Question

Account for the following:
The acidic strength decreases in the order $HCl>{{H}_{2}}S>P{{H}_{3}}$

Answer 1

A strong acid ionises entirely in a solution, whereas a weak acid only ionises partially. When strong acids come into touch with skin, they are caustic and inflict serious burns. The weak acids, on the other hand, are just moderately corrosive and are found in our food and bodies. Strong acids include hydrochloric acid and sulphuric acid, to name a few.

Complete answer:
The tendency of an acid, represented by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-, is known as acid strength. Except in its most concentrated concentrations, a strong acid's dissociation in solution is nearly complete.
$HA\to {{H}^{+}}+{{A}^{-}}$
As previously stated, various acids have varying acid strengths. A stronger acid is one that has a higher degree of dissociation. Let's look at the elements that influence the acid's strength. The degree of dissociation of an acid is determined by the two parameters listed below.
The H-A bond's tensile strength
The H-A bond's polarity
In general, the weaker the H-A link, the stronger the acid. Similarly, the stronger the acid, the higher the polarity of the H-A bond. Both of these variables make it simpler for acid molecules to dissociate into ${{H}^{+}}$and ${{A}^{-}}$, increasing acidity.
The bond dissociation enthalpy of the H—Cl bond is lower than that of the H—S bond, which is lower than that of the P—H bond. Because the polarity of the E—H bond changes from H—Cl to P—H as the electronegativity of E drops, the acidic strength decreases.
The bond pair of electrons is more strongly drawn by the electronegative atom as the electronegativity increases from P to Cl. The connection grows weaker as a result of this. As a result, HCl has a higher proclivity for proton donation than phosphine. The anion produced can also be easily maintained by more electronegative Cl atoms than by less electronegative
P atoms following deprotonation.
Hence $HCl > {{H}_{2}}S > P{{H}_{3}}$ is the correct order.

Note:
It is determined by the H-A bond's strength. The lower the bond's strength, the less energy is necessary to break it. As a result, acid is a powerful substance. The acid strength of the H-A bond is affected by its polarity. When a bond is extremely polar, the proton is more likely to exit the molecule, making it a strong acid. When utilising the above two variables to compare acid strengths of elements in the same group of the periodic table, bond strength is more relevant. When comparing the acid strengths of elements in the same row, however, the polarity of the H-A bond takes precedence.
The acid strength is also affected by the atomic size of A. The connection weakens as the atom grows larger. As a result, the acid strength rises.