MHS  AP Chemistry
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Period ____ Date _________
16  Acid-Base Equilibrium
#2: STRENGTHS OF
List the 7 strong acids. These give some clues to the strengths of acids.
HCl
HBr
HI
HNO3
H2SO4
HClO3
ACIDS
HClO4
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:
F
Cl
HF
HCl
HBr
I
Br
I
Rank the halogens in order of increasing Coulombic attraction:
Circle the acids that are on your strong acid list:
Br
Cl
F
HI
HBr HI
For “binary acids”, the greater the size or the weaker the Coulombic attraction, the
Rank the acids in order of increasing strength:
HF
HCl
Rule:
stronger the acid.
Consider the acids, H2O, H2S, H2Se and H2Te.
Rank these acids in order of increasing strength:
H2O H2S
H2Se H2Te
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:
I
Br
Cl
HOI HOBr HOCl
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
Oxidation #:
+1
HClO2
HClO3
+3
+5
HClO4
+7
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:
HOCl HClO2 HClO3 HClO4
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
HClO4