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Acids Bases Part II: Strong vs. Weak Acids

2. 2. Why Some Acids are Stronger Than Others. A binary acid (HnX) is an acid composed of hydrogen and a nonmetal.e.g. HF, HCl, H2S, H2S etc.The strengths of binary acids increase from left to right within the same period.The strengths of binary acids increase from top to bottom within the same g

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Acids Bases Part II: Strong vs. Weak Acids

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    1. 1 Acids & Bases Part II: Strong vs. Weak Acids Dr. C. Yau Spring 2012

    2. 2 Why Some Acids are Stronger Than Others

    3. 3 Acid Strengths within a Period

    4. 4 Acid Strengths within a Group The strengths of binary acids increase from top to bottom within the same group. WHY? HF < HCl < HBr < HI and H2O < H2S < H2Se < H2Te This is opposite to what one might expect based on electronegativity. The trend can be explained in 2 ways: 1) The smaller atoms would have H closer to the nucleus of the nonmetal. Smaller atoms have stronger bonds.

    5. 5 Acid Strengths within a Group 2) The larger the anion, the more stable it is because it can accommodate a negative charge better. F- < Cl- < Br- < I- smallest largest least stable most stable HX H+ + X- The more stable the anion, the stronger is the acid.

    6. 6 Which is stronger? H2S or H2O CH4 or NH3 HF or HI

    7. 7 An oxoacid is an acid that contains H, O and one other element. e.g. HNO3 H2SO4 HClO3 The acidity is dependent on 1) Electronegativity of the central atom. WHY? 2) The number of oxygen atoms in the acid. WHY?

    8. 8 Relative Strengths of Oxoacids HIO4 < HBrO4 < HClO4 central atom - least most electronegative electronegative least acidic most acidic The more electronegative central atom pulls electrons towards itself, thus weakening the H-O bond.

    9. 9 Relative Strengths of Oxoacids H3PO4 < H2SO4 < HClO4 central atom least most electronegative electronegative least acidic most acidic

    10. 10 Relative Strengths of Oxoacids HClO < HClO2 < HClO3 < HClO4 least most number of O number of O least acidic most acidic The larger the number of O, the more stable is the anion due to resonance. The negative charge of the anion is spread over a larger area, thus making it more stable.

    11. 11 Stability of Anions Delocalization of the negative charge of the anion stabilizes the anion.

    12. 12 Stability of Anions Other groups that affect the stability of anions:

    13. 13 Acid Strength Examine the following organic carboxylic acids: CH3COOH < CH2ClCOOH < CHCl2COOH < CCl3COOH increase in acid strength

    14. 14 Summary Binary acids: HF < HCl < HBr < HI and H2O < H2S < H2Se < H2Te Oxoacids: HIO4 < HBrO4 < HClO4 H3PO4 < H2SO4 < HClO4 Increase of O increases acidity: HClO < HClO2 < HClO3 < HClO4 Presence of Electronegative Elements: CH3COOH < CH2ClCOOH < CHCl2COOH < CCl3COOH

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