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Question 15.4: Identify the stronger acid in each of the following pairs: (......

Identify the stronger acid in each of the following pairs:

(a) H_{2}S  o r HCl                         (b) HClO_{3}  or  HBrO_{3}

(c) H_{2}SO_{4}  or  H_{2}SO_{3}

STRATEGY

For a given acid, where an element A is bonded to an acidic hydrogen atom, evaluate the strength and polarity of the H—A bond. Acid strength increases as the strength of the H—A bond decreases and the polarity of the H—A bond increases.

Step-by-Step
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(a) In comparing H_{2}S and HCl, note that S and Cl are in the same row of the periodic table. Therefore, the H—S bond should have similar strength to the H—Cl bond and bond polarity is the most important determinant of acid strength. Cl is more electronegative than S, making the H—Cl bond more polar than the H—S bond. HCl is the stronger acid.

(b) HClO_{3} and HBrO_{3} are oxoacids that contain the same number of O atoms and acid strength is determined by the electronegativity of the central atom. Because Cl is more electronegative than Br it can pull more electron density from the O—H bond resulting in a weaker and more polar bond in HClO_{3}. Therefore, HClO_{3} is the stronger acid.

(c) H_{2}SO_{4} and H_{2}SO_{3} are oxoacids with different numbers of oxygen atoms and different oxidation states of sulfur. The oxidation state of sulfur in H_{2}SO_{4} is +6 which is higher than +4 in H_{2}SO_{3}. The higher positive charge on the S atom is more effective in pulling electron density from the O—H bond, resulting in a weaker and more polar bond in H_{2}SO_{4}. Therefore, H_{2}SO_{4} is the stronger acid.

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