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Acidity and basicity of compounds are indications of the pH. Acidity of a medium is caused by acidic compounds, which can release hydrogen ions (H+), resulting in a low pH in that medium. Basicity of a medium is caused by basic compounds, which can release hydroxide ions (OH–), resulting in a high pH in that medium. The key difference between acidity and basicity is that acidity causes a low pH whereas basicity causes a high pH in an aqueous medium.
What is Acidity?
Acidity is the level of acid in substances. The concentration of hydrogen ions (H+) is the major parameter used to identify acidity. The hydrogen ion concentration is expressed as a pH value. pH is the negative logarithm of hydrogen ion concentration. Hence, higher the hydrogen ion concentration, lower the pH. A low pH value indicates a higher acidity.
According to the acidity of substances, there are two types of acids as strong acids and weak acids. Strong acids cause a higher acidity level in aqueous medium whereas weak acids result in a low acidity. Strong acids can completely dissociate into ions, releasing all possible hydrogen ions (H+). In contrast, a weak acid partially dissociates, releasing only some hydrogen ions. Acids can also be categorized as monoprotic acids and polyprotic acids; monoprotic acids release one hydrogen ion per molecule whereas polyprotic acids release more hydrogen ions per molecule.
The acidity of acids is determined by the pKa of the acid. pKa is the negative logarithm of Ka. Ka is the acid dissociation constant of a solution. It is a quantitative measurement of the strength of an acid in a solution (or acidity). Lower the pKa, the stronger the acid is. Higher the pKa, the weaker the acid is.
The periodic trends of acidity of chemical elements basically depend on their electronegativity values.The electronegativity of chemical elements increases from left to right of a period. If the electronegativity of an atom is higher, it can stabilize a negative atom on it very easily because it has a higher affinity for electrons. Therefore, the hydrogen ions associated with high electronegative atoms are easily released than low electronegative atoms, resulting in higher acidity. When going down a group in the periodic table, the acidity increases. This is because the size of atoms increases down the group. Large atoms can stabilize negative charges on them (by charge distribution); hence a hydrogen ion associated with a large atom can easily be released.
What is Basicity?
Basicity of a substance is the number of hydrogen atoms replaceable by a base in a particular acid. In other words, the basicity of a compound is the number of hydrogen ions that can react completely with hydroxide ions released by a base.
Electronegativity of an atom refers to its affinity for electrons. An atom having a high electronegativity can attract electrons when compared to low electronegative atoms. Higher the electronegativity, lower the basicity. In order to release hydroxide ion, the bond electrons between the oxygen atom and the rest of the molecule should be completely attracted by the oxygen atom (the oxygen atom in hydroxide group should be more electronegative than the other atom it is bonded to). Ex: if the basicity of ROH is high, the electronegativity of R is smaller than that of the oxygen atom.
Atomic radius is another factor that affects the basicity of a compound. If the atomic radius is small, the electron density of that atom is high. Hence, the hydroxide ion can easily be released. Then the basicity of that compound is comparatively high.
Formal charges are generally either positive charges or negative charges. A positive formal charge indicates a less electron density. Hence, bond electrons cannot be completely attracted by the hydroxide ion. Then it cannot be easily released (the hydroxide ion), indicating lower basicity. In contrast, a negative formal charge causes higher basicity.