Strength of acids. Study on the strength of acids

You acids are very important inorganic substances for various branches of society, such as industries cosmetics, soft drink manufacturers, automotive battery producers, among many others. They are defined as substances that, in water, have the ability to suffer the phenomenon of ionization, producing a cation (H₃O⁺) and an anion (X^-), as in the following equation:

HX + H₂O → H₃O⁺ + X^-

Like the acids have the ability to produce ions in water, the solution that contains them, in addition to having a sour taste, presents as main characteristic the fact that it conducts electric current. The production of ions is called strength, and acids, based on this item, are classified as strong, weak, or moderate.

Observation: the presence of ions in water is the factor that allows water to conduct electrical current.

Consider an acid as strong means that, in water, it is capable of producing a large amount of ions; already the acid moderate is one that produces an amount of ions neither too large nor too small; it's the

weak, in turn, is one that produces a very small amount of ions. This classification described depends on the type of acid we are dealing with, whether a hydracid (has no oxygen in its composition) or an oxyacid (has oxygen in its composition).

a) Hidracids

• The only hydrates strong they are:

- HCl (hydrochloric acid);

- HBr (hydrobromic acid);

- HI (hydriodic acid).

• The only moderate hydrate is HF (hydrofluoric acid).

• All other hydrates that were not mentioned are weak, such as:

- HCN (hydrocyanic acid);

- H₂S (hydrogen sulphide);

- HNC (isocyanic acid).

b) Oxyacids

To find the strength of an oxyacid, simply subtract the amount of oxygen from its formula by the number of ionizable hydrogens available in the acid:

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• Strong: When the result of the subtraction is equal to or greater than 2. See some examples:

- H₂ONLY₄ (sulfuric acid)→ 4 - 2 = 2

- H₄P₂O₇ (pyrophosphoric acid)→ 7 - 4 = 3

• Moderate: When the result of subtraction is 1. See some examples:

- H₃DUST₄ (phosphoric acid)→ 4 - 3 = 1

— HClO₂ (chlorous acid) → 2 - 1 = 1

• Weak: When the result of subtraction is 0. See some examples:

- H₃BO₃ (Boric acid)→ 3 - 3 = 0

— HBrO(hypobromous acid)→ 1 - 1 = 0

Observation: the H₂CO₃ is an exception to the rule for determining the strength of an oxyacid. Subtract the number of oxygens from the number of hydrogens equals 1, but it's a weak acid. This fact has been proven experimentally.

The strength of acids can also be evaluated through the calculation of the so-called degree of ionization, represented by the letter α. To calculate this degree, simply divide the number of ionized molecules by the number of molecules added to the water initially. Then just multiply by 100 to make a percentage.

α = no^O of ionized molecules
no^O of initial molecules

See an example:

If 50 molecules of an acid were added to water and 20 of them ionized, then its α is:

α = 20
​50

α = 0,4

α = 0,4.100

α = 40%

By finding α, we can evaluate the strength of the acid according to the following pattern:

• Strong: α ≥ 50%

• Moderate: 5% < α < 50%

• Weak: 5% ≤ α

​
By Me. Diogo Lopes Dias

Would you like to reference this text in a school or academic work? Look:

DAYS, Diogo Lopes. "Strength of Acids"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/forca-dos-acidos.htm. Accessed on June 28, 2021.