Acetic acid is an organic compound belonging to the carboxylic acid group. Its formula is CH₃COOH where a methyl group is attached to a carboxyl functional group. Its IUPAC name is ethanoic acid. Acetic acid is the main constituent of vinegar and is known since ancient times. Vinegar solution contains acetic acid and water where ethanoic acid is between 5% to 20% by volume. The acetic acid in vinegar is responsible for its pungent smell and the sour taste. It also has various other important uses. Here we will also learn about the physical and chemical properties of acetic acid.
Ethanoic acid is the second simplest carboxylic acid. Here is the structure of acetic acid.
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The structure of ethanoic acid is given by CH₃COOH.
In the solid-state of acetic acid, there is a chain of molecules with individual molecules connected through hydrogen bonds.
In its vapour state, its dimers are found at a temperature of about 120°c.
When in the liquid phase, its dimers are found when present in a dilute solution.
By calculation, the molar mass of acetic acid comes out to be 60.05 g/mol. It can be calculated as:
(2 × 12.011) + (4 × 1.00794) + (2×15.999) g/mol
= 60.05 g/mol
Some important physical properties of acetic acid are listed below.
Ethanoic or acetic acid has a pungent vinegar odour and sour taste.
It is a colourless liquid.
It boils at 391K.
Its density in liquid form is 1.049 g/cm³.
It can mix with water, alcohol, ether in all proportions.
In water, it dissolves with the evolution of heat and contraction in volume.
It is corrosive in nature and produces blisters when in contact with the skin.
Sulphur, iodine, and many other organic compounds are dissolved in it.
The melting point of acetic acid is 289.5K or 16.5°C.
On cooling below this temperature, it forms ice-like crystals. Therefore, pure and anhydrous acid is usually referred to as glacial acetic acid where glacial means ice-like.
Some important chemical properties of acetic acid are given below:
The carboxyl functional group in ethanoic acid causes ionization of the compound, given by the reaction: CH₃COOH ⇌ CH₃COO⁻ + H⁺
The acidic quality of acetic acid comes from the release of the proton, described by the equilibrium reaction above.
In a solution of water, the acid dissociation constant (pKa) of ethanoic acid 4.76.
CH₃COO⁻, acetate is the conjugate base of acetic acid.
Acetic acid doesn't dissociate completely as it can be seen that the pH of an ethanoic acid solution of 1.0M concentration is 2.4.
Acetic acid is a polar, protic solvent, with a dielectric constant of 6.2 in its liquid form.
Acetic acid undergoes nearly all carboxylic acid reactions.
Acetic acid undergoes decomposition when heated above 440°C to yield either methane and carbon dioxide or water and ethanone, given by the equations.
CH₃COOH + Heat → CO + CH4
CH₃COOH + Heat → H2C=C=O + H2O
When exposed to acetic acid, some metals such as magnesium, iron, and zinc undergoes corrosion. This reaction leads to the formation of acetate salts. When magnesium reacts with acetic acid, it gives magnesium acetate and hydrogen given in the equation.
2CH₃COOH + Mg → Mg(CH₃COO)2 (magnesium acetate) + H2
Acetate salts are formed when acetic acid reacts with alkalis as described in the equation.
CH₃COOH + KOH → CH₃COOK + H2O
Compounds formed on reaction with carbonates are acetate salts, water and carbon dioxide.
2CH₃COOH + Na2CO3 (sodium carbonate) → 2CH₃COONa + CO2 + H2O
CH₃COOH + NaHCO3 (sodium bicarbonate)→CH₃COOHNa + CO2 + H2O
Ethanoic acid is used in the manufacturing of various dyes, plastics, rayons, silk, and perfumes.
It is most simply used as table vinegar.
In medicines, it is used as a local irritant.
In the rubber industry also it is used as a coagulating agent.
It is also used as a coagulating agent in the rubber industry.
It is useful in the manufacturing of acetate, acetone, and esters used in various industries.
It is widely used in the production of VAM (vinyl acetate monomer).
In the treatment of cancer, it is directly injected into the tumour.
Acetic acid has desirable solvent properties, along with the ability to form miscible mixtures with both polar and nonpolar compounds. Hence, it is a very important industrial solvent. It is widely used in the industrial preparation of dimethyl terephthalate (DMT).
It is an active micro bactericidal disinfectant that is also effective against most other bacterias.
1. Give Some Preparation Methods of Acetic Acid.
Industrially, acetic acid is prepared by carbonylation of methanol. These two compounds react to form methyl iodide intermediate which is then reacted with carbon monoxide. The resulting compound reacts with water to yield acetic acid. Here carbonyl complex is necessary as a catalyst in the second step. The equations of the preparation process are
CH₃OH (methanol) + HI (hydrogen iodide) → CH₃I (methyl iodide intermediate) + H2O
CO (carbon monoxide) + CH₃I → CH₃COI (acetyl iodide)
H2O+CH₃COI→ CH₃COOH (acetic acid) + HI
Another preparation method is :
It can also be prepared by oxidation of acetaldehyde where naphthalene salts of cobalt, chromium, and manganese can be employed as metal catalysts.
2CH₃CHO+ O2 → 2CH₃COOH.
2. What can be the Hazards of Acetic Acid?
Though acetic acid is a weak acid but its concentrate form is corrosive to the skin which produces blisters and burns. Prolonged inhalation exposure about eight hours to acetic acid vapors at 10 ppm can produce some irritation of eyes, nose, and throat. Lung irritation and possible damage to lungs, eyes, and skin may result at 100 ppm. Workers coming in contact with acetic acid for a long period can suffer from bronchitis, pharyngitis, erosion of exposed teeth, and conjunctivitis as well. So, mainly the hazard depends on the concentration of the acid.