Question

If you visit a gym or fitness center, you will notice that some people are stronger than others. There is a similar situation in the world of acids and bases. Some are stronger than others. For example, $HF$ is classified as weak acid and $HCl$ is a strong acid.
Which of the following statements gives the best explanation for this difference in acid strength?
A.The molar mass of $HF$is less than the molar mass of $HCl$
B.The $H - F$bond requires more energy to break than the $H - Cl$bond.
C.A solution of 1 M $HF$ has a lower pH than a solution of 1M $HCl$.
D.The solubility of $HF$in water is less than the solubility of $HCl$ in water.

Answer 1

Acid are those substances which give out Hydrogen ion i.e. ${H^ + }$ when dissolved in a solution and stronger will be the one which gives out hydrogen ions faster and easily. If the bond is breaking easily, then it will give out the ion very easily. So, these factors can decide which one will be stronger and which is not.

Complete step by step answer:
In the given question, the two acids i.e. $HF$ and $HCl$ are compared and we have to explain the acidic strength of both the acids i.e. which one is stronger and why.
An acid is a substance which gives out Hydrogen ions i.e. ${H^ + }$ ions in the solution when it is dissolved and the base is a substance which gives out $O{H^ - }$ ions when dissolved in a solution. In the given question, we see that both contain Hydrogen, so they will give Hydrogen ions when they will be dissolved in the solution. Hence, they will act as an acid.
Now both are acids, the stronger acid will be the one which can give out Hydrogen ion $\left( {{H^ + }} \right)$ more easily as compared to others.
In this case, $HCl$ will give more fast and easily hydrogen ions as compared to $HF$ and therefore will be the stronger acid.
This is because as we move down from Fluorine to Bromine (in the halogen family) the atomic size goes on increasing and therefore, the bond strength between Hydrogen and Halogen atoms will decrease because they will move apart and become weaker.
$Bond{\text{ }}strength{\text{ }} \prec \dfrac{1}{{Bond{\text{ }}length}}$
When the bond strength between the two is decreasing, it will break the bond i.e. Hydrogen-Halogen bonds more easily (because the distance is more and strength is weak) so which will ultimately release ${H^ + }$ ion.
So, $Cl$ has a bigger size than $F$, so $H - Cl$ bonds will break more easily and therefore will release Hydrogen ions in solution more readily compared to $HF$and therefore, will become stronger acid.

So, the correct answer is B.

Note:
In the laboratory, it is warned not to use $HF$ because it is very corrosive in nature which can dissolve any metal or substance in it. Though it is less acidic, it is very corrosive (there is a difference between being acidic and corrosive). There are other properties of halogen also like electron gain enthalpy, ionization energy etc. and also Fluorine is considered as the most electronegative element of the periodic table.