6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* (2023)

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    • 6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* (1)
    • Stephen M. Contakes
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    Oxyacids (also known as oxoacids) are compounds of the general formula \(\ce{H_{n}EO_{m}}\), where \(\ce{E}\) is a nonmetal or early transition metal and the acidic hydrogens are attached directly to oxygen (not \(\ce{E}\)). This class of compounds includes such well-know acids as nitric acid (\(\ce{HNO_2}\)) and phosphoric acid, (\(\ce{H_3PO_4}\)).

    6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* (2)

    The acidity of oxyacids follows three trends:

    Trend 1: In a homologous series the acidity increases with the electronegativity of the central atom

    Elements in the same group frequently form oxyacids of the same general formula. For example, chlorine, bromine, and iodine all form oxyacids of formula \(\ce{HOE}\): hypochlorous, hypobromous and hypoiodous acids. In the case of these homologous oxyacids, the acidity is largely determined by the electronegativity of the central element. Central atoms that are better able to inductively pull electron density towards themselves make the oxygen-hydrogen bond that is to be ionized more polar and stabilize the conjugate base, \(OE^-\). Thus the acid strength in such homologous series increases with the electronegativity of the central atom. This may be seen from the data for the hypohalous acids in Table \(\PageIndex{1}\), in which the acid strength increases with the electronegativity of the halogen so that the order of acidity is:

    \[\ce{HClO>HBrO>HIO} \nonumber \]

    Table \(\PageIndex{1}\): Relationship of central atom's electronegativity to acid ionization constant in the hypohalous acids.
    HOX Electronegativity of X Ka
    HOCl 3.0 4.0 × 10−8
    HOBr 2.8 2.8 × 10−9
    HOI 2.5 3.2 × 10−11

    Note that the influence of central atom electronegativity on the strength of oxyacids is exactly the opposite to that observed for the binary hydrides in Table \(\PageIndex{5}\), for which the acidity increased down a group, giving the order of acidity:

    \[\ce{HI>HBr>HCl \gg HF} \nonumber \]

    The reason for this is that in the hydrogen halides, the bond to be broken (H-E bond) decreased in strength down the group, while in oxyacids the bond to be broken is always an O-H bond and so varies much less in strength with the electronegativity of the central atom.

    Trend 2: For oxoacids of a given central atom the acidity increases with the central element's oxidation state or, in other words, the number of oxygens bound to the central atom.

    Here we are looking at the trend for acids in which there are variable numbers of oxygen bound to a given central atom. An examples is the perchloric (\(\ce{ClO_4^{-}}\)), chloric (\(\ce{ClO_3^{-}}\)), chlorous (\(\ce{ClO_2^{-}}\)), and hypochlous (\(\ce{ClO^{-}}\)) acid series. In such a series, the greater the number of oxygens, the stronger the acid. This can be explained in several ways. From the viewpoint of the acid itself the key factor is again the inductive effect, in this case involving the ability of the oxygens attached to the central atom to pull on electron density across the OH bond. This is seen from the charge density diagram for the chlorine oxoacids shown in Figure \(\PageIndex{1}\), in which the partial positive charge on the acidic hydrogen increases with the number of oxygens present.

    6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* (3)

    The increase in oxoacid acidity with the number of oxygens bound to the central atom may also be seen by considering the stability of the conjugate oxyanion. That the stability of the conjugate base increases with the number of oxygens may be seen from the charge distribution diagrams and Lewis bonding models for the chlorine oxyanions shown in figure \(\PageIndex{2}\) . As the negative charge is spread over more oxygen atoms it becomes increasingly diffuse.

    6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* (4)
    Exercise \(\PageIndex{1}\)

    Sulfur and selenium both forms oxoacids of formula \(\ce{H_2EO_3}\), where E is either S or Se. These are called sulfurous and selenous acid, respectively. Which oxoacid would you expect to be more acidic: selenous acid or sulfurous acid?

