- What are weak acids?
- Dissociation of weak acids
- Properties
- Polarity and inductive effect
- Atomic radius and bond strength
- Examples of weak acids
- References
The weak acids are only partially dissociates in water. After their dissociation, the solution where they are found reaches equilibrium and the acid and its conjugate base present simultaneously are observed. Acids are molecules or ions that can donate a hydronium ion (H +) or can form a covalent bond with a pair of electrons.
These in turn can be classified by their strength: strong acids and weak acids. When talking about the strength of an acid, this is the property that measures the degree of ionization of these species; that is, the ability or tendency of an acid to lose a proton.
Speciation graph for a weak acid that dissociates HA + H2O ↔ A- + H3O +
A strong acid is one that dissociates completely in the presence of water; that is, one mole of strong acid dissolved in water will result in the separation of one mole H + and one mole of conjugate base A -.
What are weak acids?
Weak acids, as mentioned above, are those that partially dissociate in water. Most acids are weak acids, and are characterized by releasing only a few hydrogen atoms into the solution where they are found.
When a weak acid dissociates (or ionizes) the phenomenon of chemical equilibrium occurs. This phenomenon is the state in which both species (that is, reactants and products) are present in concentrations that tend not to vary with time.
This state occurs when the rate of the forward reaction equals the rate of the reverse reaction. Therefore, these concentrations neither increase nor decrease.
The "weak" classification in a weak acid is independent of its dissociation capacity; an acid is considered weak if less than 100% of its molecule or ion dissociates incompletely in aqueous solution. Therefore, there is also a degree of dissociation between weak acids themselves called the acid dissociation constant Ka.
The stronger an acid, the higher its Ka value. The strongest weak acid is the hydronium ion (H 3 O +), which is considered the border between weak acids and strong acids.
Dissociation of weak acids
Weak acids ionize incompletely; that is, if this weak acid is represented in a general solution formula as HA, then a significant amount of undissociated HA would be present in the aqueous solution formed.
Weak acids follow the following pattern when dissociating, where H + is the hydronium ion in this case, and A - represents the conjugate base of the acid.
The strength of a weak acid is represented as an equilibrium constant or as a percentage of dissociation. As previously stated, the expression Ka is the dissociation constant of an acid, and this is related to the concentrations of reactants and products in equilibrium in the following way:
Ka = /
The higher the Ka value, the more the H + formation will be favored, and the lower the pH of the solution. The Ka of weak acids varies between the values of 1.8 × 10 -16 to 55.5. Those acids with a Ka less than 1.8 × 10 -16 have less acid strength than water.
The other method used to measure the strength of an acid is studying its percentage of dissociation (α), which varies from 0% <α <100%. Is defined as:
α = / +
Unlike Ka, α is not a constant and will depend on the value of. In general, the value of α will increase as the value of. In this sense, acids become stronger depending on their degree of dilution.
Properties
There are a number of properties that determine the strength of an acid and make it more or less strong. Among these properties are polarity and inductive effect, atomic radius, and bond strength.
Polarity and inductive effect
Polarity refers to the distribution of electrons in a bond, which is the region between two atomic nuclei where a pair of electors share.
The more similar the electronegativity between two species, the more equivalent the electron sharing; but the more different the electronegativity, the more time the electrons will spend in one molecule than in the other.
Hydrogen is an electropositive element, and the higher the electronegativity of the element to which it binds, the higher the acidity of the compound formed. For this reason, an acid will be stronger if it occurs between the hydrogen bond and a more electronegative element.
Furthermore, the inductive effect means that the hydrogen does not need to be directly attached to the electronegative element for the compound to increase its acidity. Because of this, some isomers of substances are more acidic than others, depending on the configuration of their atoms in the molecule.
Atomic radius and bond strength
The strength of the bond that binds hydrogen to the atom that governs the acid is another important factor in defining the acidity of a molecule. This, in turn, is dependent on the size of the atoms that share the bond.
For an acid called HA, the more you increase the size of its A atom, the more the strength of its bond will decrease, so this bond will be easier to break; this makes the molecule more acidic.
Atoms with higher atomic radii will benefit in acidity thanks to this detail, since their bond with hydrogen will be less strong.
Examples of weak acids
There are a large number of weak acids (most of all acids). These include:
- Sulfurous acid (H 2 SO 3).
- Phosphoric acid (H 3 PO 4).
- Nitrous acid (HNO 2).
- Hydrofluoric acid (HF).
- Acetic acid (CH 3 COOH).
- Carbonic acid (H 2 CO 3).
- Benzoic acid (C 6 H 5 COOH).
References
- Weak acid. (sf). Retrieved from en.wikipedia.org
- Essential Biochemistry. (sf). Retrieved from wiley.com
- CliffNotes. (sf). Retrieved from cliffsnotes.com
- Science, F. o. (sf). University of Waterloo. Retrieved from science.uwaterloo.ca
- Anne Marie Helmenstine, P. (nd). ThoughtCo. Retrieved from thoughtco.com