For the series of oxoacids H3PO4 , H2SO4 , and HClO4 , which acid is strongest? Why?
The relative strength is determined by (a) the electronegativity of the central atom and (b) by the number of oxygen atoms. Here is a good description. For this series HClO4 is the strongest followed by H2SO4 followed by H3PO4.
https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Miessler_Fischer_Tarr)/6%3A_Acid-Base_and_Donor-Acceptor_Chemistry/6.3%3A_Br%C3%B8nsted-Lowry_Concept/6.3.2_Br%C3%B8nsted-Lowry_Superacids_and_the_Hammett_Acidity_Function/6.3.3_Thermodynamics_of_Solution-Phase_Br%C3%B8nsted_Acidity_and_Basicity/6.3.4_Thermodynamics_of_Gas_Phase_Br%C3%B8nsted_Acidity_and_Basicity/6.3.5._Non-nucleophilic_Br%C3%B8nsted-Lowry_Superbases/6.3.6._Rules_of_Thumb_for_thinking_about_the_relationship_between_Molecular_Structure_and_Br%C3%B8nsted_Acidity_and_Basicity*/6.3.7.__The_acid-base_behavior_of_binary_element_hydrides_is_determined_primarily_by_the_element's_electronegativity_and_secondarily_by_the_element-hydrogen_bond_strength.*/6.3.8._The_Acidity_of_an_Oxoacid_is_Determined_by_the_Electronegativity_and_Oxidation_State_of_the_Oxoacid's_Central_Atom*
To determine which of the oxoacids H3PO4, H2SO4, and HClO4 is the strongest, we need to consider their relative acid strengths. The acid strength of an oxoacid depends on the electronegativity and electronegativity difference between the central atom and the surrounding oxygen atoms, as well as the number of oxygens bonded to the central atom.
The electronegativity difference between the central atom and the surrounding oxygen atoms helps to determine the polarity of the O-H bond. Generally, the higher the electronegativity difference, the more polar the bond, making it easier for the hydrogen to dissociate as a proton, leading to a stronger acid.
Additionally, the number of oxygens bonded to the central atom affects the acid strength. The presence of more oxygen atoms leads to greater delocalization or stability of the negative charge, making it easier for the central atom to release the proton and making the acid stronger.
Now, let's compare the three oxoacids:
1. H3PO4 (phosphoric acid): Phosphoric acid contains phosphorus (P) as the central atom and three oxygen atoms. The electronegativity difference between P and O is significant, and the presence of multiple oxygen atoms provides sufficient delocalization of the negative charge. As a result, H3PO4 is a moderate-strength acid.
2. H2SO4 (sulfuric acid): Sulfuric acid contains sulfur (S) as the central atom and four oxygen atoms. Sulfur has a higher electronegativity than phosphorus, resulting in a more polar O-H bond. Additionally, the fourth oxygen helps to stabilize the negative charge, further enhancing the acid strength. H2SO4 is a strong acid.
3. HClO4 (perchloric acid): Perchloric acid contains chlorine (Cl) as the central atom and four oxygen atoms. Chlorine has a higher electronegativity than sulfur, which means the O-H bond is even more polar, making proton dissociation easier. Combined with the four oxygen atoms, which increase the stability of the negative charge, HClO4 is the strongest acid among the three.
In summary, HClO4 (perchloric acid) is the strongest acid among H3PO4, H2SO4, and HClO4 due to the high electronegativity difference and the presence of four oxygen atoms.