Question

Which one of the following is the strongest acid?
(a) $ClC{H_2}COOH$
(b) $BrC{H_2}COOH$
(c) $FC{H_2}COOH$
(d) $IC{H_2}COOH$

Answer 1

Hint : Strong acids are those acids that completely dissociate into its constituent ions in water. Weak acids do not completely dissociate in water, that means, it does not donate all of its hydrogen ions in a solution.

Complete Step By Step Answer:
The given acids are $\alpha -$ halo acids. As we know that the electron withdrawing inductive effect ( $- I$ effect) of the halogens decreases in the order $F > Cl > Br > I$ , therefore the acidic strength of $\alpha -$ halo acids decreases in the same order.
$FC{H_2}COOH > ClC{H_2}COOH > BrC{H_2}COOH > IC{H_2}COOH$
The stronger the electron withdrawing group, the greater is the strength of the substituted carboxylic acid.
Factors that affect the strength of an acid are:
Acid strength is the tendency of an acid to dissociate into a proton ${H^ + }$ and an anion ${A^ - }$ .
Acidic strength depends on the strength of the $HA$ bond. When the bond is weak, less energy is required to break the bond. Hence, the acid is strong.
The polarity of the bond affects the strength of the acid. If the bond is highly polar, the proton leaves the molecule more easily, making it a strong acid.
When we compare the strength of the acid in the same row, priority is given to the polarity of the $HA$ bond.
The atomic size of the anion also affects the acidic strength .AS the size of the atom increases, the bond gets weaker. Hence, the acid strength increases.
Therefore, from the above factors we can conclude that $FC{H_2}COOH$ is the strongest acid amongst the $\alpha -$ halo acids.
Hence, option (c) $FC{H_2}COOH$ is correct.

Note :
The strength of an acid depends on the concentration of hydronium ions present in the solution. Strong acids have weakly bound hydrogen atoms and therefore the molecules easily separate from them in the solution.