Determine the strength of an acid it is the same as pointing out the capacity that this compound has to ionize when dissolved in water. Knowledge about the strength of acid shows whether the concentration of hydronium ions in water will be high or low.
Classification of acids
About strength, an acid Can be:
Strong (acid that produces a high amount of hydronium cations (H3O+) in the water);
Moderate or Semi-strong (acid that produces an intermediate amount of hydronium cations (H3O+) in the water);
Weak (acid that produces a low amount of hydronium cations (H3O+) in the water).
Now see some tips for determining the strength of an acid:
1st Tip: Acid Molecular Formula
- If we have a hydrate (acid without oxygen in its formula):
Strong acid: Only HCl, HBr or HI;
Moderate or semi-strong acid: HF only;
Weak acid: any other hydroxide.
- If we have an oxyacid (acid with oxygen in its formula):
In this case, we must perform the subtraction between the number of oxygens and the number of ionizable hydrogens present in the acid formula:
x = O - H
So, if the x has:
Result ≥ 2 → Strong acid
Result = 1 → Moderate or semi-strong acid
NOTE: The H acid3DUST3 has three hydrogens in the formula, but only two are ionizable, so your result is 1. The acid H3DUST2 has three hydrogens in the formula, but only one is ionizable, so its result is 1.
Result = 0 → Weak acid
NOTE: The H acid2CO3, even with a result of 1, is an exception, as it is weak.
Examples:
H2s
It is a weak acid because it is not one of the strong hydra acids (HCL, HBr and HI) and it is not the moderate hydra acid (HF).
H3DUST4
It is a moderate acid because, since it is an oxyacid, in the subtraction between the number of oxygens (4) and the number of hydrogens (3), the result is 1.
H4P2O7
It is a strong acid because, since it is an oxyacid, in the subtraction between the number of oxygens (7) and the number of hydrogens (4), the result is 3.
2nd Tip: degree of ionization (α)
The degree of ionization indicates the percentage of acid ionization when dissolved in water. It is the relationship established between the number of ionized molecules and the number of initial acid molecules:
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α = number of ionized molecules
number of starting molecules
After performing the division between the number of molecules, we must multiply the result by 100 to find the percentage of ionization. With that, if:
α ≥ 50% → Strong acid
50% < α > 5% → Moderate or semi-strong acid
α ≤ 5% → Weak acid
Example: 50 molecules of a certain HX acid were added to the water, but only 20 molecules were dissociated.
α = number of ionized molecules
number of starting molecules
α = 20
50
α = 0,4
As we must multiply α by 100, we have:
α = 0,4.100
α = 40% - moderate acid
3rd Tip: Ionization Constant (Ki)
For an acid, we have the following ionization equation general:
HX + H2O → H3O+ + X-
Its ionization constant (Ki) is the relationship between product concentrations and acid molar concentration:
Ki = [H3O+].[X-]
[HX]
NOTE: Water does not enter the expression because, for the ionization happens, it must be present, that is, it is a constant in the process.
Analyzing the expression, we can see that the concentration of hydronium [H3O+] is in the numerator and the acid concentration [HX] is in the denominator. Thus, the greater the concentration of hydronium, the greater the Ki value.
Through the Ki of an acid, we can know if there are too many hydroniums in the medium and vice versa. To classify the acid, we will take into account the following reference:
Ki > 10-3 → Strong acid
Ki = 10-3 or 10-4 → Moderate or semi-strong acid
Ki ≤ 10-5 → Weak acid
Examples:
Sulfuric acid (H2ONLY4) - Ki = 1.2.10-2
It is a strong acid because its Ki is greater than 10-3.
Nitric acid (HNO2) - Ki = 4.10-4 → Moderate Acid
It is a moderate acid because its Ki is 10-4.
hydrocyanic acid (HCN) - Ki = 6.2.10-10
It is a weak acid because its Ki is less than 10-5.
By Me. Diogo Lopes Dias
Would you like to reference this text in a school or academic work? Look:
DAYS, Diogo Lopes. "Tips for Determining the Strength of an Acid"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/dicas-para-determinar-forca-um-acido.htm. Accessed on June 27, 2021.
Ostwald's law, ionization constant, molar concentration, degree of ionization, weak electrolyte, number of mol ionized, Friedrich Wilhelm Ostwald, monoacids, monobases.
