NAME _________________________________
PREDICTING THE STRENGTH OF ACIDS AND BASES
I RECOGNIZED SOME DIFFICULTY WITH ERIQ PRIOR TO OUR LAST QUIZ … SO, I’VE REDESIGNED AND THIS IS A SUPPLEMENT FOR YOUR NOTES
First, understand, that NO STRONGER BASE THAN OH- CAN EXIST IN WATER (per the values of this
table)… BUT NATURE IS MORE COMPLICATED THAN THAT….So…
Each of these is an accurate statement …and we each like certain concepts couched with a particular parlance.
So, pick your personal poison … Which do you prefer?
The stronger the original parent acid, the more stable the conjugate base, and thus the weaker, the base.
The larger the Ka, the weaker the base.
The larger the pKa of the conjugate acid, the stronger the base…. (the more likely it will accept and bind up H+)
The weaker the acid, the stronger the conjugate base.
Acid Name
Hydroiodic
Hydrobromic
Perchloric
Hydrochloric
Chloric
Sulfuric (1)
Nitric
Hydronium ion
Iodic
Chlorous
Hydrofluoric
Nitrous
Formic (Methanoic)
Lactic
Hydrosulfuric
Hypochlorus
Hydrocyanic
Boric (1)
Ammonium ion
Phenol
Water
Acid Form:
HA
HI
HBr
HClO4
HCl
HClO3
H2SO4
HNO3
H3O+
HIO3
HClO2
HF
HNO2
HCOOH
CH3CH(OH)CO2H
H2S
HClO
HCN
H3BO3
NH4+
C6H5OH
H2O
Conjugate Base:
AIBrClO4ClClO3HSO4NO3
H2O
IO3ClO2FNO2HCOOCH3CH(OH)CO2HSClOCNH2BO3NH3
C6H5OOH-
Ka
pKa
Ka >1
while pKa <1
1
1.6 x 10-1
1.1 x 10-2
7.2 x 10-4
4.0 x 10-4
1.77 x 10-4
1.38 x 10-4
9.1 x 10-8
3.0 x 10-8
6.17 x 10-10
5.8 x 10-10
5.6 x 10-10
1.6 x 10-10
1.0 x 10-14
0.0
0.80
1.96
3.14
3.39
3.75
3.86
7.04
7.53
9.21
9.23
9.25
9.80
14.0
1) Use the data to arrange the following species according to their strength as bases (from weakest to strongest)
H2O, F-, Cl-, NO2-, and CN-
* Cl-, H2O, F-, NO2-, CNARE YOU USING YOUR PERSONAL RULE?
2) Use the data to arrange the following species according to their strength as bases (from weakest to strongest)
HS-, HCOO- , F-, Br-, NH3
* Br-, F-, HCOO-, HS-, NH3
ARE YOU USING YOUR PERSONAL RULE?
3) Use the data to arrange the following species according to their strength as bases (from weakest to strongest)
ClO2-, ClO4-, IO3-, NO2-, H2O
* ClO4-, H2O, IO3-, ClO2-, NO2ARE YOU USING YOUR PERSONAL RULE?
Keep going….
4) According to the data, which acid has the strongest conjugate base?
1) HIO3
2) HNO2
3) NH4+
4) HI
ans:*3 the weakest acid …has the strongest base
5) Given the following data, identify the species which produces the strongest conjugate base.
1) H – F
pKa = 3.14
2) H2N – H
pKa = 38
3) CF3SO3 – H
pKa = -14
4) CH3 – H
pKa = 48
QUESTIONS 6 – 10 FOCUS UPON THE USE OF ERIQ …IN TERMS OF ELEMENT AND INDUCTION
6) Consider the concept of family trend and strength of an acid … Which of the following is the strongest acid?
1)
2)
3)
4)
H2O
H2S
H2Se
H2Te
ans: *4
Defend your reasoning: *In a family, the element, having the largest radius, bonded to the H+, yields the
most stable conjugate base (hence the weaker base …. tending not to accept a H+), meaning that
it is the strongest acid. In family trends, the radius of the Element bonded to the H+ dominates. (setting)
Tellurium has the greatest number of principal energy levels, and is thus, the largest in radius (proof)
7) Consider the concept of period trend and strength of an acid … Which of the following is the strongest acid?
1) CH4
2) NH3
3) H2O
4) HF
ans: *4
Defend your reasoning: *Given central atoms in a common period of the periodic table, the greater the
electronegativity, of the anion, the stronger the resulting acid.(setting). Fluorine has the greatest
electronegativity value (3.98 to 4.0).
8) Given:
Which represents the stronger acid? Why (use ERIQ)
* propanol … this is a period trend issue.
In butane the H is bonded to C, while in propanol the H is bonded to the more electronegative O atom.
In a period trend the Element of greater electronegativity impacts the acidity of the molecule to the
greatest extent.
9) Given:
Which represents the stronger acid? Why? (Use ERIQ)
*trichloroacetic acid, is the stronger acid. The
conjugate base is highly stabilized due to induction …as the highly electronegative Cl atoms draw
electrons via the sigma (single bonds) across the molecule, stabilizing the conjugate base to a greater
extent compared to the effect in acetic (ethanoic) acid
10) List the molecules (a, b, and c) in order of weakest to strongest acid. Defend your thinking.
O–H
Order weakest to strongest: * c < a < b (argument)
a)
The dominant factor is the
inductive effect. Choice “b” is the strongest acid, because the conjugate base will be
CH3
the most stable, due to the draw of electrons towards the oxygen of the O-H group
O–H
and the nitrogen of the NO2 group… and the more stable the conjugate base, the
b)
stronger the acid. Choice “a” is not as strong an acid, due to a slightly lesser
NO2
O–H
c)
inductive effect between the O-H and the CH3 groups.
11) Using some portion of ERIQ, explain the differences in pKa values.
http://www.laney.edu/wp/corlett/files/2012/01/ch03.pdf
*The smaller the pKa the stronger the acid. (setting) The induction effect will help to explain the
differences. The fourth molecule has the greatest inductive (induction) effect, due to the highly electronegative Cl atoms, spreading the electron cloud over the molecule more effectively and thus creating
a far more stable conjugate base … and the Golden Rule is the more stable the conjugate base, the
stronger the acid. Hence each of the four molecules undergoes a progressively stronger inductive
effect (from left to right) and this therefore indicates a stronger acid, thus a lower pKa value.