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

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 group. 2

3. 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

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

7. Relative Strengths of Oxoacids
An oxoacid is an acid that contains H, O and one other element.
e.g. HNO H2SO HClO3
The acidity is dependent on…
1) Electronegativity of the central atom. WHY?
2) The number of oxygen atoms in the acid.
WHY?

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

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. 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

12. 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?

13. 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.

14. 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