common household substances that contain acids and bases. vinegar is a dilute solution of acetic...
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Common household substances that contain acids and bases. Vinegar is a dilute solution of acetic acid. Drain cleaners contain strong bases such as sodium hydroxide.
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Models of Acids and Bases
Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion.Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors.
HCl + H2O Cl + H3O+
acid base
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Conjugate Acid/Base Pairs
HA(aq) + H2O(l) H3O+(aq) + A(aq) conj conjacid base acid base
conjugate base: everything that remains of the acid molecule after a proton is lost.
conjugate acid: formed when the proton is transferred to the base.
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Figure 14.1: The reaction of HCl and H2O.
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Base Acid conj. acid conj. base
14.2 Acid Strength
Its equilibrium position lies far to the right. (HNO3)
Yields a weak conjugate base. (NO3)
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Strong Acid:Strong Acid:
Acid Strength(continued)
Its equilibrium lies far to the left. (CH3COOH)
Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO)
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Weak Acid:Weak Acid:
Figure 14.6: (a) A strong acid HA is completely ionized in water. (b) A weak acid HB exists mostly as undissociated HB molecules in water. Note that the water molecules are not shown in this figure.
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Figure 14.4: Graphic representation of the behavior of acids of different strengths in aqueous solution. (a) A strong acid. (b) A weak acid.
Polyprotic Acids
. . . can furnish more than one proton (H+) to the solution.
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H CO H HCO ( )
HCO H CO ( )
2 3 3 a
3 32
a
1
2
K
K
Oxyacids and Organic Acids Oxyacids are acids where the proton is
attached to a oxygen. Organic Acids are acids with a carbon
backbone, with a carboxyl group attached.
Sulfuric Acid
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Acid Dissociation Constant (Ka)
HA(aq) + H2O(l) H3O+(aq) + A(aq)
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Ka3H O A
HA
H A
HA
Monoprotic Acids
Have one acidic proton. Notice that strong acids do not have a
Ka!
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Water as an Acid and a Base
Water is amphoteric (it can behave either as an acid or a base).
H2O + H2O H3O+ + OH
conj conj acid base acid base
Kw = 1 1014 at 25°C
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For any aqueous solution at 25°C the product of [H+] and [OH-] must equal 1 x 10-14.
So this means that a soln: [H+] = [OH-] is neutral [H+] > [OH-] is acidic [H+] < [OH-] is basic
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pH and sig figs
[H+] = 1.0 x 10-9
pH = 9.00
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2 sig figs
2 decimal places
Figure 14.9: pH meters are used to measure acidity.
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Solving Weak Acid Equilibrium Problems
List major species in solution.
Choose species that can produce H+ and write reactions.
Based on K values, decide on dominant equilibrium.
Write equilibrium expression for dominant equilibrium.
List initial concentrations in dominant equilibrium.
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Solving Weak Acid Equilibrium Problems (continued) Define change at equilibrium (as “x”).
Write equilibrium concentrations in terms of x.
Substitute equilibrium concentrations into equilibrium expression.
Solve for x the “easy way.”
Verify assumptions using 5% rule.
Calculate [H+] and pH.
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How does initial concentration affect percent
dissociation?
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Go to page 674
Figure 14.10: The effect of dilution on the percent dissociation and [H+] of a weak acid solution.
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Percent Dissociation (Ionization)
% dissociationamount dissociated( )initial concentration( )
100% MM
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Bases“Strong” and “weak” are used in the same sense for bases as for acids.
strong = complete dissociation (hydroxide ion supplied to solution)
NaOH(s) Na+(aq) + OH(aq)
weak = very little dissociation (or reaction with water)H3CNH2(aq) + H2O(l) H3CNH3
+(aq) + OH(aq)
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Acid-Base Properties of Salts
Cation AnionAcidic
or Basic Exampleneutral neutral neutral NaCl
neutral conj base ofweak acid
basic NaF
conj acid ofweak base
neutral acidic NH4Cl
conj acid ofweak base
conj base ofweak acid
depends onKa & Kb
values
Al2(SO4)3
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Structure and Acid-Base Properties
Two factors for acidity in binary compounds:
Bond Polarity (high is good)
Bond Strength (low is good)
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Oxides
Acidic Oxides (Acid Anhydrides): OX bond is strong and covalent.
SO2, NO2, CrO3
Basic Oxides (Basic Anhydrides): OX bond is ionic.
K2O, CaO
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Figure 14.11: The effect of the number of attached oxygens on the O—H bond in a series of chlorine oxyacids.
Lewis Acids and Bases
Lewis Acid: electron pair acceptor Lewis Base: electron pair donor
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Al3+ + 6H
OH
Al OH
H6
3+
Figure 14.12: Reaction of BF3 with NH3.
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