Consider each acid formula starts with hydrogen then has a non-metal element and sometimes oxygen. Consider the location of these non-metals (not oxygen) on the periodic table. Describe the link between the position of the non-metal and acid strength.
Acids under question, Hydrochloric, Sulfuric and Phosphoric Acid.
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For the acids in question (hydrochloric acid, sulfuric acid and phosphoric acid), the corresponding non-metals are period 3 elements (chlorine, sulfur and phosphorus).
The acid strength of non-metal hydrides is a function of two characteristics: polarity of X-H bonds and strength of H-X bond. Polarity is a function of electronegativity of non-metal and increases with an increase in electronegativity. Higher electronegativity means easy dissociation of electrons from hydrogen and thus easier transfer of protons (H+). Therefore, higher electronegativity causes higher acid strength. The atomic radius governs the strength of H-X bond, the greater the radius of the non-metal atom, the easier it is to break the hydrogen atom away from it. Hence, a larger atomic radius results in lower bond-strength and higher acidic strength.
In the given case, acid strength varies as hydrochloric acid>sulfuric acid>phosphoric acid. Electronegativity also varies as Cl>S>P, while the atomic radius varies as Cl<S<P. Hence, the atomic radius reduces the acidic strength, while electronegativity increases it as we move from phosphorus to chlorine. However, the smaller change in atomic radius is insignificant when compared to a large change in electronegativity and hence the acidic strength increases as we go across the row (from P to Cl).