    Answer

    Sulfurous acid should be more acidic. Since sulfur is more electronegative than selenium sulfur will polarize OH bonds to a greater extent, making them more acidic. This prediction is borne out by a comparison of the \(pK_a\) values for the acids:

    Acid \(pK_{a1}\) \(pK_{a2}\)
    sulfurous acid, \(H_2SO_3\) 1.85 7.2
    selenous acid, \(H_2SeO_3\) 2.62 8.32

    Trend 3: For polyprotic oxoacids the acidity decreases as each successive proton is removed

    Oxoacids with multiple O-H bonds are called polyprotic since they can donate more than one hydrogen ion. In this case hydrogen ions are removed in successive ionization reactions. Examples include phosphoric and carbonic acid:

    \[\ce{H_3PO_4 ⇌ H^{+} + H_2PO_4^{-}} \quad \quad pK_{a1} = 2.2 \nonumber \]

    \[\ce{H_2PO_4^{-} ⇌ H^{+} + HPO_4^{2-}} \quad \quad pK_{a2} = 7.2 \nonumber \]

    \[\ce{HPO_4 ⇌ H^{+} + PO_4^{3-}} \quad \quad pK_{a3} = 12.4 \nonumber \]

    \[\ce{H_2CO_3 ⇌ H^{+} + HCO_3^{-}} \quad \quad pK_{a1} = 3.6 \nonumber \]

    \[\ce{HCO_3^{-} ⇌ H^{+} + CO_3^{2-}} \quad \quad pK_{a1} = 10.3 \nonumber \]

    The dissociation constants for successive ionization constants decrease by about five orders of magnitude between successive hydrogen ions. This is reflected in Linus Pauling's rulesfor oxoacids and their oxyanions:

    Pauling's Rules
    1. The \(pK_a\) for an oxyacid of general formula \(\ce{E(OH)_{q}(O)_{p}}\) is given by \[pK_a = 8 - 5 \times p \label{PaulingRules} \]
    2. As an oxoaxid undergoes successive ionizations the \(pK_a\) increases by five each time.

    The central theme of Pauling's Rules is that the more oxygens there are on the central atom, the more resonance structures that can be constructed for the conjugate base, which increases its stability and increases the acidity of the acid. However, as the acids successively ionize, they have fewer resonance structures. Pauling's Rules are phenomenological (i.e., not based on a theoretical framework). However, asempirical rules, they often workquite well, but it should be borne in mind that theyareapproximate.

    Exercise \(\PageIndex{2}\): How well do Pauling's rules for oxoacids work?

    Calculate the theoretical \(pK_a\) values for phosphoric and carbonic acid and their dissociation produces and compare the results with the experimental \(pK_a\) values.

    Answer

    For phosphoric acid, Pauling's rules (Equation \ref{PaulingRules}) predict the \(pK_a\) values quite well:

    • \(H_3PO_4\): \(p = 3\) and \(q =1\) and \[pK_{a1, predicted} = 8 - 5 \times 1 = 3 \nonumber \] This is slightly greater than the observed value of 2.2.
    • \(H_2PO_4^-\): \[pK_{a2, predicted} = pK_{a1, experimental} + 5 = 7.2 \nonumber \] This is spot on with experiment.
    • \(HPO_4^{2-}\): \[pK_{a3, predicted} = pK_{a2, experimental} + 5 = 12.2 \nonumber \] This is slightly less than the experimental value of 12.4.

    For carbonic acid Pauling's rules predict \(pK_{a1}\) reasonably well, but \(pK_{a2}\) less so:

    • \(H_2CO_3\): \(p = 2\), \(q =1\) and \[pK_{a1, predicted} = 8 - 5 \times 1 = 3 \nonumber \] This is slightly lower than the observed value of 3.6.
    • \(HCO_3^-\): \[pK_{a2, predicted} = pK_{a1, experimental} + 5 = 8.6 \nonumber \] This is 1.7 units less than the experimental value of 10.3.

    In some cases discrepancies between experimental \(pK_a\) values and those predicted by Pauling's rules suggest that structural rearrangements may be taking place upon ionization or else that the reported \(pK_a\) values do not really represent the ionization in question because they do not fully account for all the equilibria taking place in solution. In the case of carbonic acid, however, the reason for the discrepancy between the predicted and experimental \(pK_{a2}\) values is not entirely clear.

