Acids and Bases
Acids & Bases
 These were introduced in Chapter 4
 Arrhenius:
 Acid = any substance that produces H+ in soution.
 Base = any substance that produces OH- in solution.
Acid
Base
Sour Taste
Bitter Taste
Turn Blue Litmus to Red
Turn Red Litmus to Blue
Attack Most Metals
Slippery, Soapy Feel
Neutralize Bases
Neutralize Acids
Bronsted-Lowery Theory
 Definition of acids and bases focused on the transfer
of a single proton.
 Acid = any substance that donates a proton (H+).
 Base = any substance that accepts a proton (H+).
 Expands the number of acids and bases.
 Not restricted to aqueous solutions.
Hydronium Ion
 In water, HCl dissociates
completely to:
HCl(aq)  H+(aq) + Cl-(aq)
 In essence, this shows that a free
proton is present.
 In reality, this does not ever
happen!
HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq)
Bronsted-Lowery Theory
 As mentioned, acid-base reactions do not have to be
aqueous. NH3(g) + HCl(aq)  NH4+(s) + Cl-(s)
Acid-Base Equilibrium
 In any acid-base equilibrium reaction, both the
forward and the reverse reactions involve the transfer
of a proton.
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
HF(aq) + H2O(l)  H3O+(aq) + OH-(aq)
Acid-Base Equilibrium
 Any two substances on either side of the double arrow
that differ by a proton are called a conjugate acid-base
pair.
 Thus, every acid has a conjugate base made by
removing the proton and every base has a conjugate
acid made by adding a proton.
 What did you notice about the role of water in the
previous two reactions?
Strong Acids
 Strong acids completely ionize, leaving no un
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dissociated molecules in solution.
Only six acids are considered “strong.”
The conjugate base of a strong acid has no base
properties.
Acid strengths sheet shows the order of strengths.
Weaker the acid, the stronger the conjugate base.
HCl(aq) + H2O(aq)  H3O+(aq) + OH-(aq)
Strong Bases
 Strong Bases completely ionize, due to the fact that
they are ionic.
 Only the group 1A and 2A metal hydroxides are
considered to be “strong”.
 All other hydroxides are insoluble in water.
 NaOH(aq)  Na+(aq) + OH-(aq)
Auto-Ionization of Water
 On rare occasion, two water molecules react with each
other.
Auto-Ionization of Water
 Because water is a liquid, this is a heterogeneous
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equilibrium.
Kw =
At 25oC, the value of Kw = 1.0 E-14.
This is called the ion-product constant.
Allows us to calculate an unknown [H3O+] or [OH-].
pH Scale
 Uses a logarithm in base 10, which is different than the
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natural logarithm.
The letter “p” always represents the negative logarithm
in chemistry.
pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14
pH Scale
 A word about significant figures.
 Suppose that the [H3O+] = 2.4 x 10-3 M
 pH = -log(2.4 E-3) = 2.619788…
 2.4 x 10-3 M
pH = 2.62
 Red numbers are the significant digits.
 Blue numbers are exact numbers.
Acidic or Basic?
When the [H3O+] > [OH-], the
solution is acidic.
When the [OH-] > [H3O+], the
solution is basic.
What about pH?
pH of Strong Acids and Bases
 Since these dissociate 100%, these are relatively easy.
 0.10M HCl would produce ____M H3O+
 0.025M NaOH would produce ____M OH 0.030M Ca(OH)2 would produce ____M OH Can also have gram and volume amounts.
 Can also do partial neutralization.
Weak Acids
 A weak acid only partially ionizes.
 Too many to list, but formula generally begins with an
“H”.
 Appendix D1 lists many weak acids.
 Equilibrium constant is called Ka.
 Will require an ICE table to find the pH.
Weak Acids
 General set-up is:
 HA + H2O  H3O+ + AI
C
E
 Appendix D1 used to find the Ka values.
 Problem will look like a quadratic problem, but …
Weak Acids
 Problems may include:
 Finding a pH from starting concentration.
 Finding a Ka value given pH and starting concentration.
 Finding a percent ionization.
Polyprotic Acids
 A polyprotic acid contains two or more acidic protons
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per molecule.
Some common ones include: H3PO4 and H2CO3.
One is also a strong acid: H2SO4.
All ionize in successive steps and have multiple Ka
values.
Intuitively, each H+ is more and more difficult to
remove.
Weak Bases
 A weak base reacts with water, which donates a proton
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to the N atom.
N atom has lone pair to bond the proton.
Table D2 lists some examples of the weak bases.
The equilibrium constant is called Kb.
Will require an ICE table for most problems.
Weak Bases
 Generally, the set-up is:
B + H2O  BH+ + OHI
C
E
 Appendix D2 has Kb values.
 Will look like a quadratic, but…
Weak Bases
 Problems may include:
 Finding a pH from starting concentration.
 Finding a Ka value given pH and starting concentration.
 Finding a percent ionization.
Conjugate Pairs
 NH3 is a base and NH4+ is its conjugate acid.
NH3 + H2O  NH4+ + OHNH4+ + H2O  H3O+ + NH3
 What if we need the Ka for NH4+?
Kb = 1.8 x 10-5
Ka = ???
Conjugate Pairs
 Like a pH = -log[H+], there is also a pKa and a pKb.
 For any conjugate pair, the pKa + pKb = __.
 Can also convert a pKa back to a Ka.
Salts
 Solutions of NaF are always basic and solutions of
NH4Cl are always acidic.
 On the other hand, a solution of NaCl is neutral.
 Why???
 Predicting and calculating a pH of various salts based
on the hydrolysis of certain ions.
Salts
1.
2.
3.
4.
5.
Salts containing the cation of a _________ and an
anion of a _________ will be neutral.
Salts containing the cation of a _________ and an
anion of a _________ will be __________.
Salts containing the cation of a _________ and an
anion of a __________ will be _________.
Salts containing the cation of a _________ and an
anion of a _________ can be ____________.
Transition metal and Al ions of high charge will be
____________.
Structure and Strength
 Acid strength can be predicted based on
two factors.
 Factor #1: Degree of polarity of the H-X
bond. This depends on E.N. difference.
 Rule:
 Factor #2: Bond Strength
 Ch. 8: HI = 299kJ, HBr = 366kJ, HCl =
431kJ, and HF = 567kJ
 Rule:
Structure and Strength
 Binary Acids – hydrogen is bonded to only one other
element.
 Bond strength (factor #2) is most important.
 Which group 7A acid is the strongest? Weakest?
 General Rule =
Structure and Strength
 Oxy-acids are very common.
 General structure is:
H–O–X
 For all of these, factor #1 is dominant.
1. For oxy-acids where the number of
oxygen atoms differs and X is the
same.
2. For oxy-acids where the number of
oxygen atoms are identical, but the X
atom varies.
Structure and Strength
 Carboxylic acids are organic acids
containing the –COOH group.
 Acidic due to stability of –COO- group.
 Increasing strength by adding
electronegative atoms.
 CH3COOH vs. CF3COOH