounds.

Individual evidence

1. ^ ^{A b c} Charles E. Mortimer, Ulrich Müller: Chemistry . Thieme, Stuttgart 2010, ISBN 978-3-13-484310-1 , p. 310 f .
2. ↑ ^{a b} T. L. Brown; HE LeMay; BE Bursten: Chemistry, study compact . Pearson Verlag, Munich 2011, ISBN 978-3-86894-122-7 .
3. ↑ 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.
4. ↑ 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.
5. ^ 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.
6. ↑ 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.
7. ↑ chem.wisc.edu: pKa Data , Compiled by R. Williams (PDF, 78 kB).