MHS  AP Chemistry
Name____________________________
Period ____ Date _________
16  Acid-Base Equilibria
#2: STRENGTHS OF
ACIDS
Overview:
There are three trends in acids that you should know and be able to explain.
 HF, HCl, HBr, HI or H2O, H2S, H2Se, H2Te
 HOCl, HOBr, HOI
 HOCl, HClO2, HClO3, HClO4
Background Ideas:
There are two kinds of acids, “binary acids” and “oxyacids”. Which are which in your strong acid list?
“Electronegativity” is a number that indicates how well an element holds onto its electrons. Note the patterns
of the electronegativities of the following elements.
H
He
2.1
--N
O
F
Ne
3.0 3.5 4.0 --S
Cl Ar
2.5 3.0 --Br Kr
2.8 --Acids all contain a bond between a hydrogen atom and some other atom. Note that for “oxyacids” the hydrogen
is attached to the oxygen.
The Big Idea:
When an acid dissociates, the proton (H+) has the “choice” of sharing electrons with an atom in the acid or
sharing electrons with a passing water molecule. The less that the proton “sees” the electrons in the acid, the
more likely the proton will dissociate from the acid… the stronger the acid will be.
Trend #1: Binary Acids: HF, HCl, HBr, HI
Rank the halogens in order of increasing size:
Rank the halogens in order of increasing Coulombic attraction:
Circle the acids that are on your strong acid list:
HF
HCl
HBr
HI
Rank the acids in order of increasing strength:
Rule: For “binary acids”, the greater the ________________ or the weaker the _________________, the
stronger the acid.
Consider the acids, H2O, H2S, H2Se and H2Te.
Rank these acids in order of increasing strength:
Trend #2: Oxyacids: HOCl, HOBr, HOI
Consider the structural formula for the acid, HOCl.
For oxyacids with the same number of oxygens, the more electronegative the halogen, the less the proton
(H+) will “see” the electron pair.
Rank the elements, Cl, Br, and I in increasing order of electronegativity:
Rank the three acids in order of increasing strength:
Rule: For oxyacids with the same number of oxygens, the more electronegative the halogen, the stronger the
acid will be.
Trend #3: Oxyacids: HOCl, HClO2, HClO3, HClO4
Draw the structural formulas for these acids:
HOCl
HClO2
HClO3
HClO4
Identify the oxidation numbers of the chlorine atom in each structure above.
Increasing oxidation number causes the chlorine to increase in electronegativity. The chlorine pulls more
strongly on the electrons in the oxygen atoms, weakening the bond between the hydrogen and the oxygen.
In which acid will the H+ be most likely to attach to a passing water molecule and dissociate? _______
[Hint: Which acid(s) are on your strong acid list?]
Rank the four acids in order of increasing strength:
Rule: The more oxygens in an oxyacid, the stronger the acid.
Write notes to yourself about acid strength:
 HOCl, HOBr, HOI

HOCl, HClO2, HClO3, HClO4

HF, HCl, HBr, HI or H2O, H2S, H2Se, H2Te