Strong acids

In chemistry, strong acids are a subgroup of acids . They are mostly ionized in aqueous solutions , making them strong electrolytes . The acid strength always relates to the aqueous solutions of the acid, not to the pure substances.

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 . Strong acids are mostly protolyzed / ionized in aqueous solution. The reaction equilibrium is on the side of the products . 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 strong acid, this equilibrium is, as mentioned above, on the right-hand side, the product 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 strong acids essentially depends on the concentration of the oxonium ions formed. In addition, the reactivity also depends on the base of the acid-base pair.

pK _s and pK _b values

One way to find out how strong an acid is is to use the protochemical series in which the pK _s and pK _b values ​​of some acid-base pairs are listed. The pK _b values ​​here denote the base strength and the pK _s values ​​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 smaller the value, the stronger the acid / base. Very strong acids have a pK _s value of less than -0.35. Medium to strong acids have a pK _s value of ± 0.35. Strong acids are always weak bases too. The following table lists the pK _s and pK _b values ​​of some very strong to very weak acids and their corresponding bases under standard conditions. Medium-strength acids and bases are highlighted in light gray, while weak to very weak 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

Strong inorganic acids

The list of strong acids is relatively short compared to the list of weak acids. The inorganic acids are mainly hydrogen halides dissolved in water , primarily hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI). In addition, the oxygen acids sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ) and perchloric acid (HClO ₄ ) are known strong acids.

Strong organic acids

In the case of 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 usually of medium strength. However, if there is an electron withdrawing substituent in the vicinity of the carboxy group , e.g. B. halides, then it is a question of strong acids. For example, the trifluoroacetic acid (CF ₃ COOH) having a pK _s value of 0.23 substantially stronger than the unsubstituted acetic acid (CH ₃ COOH) having a pK _s value of 4.75. In addition, the sulfonic acids , phosphonic acids , and incompletely esterified sulfuric acid esters and phosphoric acid esters are considered strong acids.

Individual evidence

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