Acids & Bases Part II: Strong vs. Weak Acids Jespersen Chap. 16 Sec 3 Dr. C. Yau Spring 2014 1

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 group. 2

Acid Strengths within a Period The strengths of binary acids increase from left to right within the same period. WHY? H2O < HF and H2S < HCl This can be explained by the trend in electronegativity: N O F Least most electronegative electronegative F is more electronegative than O. The pair of electrons in the H-F bond is spending more time near F than H. Thus, it is easier to pull H+ off, making HF the stronger acid than H2O. 3

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.

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.

Which is stronger? H2S or H2O CH4 or NH3 HF or HI δ- δ+ Which is stronger? H2S or H2O CH4 or NH3 HF or HI H2S H2O δ- δ+ NH3 CH4 δ- δ+ HI HF Do Pract Exer 11 & 12 p. 751 6

Relative Strengths of Oxoacids 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?

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.

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

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.

Stability of Anions Delocalization of the negative charge of the anion stabilizes the anion. The more stable the anion, the stronger is the acid. Do Pract Exer 13, 14, p. 753 & Pract Exer 15, 16, p. 754

Stability of Anions Other groups that affect the stability of anions: Which do you think would be a stronger acid and why? How does the Cl affect the acid? How does it affect the anion?

Examine the following organic carboxylic acids: Acid Strength Examine the following organic carboxylic acids: CH3COOH < CH2ClCOOH < CHCl2COOH < CCl3COOH increase in acid strength The highly electronegative Cl withdraw electron density from the O-H bond, making it easier for the proton to leave. In addition, the Cl stabilizes the resulting anion by spreading the negative charge further. The increasing number of Cl on the above acids increases the effect, resulting in an increase in acidity as shown.

Summary Binary acids: Oxoacids: 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