Acids Bases Part II Strong vs Weak Acids

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 (Hn. X) is an acid composed of hydrogen and a nonmetal. e. g. HF, HCl, H 2 S 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? H 2 O < HF and H 2 S < HCl This can be explained by the trend in electronegativity: N O F Least most 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 H 2 O. 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 H 2 O < H 2 Se < H 2 Te 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. 4

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- < Ismallest largest least stable most stable HX H + + XThe more stable the anion, the stronger is the acid. 5

δ- • Which is stronger? • H 2 S or H 2 O δ- δ+ δ+ H 2 S • CH 4 or NH 3 H 2 O δδ+ NH 3 CH 4 δ+ • HF or HI δ+ δ- δ- HI Do Pract Exer 11 & 12 p. 751 HF 6

Relative Strengths of Oxoacids An oxoacid is an acid that contains H, O and one other element. e. g. HNO 3 H 2 SO 4 HCl. O 3 The acidity is dependent on… 1) Electronegativity of the central atom. WHY? 2) The number of oxygen atoms in the acid. WHY? 7

Relative Strengths of Oxoacids HIO 4 < HBr. O 4 < HCl. O 4 central atom - least most electronegative least acidic most acidic The more electronegative central atom pulls electrons towards itself, thus weakening the H-O bond. 8

Relative Strengths of Oxoacids H 3 PO 4 < H 2 SO 4 < central atom least electronegative least acidic HCl. O 4 most electronegative most acidic 9

Relative Strengths of Oxoacids HCl. O < HCl. O 2 < HCl. O 3 < HCl. O 4 least most 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. 10

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 11

Stability of Anions Other groups that affect the stability of anions: A B Which do you think would be a stronger acid and why? How does the Cl affect the acid? How does it affect the anion? 12

Acid Strength Examine the following organic carboxylic acids: CH 3 COOH < CH 2 Cl. COOH < CHCl 2 COOH < CCl 3 COOH 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 13 acidity as shown.

Summary Binary acids: HF < HCl < HBr < HI and H 2 O < H 2 Se < H 2 Te Oxoacids: HIO 4 < HBr. O 4 < HCl. O 4 H 3 PO 4 < H 2 SO 4 < HCl. O 4 Increase of O increases acidity: HCl. O < HCl. O 2 < HCl. O 3 < HCl. O 4 Presence of Electronegative Elements: CH 3 COOH < CH 2 Cl. COOH < CHCl 2 COOH < CCl 3 COOH 14