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Lecture 1

CHEM 112 Lecture Notes - Lecture 1: Chalcogen, Acid Strength, Conjugate Acid


Department
Chemistry
Course Code
CHEM 112
Professor
John Carran
Lecture
1

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Week 22 Lecture 1
Acid Base Chemistry
-The stronger the acid, the weaker the conjugate abse
-the stronger base, the weaker its conjugate acid
Ka x Kb = Kw
logKa +logKb = log Kw
pKa + pKb = pKw
-For strong acids, such as HCl, HNO3 , and HClO4 , their proton donating ability is greater than H3O+ ,
such that the proton transfer to water is considered to be complete, leaving the solution with essentially
only H3O+ and the conjugate base of the acid.
-These acids are said to be leveled in water to the strength of H3O+ , so that these are all considered to
have the same strength in water.
-Similarly, for bases such as NH2 - , CH3 - , and O2- , their proton accepting ability is greater than OH- ,
such that proton transfer from water to the base is considered to be complete, leaving a solution with
only OH- and the conjugate acid of the base.
Note: in the gas phase, in the absence of solvent, the acid or base strength of these species can be
determined, because there is no leveling effect.
Molecular Structures and Acid Strengths
-Acid strength must be related to the strength of the bond between the proton which is being removed
and the atom to which it is attached.
-the acid-base reaction in water is an equilibrium, so we must consider the ability of the conjugate base
to accept a proton and form a new bond with the proton
-the reaction takes place in water, which not only acts as a proton acceptor and proton donor, but
solvates the acid and its conjugate base
-For the overall process of HA(aq) + H2O(l) → H3O+ (aq) + A- (aq), the enthalpy change is:
DH = -DHsolv(HA) + DHb (H-A) + I(H) - Ea (A) + DHhyd(H+ ) + DHhyd(A- )
Binary Acids
-Binary acids (HnA) consist of hydrogen atoms and one other type of atom (A). Examples include HCl,
H2O, H2 S, and NH3 .
-The common binary acids involve atoms (A) from the p-block or “main group” of the elements in the
periodic table.
-Trends in the acidity of the binary acids (HnA) can be observed by following the nature and properties
of A across a row (period) or down a column (group).
-Acid strengths of binary acids across a period correlate with electron affinities.
-By using an atom A from another period, we find that the pKa does not follow the trend between the
pKa and the electronegativity difference. Clearly another factor is important in this case.
-Acid strengths of binary acids across a period correlate with electron affinities, while acids strengths
down a group correlate with bond strength.
-the more polar the hydrogen and group bond the stronger the acid
-this effect is dominant for acids of the same period
-the weaker the H-A bond the stronger the acid
-this effect is dominant for acids of the same group
-ex. acids from group 16 elements have the H-A bond dissociation decrease so the acid strength
increases and this can be seen in the pKa values
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