It is important to be able to identify strong acids from weak ones, both for academic purposes and when we use them in the laboratory. Strong acids are very rare, so one of the easiest ways to tell them apart from weak ones is to memorize what they are. Any other acid not listed will be a weak acid.
Key aspects of strong acids and weak acids
- Strong acids completely dissociate in aqueous solutions, so that all their molecules lose at least one proton (H + ) that forms a hydronium ion (H 3 O + ) with the water molecule, while weak acids only dissociate partially.
- The strong acids are very few, there are only seven strong inorganic acids, so to easily differentiate them from the weak ones, they can be memorized; if it is not listed, it will be a weak acid.
- Strong acids are hydrochloric acid (HCl), nitric acid (HNO3 ) , sulfuric acid (H2SO4 ) , hydrobromic acid ( HBr ) , hydroiodic acid (HI), perchloric acid ( HClO4 ) and chloric acid (HClO 3 ).
- The only weak acid formed from the reaction of a halogen element is hydrofluoric acid (HF). While technically a weak acid, hydrofluoric acid is highly corrosive.
strong acids
Strong acids completely dissociate in aqueous solutions, each molecule releasing at least one proton (H + cation ) which forms a hydronium ion (H 3 O + ) with the water molecule. The most common inorganic strong acids are seven.
- Hydrochloric acid (HCl)
- Nitric acid (HNO 3 )
- Sulfuric acid (H 2 SO 4 )
- Hydrobromic acid (HBr)
- Hydroiodic acid (HI)
- Perchloric acid (HClO 4 )
- Chloric acid (HClO 3 )
The following examples are typical ionization reactions of strong acids in aqueous solutions.
HCl → H + + Cl –
HNO 3 → H + + NO 3 –
H 2 SO 4 → 2H + + SO 4 –
It is necessary to make it clear that in these reactions the hydrogen ions have a positive charge, they are cations, and that the direction of the reaction is only towards the products, which indicates that all the reactant molecules dissociate.
weak acids
Weak acids do not completely dissociate in aqueous solutions; that is, a certain number of reactant molecules remain with their original composition. This is the case of hydrofluoric acid (HF). There are much more weak acids than strong ones. Most organic acids are weak, with a few exceptions such as p-toluenesulfonic acid, which dissociates quite a bit, but not completely. Some weak acids are listed below in order of decreasing acidity.
- HO 2 C 2 O 2 H – oxalic acid
- H 2 SO 3 – sulfurous acid
- HSO 4 – – hydrogen sulfate ion
- H 3 PO 4 – phosphoric acid
- HNO 2 – nitrous acid
- HF – hydrofluoric acid
- HCO 2 H – methanoic acid
- C 6 H 5 COOH – benzoic acid
- CH 3 COOH – acetic acid
- HCOOH – formic acid
An example of a weak acid ionization reaction is acetic acid, which generates hydronium cations and acetate anions.
CH 3 COOH + H 2 O ⇆ H 3 O + + CH 3 COO –
It is noteworthy that in this case, unlike strong acids, the reaction occurs in both directions, as indicated by the double arrow. In this case, because the reverse reaction is more likely, only about 1% of the acetic acid molecules dissociate, while the rest remain in their original composition.
How to distinguish between strong acids and weak acids
The value of the equilibrium constant of the dissociation reaction indicates whether an acid is strong or weak . That is, the acid dissociation constant K a , is the equilibrium constant of the acid dissociation reaction in an aqueous medium (the logarithmic parameter is also used, pK a =-logK a ). The value of K a is large for strong acids (and that of pK a is small); in weak acids the value of K a is small (and that of pK a is large).
Strong or weak relative to concentrated or diluted
Care must be taken not to confuse the terms strong and weak with concentrated and diluted. A concentrated acid has a large amount of acid in the aqueous solution; a diluted one has little amount of acid. For example, if you have 12M acetic acid (12 moles per liter concentration) of an acid, you have a strong solution, but the acid is still weak. A 0.0005 M hydrochloric acid solution (0.0005 mol per liter concentration) is a dilute solution, but the acid is still strong.
Difference Between Strong and Corrosive
It is possible to drink dilute acetic acid (which in fact vinegar is), but drinking the same concentration of sulfuric acid would produce a tremendous chemical burn. The term corrosive (in this case it is sulfuric acid) refers to the damage that the acid generates in the material with which it comes into contact, while being strong or weak is a characteristic of the acid. Although acids are often corrosive, some carboranes are very strong acids, much stronger than sulfuric acid, but can be held in the hand without harming the skin, while hydrofluoric acid, even a weak acid, would destroy tissue. hand in hand with minimal contact.
Sources
- Housecroft, CE Inorganic Chemistry . (second edition) Prentice Hall. Sharpe, AG (2004). ISBN 978-0-13-039913-7.
- Porterfield, William W. Inorganic Chemistry. (1984). Addison-Wesley. ISBN 0-201-05660-7.
- Trummal, A., Lipping, L., et al. Acidity of strong acids in water and dimethyl sulfoxide . J. Phys. Chem. A. 120(20) (2016) 3663–3669. doi:10.1021/acs.jpca.6b02253
