Nature of acidity and structural features
Historically, chemists believed that oxygen was an obligatory component of any acid (hence the name of the element — "oxygen" meaning "acid-former" in Greek). Chlorine was not recognized as an independent element for a long time and was considered an oxide of a hypothetical "murium" to justify the presence of oxygen in hydrochloric acid ($HCl$).
Today it has been proven that in oxygen-containing acids, the mobility of the proton is due to a specific structure. Hydrogen bonded to oxygen becomes "acidic" only if the molecule has another oxygen atom attached to the central atom by a double bond. This second oxygen intensively pulls electron density toward itself, weakening the $O-H$ bond and allowing the hydrogen to detach easily. This is precisely why ordinary water ($H_2O$), which lacks such an additional doubly bonded atom, exhibits only negligibly weak acidity.
Classification and physical properties
In terms of their physical state, most acids are liquids or gases. However, crystalline acids also exist (a characteristic example is citric acid). In practice, they are most often used in the form of aqueous solutions, while concentrated ("strong") forms require storage in hermetically sealed containers due to volatility.
According to their composition, acids are divided into four main groups:
- Hydracids (binary): hydrogen is directly bonded to halogen atoms (fluorine, chlorine, bromine, iodine) or sulfur.
- Oxoacids (oxygen-containing): hydrogen is bonded to an oxygen atom.
- Organic acids: possess a carbon skeleton.
- Complex acids.
An important classification criterion is basicity — the number of hydrogen atoms capable of being replaced by a metal. Acids can be monoprotic, diprotic, and triprotic. In formulas of organic substances (e.g., acetic acid $CH_3COOH$), these mobile atoms are traditionally written at the very end of the molecule.
Chemical properties: substitution and exchange
The presence of mobile hydrogen determines the ability of acids to participate in two key groups of reactions:
- Substitution reactions: interaction with metals, accompanied by gas evolution ($Zn + H_2SO_4 = ZnSO_4 + H_2\uparrow$).
- Exchange (neutralization) reactions: interaction with metal hydroxides or oxides to form water ($H_2SO_4 + MgO = MgSO_4 + H_2O$).
The products of such reactions are always salts — substances in which hydrogen is replaced by a metal. At the same time, classical molecular acids are separated into an independent class precisely because they play a huge role in aqueous solutions and form their own extensive families of salts (sulfates, chlorides, nitrates, etc.).
Acid strength and displacement rules
Acid strength reflects the rate and intensity of its chemical reactions. Strong acids (e.g., hydroiodic, hydrobromic, hydrochloric, sulfuric, nitric) react extremely vigorously, whereas weak acids (carbonic, hydrosulfuric, boric) react slowly.
In exchange reactions with salts, a strict displacement rule applies: a stronger acid always displaces a weaker one from its salt. Volatility plays a key role here. Upon heating,
Examples of the rule in action:
- If strong non-volatile sulfuric acid acts on sodium acetate, weak volatile acetic acid will be released (a characteristic odor will appear).
- The reverse reaction is impossible: acetic acid will never displace sulfuric acid from sulfates.
Extended understanding of acidity
With the development of chemistry, the concept of an "acid" went beyond substances that simply contain hydrogen. In essence, an acid is any substance that creates an acidic medium.
Solutions of certain salts (e.g., aluminum sulfate $Al_2(SO_4)_3$ or potassium dihydrogen phosphate $KH_2PO_4$) turn litmus red, demonstrating an acidic reaction, although they have an ionic structure or contain no hydrogen at all. Non-metal oxides also exhibit acidic properties. For example, sulfur trioxide ($SO_3$) reacts with bases in exactly the same way as sulfuric acid. Since general exchange reactions always proceed toward the formation of weaker acids (and $SO_3$ forms sulfuric acid upon addition of water), the $SO_3$ oxide itself can be classified as a stronger acid than $H_2SO_4$, even though it does not form its own "family" of salts.
Briefly: answers to topical questions
How does the strength of binary hydrogen halide acids change in the series from fluorine to iodine?
In the series from fluorine to iodine, an aqueous solution of HF is a weak acid; aqueous solutions of HCl, HBr, and HI are strong acids. For HF at a concentration of 0.1 mol/L, the degree of dissociation is low.
To remember the rule for displacing acids from salts, imagine wrestlers in a ring: "Heavy and non-volatile expels light and volatile." Sulfuric acid will easily displace hydrochloric or acetic acid from a salt, but not vice versa.
Class and exam questions
Does carbonic acid exist as an independent substance?
No, it is extremely unstable. In ordinary carbonated water, the equilibrium is strongly shifted: 99% exists as dissolved gas ($CO_2$) and water, and only 1% of molecules exist as $H_2CO_3$.
Why does water not exhibit pronounced acidic properties even though it contains oxygen and hydrogen?
The water molecule lacks an additional doubly bonded oxygen atom that could pull electron density toward itself and make the hydrogen atoms mobile enough for cleavage.
Can substances without hydrogen atoms be acids?
Yes, in a broad sense, many non-metallic oxides (e.g., $SO_3$) exhibit acidic properties, as well as solutions of certain salts that do not contain hydrogen (e.g., $Al_2(SO_4)_3$), since they are capable of creating an acidic medium in solution.
What else to study on this topic
- Nomenclature of acid residues and salts of organic and inorganic acids (citrates, borates, acetates).
- Use of indicators to demonstrate neutralization reactions.
- Dependence of acid strength on their position in the reactivity series.
- Specific physicochemical properties of individual acids (volatility, maximum concentration in solutions).
- Comparison of acid and base properties in a historical context.
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Other topics in section «Chapter 3. Classification of Substances»
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