    FAQs

    6.3.7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom*? ›

    7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* Trend 2: For oxoacids of a given central atom the acidity increases with the central element's oxidation state or, in other words, the number of oxygens bound to the central atom.

    How do you determine the acidity of oxoacids? ›

    In general, the relative strength of oxyacids can be predicted on the basis of the electronegativity and oxidation number of the central nonmetal atom. The acid strength increases as the electronegativity of the central atom increases.

    How does electronegativity determine acidity? ›

    Thus, increasing electronegativity of the atom that shares an electron pair will decrease basicity. Weaker bases have stronger conjugate acids, so we conclude that as electronegativity of an atom increases the acidity of the attached proton also increases.

    What is the relationship between acidity and oxidation state? ›

    As the oxidation state of the central atom becomes larger, the acidity of the molecule increases. An atom becomes more electronegative as the oxidation number increases.

    Which of the following is correct for acidic strength of oxoacid? ›

    According to our question we should write in decreasing order of acid strength, thus the correct answer will in this order $HCl{O_4} > HCl{O_3} > HCl{O_2} > HOCl$. The acid strength of oxoacids can be accounted for with respect to the Bronsted lowry concept of acid and base also.

    How is acidity determined? ›

    The relative strength of an acid can be predicted based on its chemical structure. In general, an acid is stronger when the H–A bond is more polar. Acidity is also greater when the H–A bond is weaker and when the conjugate base, A⁻, is more stable.

    How do you determine acidity level? ›

    We can calculate the pH of a solution by taking the negative logarithm of the hydronium ion concentration, or pH = -log[H₃O⁺]. At 25°C, a solution with pH < 7 is acidic, a solution with pH > 7 is basic, and a solution with pH = 7 is neutral.

    How do you determine the acidity and basicity of an organic compound? ›

    When evaluating acidity / basicity, look at the atom bearing the proton / electron pair first. Then you may also need to consider resonance, inductive (remote electronegativity effects), the orbitals involved and the charge on that atom.

    How does increasing electronegativity for the central atom affect acidity? ›

    If the central atom is highly electronegative, then it pulls electron, polarizes the bond and make the molecule more stable. The more stable molecule, acidic strength becomes higher.

    Is acidity proportional to electronegativity? ›

    The acidic nature of a substance is directly proportional to the electronegativity of the groups attached. All the hydrogens are not acidic in nature. The hydrogen attached with electronegative atoms is acidic.

    What is the relation between oxidation state and electronegativity? ›

    It is found that for a given cation, the electronegativity increases with increasing oxidation state and decreases with increasing coordination number. For the transition-metal cations, the electronegativity of the low-spin state is higher than that of the high-spin state.

    What is the relationship between oxidation and? ›

    The relationship between oxidation and oxidizing agent in a redox reaction is that the oxidizing agent causes the oxidation of the substance being oxidized. It does this by accepting electrons from the substance, which becomes oxidized in the process.

    What is the relationship between oxidation and oxidation? ›

    Oxidation is the addition of oxygen or removal of hydrogen. The oxidizing agent causes oxidation of other compounds by getting reduced.

    What is acidity of oxo acids? ›

    7: The Acidity of an Oxoacid is Determined by the Electronegativity and Oxidation State of the Oxoacid's Central Atom* Trend 2: For oxoacids of a given central atom the acidity increases with the central element's oxidation state or, in other words, the number of oxygens bound to the central atom.

    What is the strongest oxo acid? ›

    What is the most powerful and least powerful oxyacid? The electronegativity of the core atom determines the oxyacid's strength. Perchloric acid is the strongest oxyacid, whereas Hypochlorous acid is the weakest. The total number of oxygen atoms bonded in the structure is the only difference between these acids.

    Which oxoacid is most acidic in nature? ›

    HClO4 is most acidic among given choices.

    Which is more acidic Oxyacid? ›

    $HCI{O_4} > HBr{O_4} > HI{O_4}$ (R) Greater is the oxidation state of a halogen, more is the acidic character of its oxyacid.

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