An acid that dissociates partially into its ions in an aqueous solution is a weak acid. In contrast, those that dissociate completely are strong acids. The conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid. A weak acid has a higher pH value than a strong acid at the same concentration. In everyday life, it is easier to find weak acids than strong acids. For example, acetic acid is in vinegar and citric acid in lemon juice, and more.
Ionization of Weak Acids
The reaction symbol used for the ionization of a strong acid in water is an arrow from left to right. Whereas, in the case of weak acids, a double arrow is used, which indicates both the forward and backward reaction taking place at equilibrium. When equilibrium is achieved, the aqueous solution contains the weak acid, its conjugate base and the hydrogen ion as shown below:
HA ⇌ H+ + A−
In the case of acetic acid, the chemical reaction is as follows:
H3COOH ⇌ CH3COO- + H+
In the above reaction, the acetate ion (CH3COO-) is the conjugate base of acetic acid.
What makes an acid weak?
The ability to dissociate of an acid in water depends on the polarity in the chemical bond between its different atoms. If the atoms in a bond have nearly the same electronegativity, the electrons are equally shared between them and thus have a non-polar bond between them. In contrast, if there is a difference in electronegativities between the participating atoms, there will be a separation of charge which makes electrons drawn more towards one atom than to the other and thus polar or ionic bond is formed between atoms in this case.
The hydrogen atom carries a slight positive charge when it is bonded to an electronegative element. If the hydrogen's electron density is less, it will be more polar and it is easy to ionize it thus it is more acidic in this case. However, when the polarity is less between hydrogen and the other atom, the removal of hydrogen ions is not easy and thus weak acid is formed.
One more factor that decides the strength of an acid is the size of the atom bonded to hydrogen. If the size is more, the strength of the bond will be less and the bond breaks easily to release the hydrogen and thus the strength of the acid increases.
Common Weak Acids
1. Formic Acid
Formic acid is one of the most basic carboxylic acids. It also goes by the name methanoic acid. Its chemical formula is HCOOH or CH2O2. It is naturally found in the bodies of some ants. It appears as a colorless liquid under standard temperature and pressure conditions.
Its density is 1.22 gm/ml; the melting point is 8.4 degrees Celsius; pKa value is 3.745; molar mass is 46.025 gm/pole and boiling point is 100.8 degrees Celsius. Further, formic acid is miscible with many organic solvents including ethanol, methanol, glycerol, etc. However, it is partially soluble in aromatic compounds such as benzene and toluene.
2. Acetic Acid
It also goes by the name ethanoic acid. It is a weak acid with the chemical formula CH3COOH. It is a main component of vinegar; a 5% to 20% acetic acid solution in water. Being a weak acid, it partially dissociates into its ions in water.
Its molar mass is 60.052 grams per mole. It exists as a colorless liquid with a strong odor under standard conditions of temperature and pressure. In the liquid state, its density is 1.049 gm per cubic centimeter. Whereas, in the solid-state, its density is 1.27 gm per cubic centimeter. Its melting point is around 16 degrees Celsius and its boiling point lies between 110 to 118 degrees Celsius.
Further, owing to its antibacterial qualities, it is used as an antiseptic. It is also used in the manufacturing of various products such as rayon fibers, rubber, perfumes, etc.
3. Benzoic Acid
It is a weak organic acid with the chemical formula C6H5CO2H. It is naturally found in many plants. It is also known as benzene carboxylic acid and carboxy benzene.
In solid state, it has no color. Its melting point is 122 degrees Celsius, while its boiling point is 250 degrees Celsius. Its molecular weight is 122.12 g/mol and its density is 1.27 gm per cubic centimeter. It is poorly soluble in water. For instance, its solubility in water at 25 degrees Celsius is 3.44 gm per litre. It is soluble in some organic solvents such as benzene, acetone, alcohol, etc.
It is used as a preservative in various foods like pickles, fruit juices, and processed foods. Further, it is used in the prevention of bacterial infections and ringworm and athlete's foot. It is also used in the manufacturing of phenol, dyes and insect repellents.
4. Oxalic Acid
Oxalic acid is a weak organic acid. Its chemical formula is C2H2O4 or HOOCCOOH. It is also known as ethanedioic and is naturally present in plants, fruits, cocoa, nuts, seeds, and more. It appears as a colorless and odorless substance in the solid-state, which is soluble in water.
Its molecular weight is 90.03 g/mol; density is 1.90 gm per centimeter cube and melting point is 190 degrees Celsius. It is the simplest type of dicarboxylic acid wherein two carboxylic acid groups (COOH) are directly bonded to each other through their carbon atoms.
It has a dibasic nature owing to its ability to donate two hydrogen ions. It undergoes decarboxylation when heated to 150 degrees Celsius. Further, it is used as a cleansing agent, in the processing of metals, to purify and dilute certain chemicals, in the rust removal process and in the preparation of dyes.
5. Hydrofluoric Acid
Hydrogen fluoride when dissolved in water forms hydrofluoric acid. This weak acid has the chemical formula HF. Like other weak acids, it also does not completely dissociate into its ions (hydrogen and fluoride ions). Despite being weak acid, it can cause severe burns if comes in contact with the body.
HF when dissolved in water results in colorless solution under standard temperature and pressure conditions. The pKa value of HF is 3.17 and the density of 48% HF solution in water is around 1.15 gm per ml.
It is used for the etching of silicon, cleaning of metals and in the manufacturing of electronics. It is also used in various industrial processes, e.g. oil refineries. It is also needed to dissolve oxides and silicates.
6. Nitrous Acid
Nitrous acid is an unstable weak acid with the chemical formula HNO2. It is prepared in cold dilute solutions. The reason for being unstable is that it can acts as both oxidizing and reducing agent as its nitrogen atom can gain or donate electrons during a chemical reaction.
Its molar mass is 47.013 g/mol; density is 1.1 g/cm3 and boiling point is around 158 degrees Celsius. It appears as a pale blue solution with a pungent smell. It is used in the formation of diazonium salts and azo dyes. It is also used to remove the toxic nature of sodium azide (an explosive compound) and as an oxidizer in liquid fuel rockets.
7. Sulphurous Acid
Sulphurous acid is also known by multiple names Sulphur dioxide solution, dihydrogen trioxosulfate, or trioxosulfuric acid. Its chemical formula is H2SO3.
It appears as a colorless liquid with a burning smell. It is the conjugate acid of a hydrogensulfite. It can corrode tissue and metals. It is a toxic compound so it can cause injury if comes in contact with the body.
Its molecular weight is 82.07 g/mol. Its molecule is made of two hydrogen atoms, three oxygen atoms and one Sulphur atom. It is used as reducing agents, disinfectants, and in the production of paper products and as an intermediate in industries.
8. Phosphoric Acid
Phosphoric acid also goes by the name orthophosphoric acid. Its chemical formula is H3PO4 which show it comprises one phosphorus atom, four oxygen atoms and three hydrogen atoms.
Its molecular weight is 98.00 g/mol, the melting point is 42.4 degrees Celsius, boiling point is 158 degrees Celsius, and its density is 1.88 g/cm3. Further, it dissolves easily in water and ethyl alcohol. It has lots of uses such as to remove rust, as a food additive, as an acidulant in the beverage industry and preventing the growth of fungi, bacteria, etc. It is also used in agriculture-related products, in pharma companies and in the preparation of personal care products.