How do we compare substances in terms of acidity?
Rule 1: For binary acids with elements in the same group, acidity increases as we go down the group (with increasing radius). For example lets compare, HCl, HF and HI. First we notice that Cl,F, and I are all in the same group, Group17. Therefore we can now compare elements in terms of their radius. Iodine has the largest radius (most down) followed by chlorine and fluorine. Therefore, HI is most acidic, followed by HCl and HF is the least acidic.
Rule 2: For compounds containing elements in the same period, acidity increases with increasing electronegativity (left to right). For example lets compare the acidity of H2O, HF and NH3. Nitrogen, Oxygen and Fluorine are all in the same period of the periodic table. Therefore, we must look at the electronegativity. Fluorine is most electronegative, followed by oxygen and then nitrogen. Thus, HF is most acidic followed by H2O and NH3 is least acidic.
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