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Acid‐Base
Acid
Base Theories
Theories
Overview
 Acids and bases ‐ properties
 Arrhenius theory
y
 Bronsted‐Lowry theory
 Strong vs. Weak acids/bases
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Acids and Bases: Review
Acid
Base
Conducts electricity?
Litmus colour
Tastes
Feels
pH value
Arrhenius theory of acids and bases
 Early theory; Svante Arrhenius
 Acids are substances that release hydrogen ions, H+, when dissolved
HCl(g)  H+(aq) + Cl‐(aq)
HNO3 (aq)  H+ (aq) + NO3‐(aq)
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 Bases are substances that produce hydroxide ions, OH‐, when dissolved
,
Ba(OH)2 (s)  Ba2+ + 2 OH‐ (aq)
 The neutralization reaction between an acid and a base is the reaction of the H+ and the OH‐:
Overall:
HCl(aq) + NaOH (aq)  H2O (l) + NaCl (aq)
Total ionic: H+ (aq) + Cl‐ (aq) + Na+ (aq)+ OH‐ (aq) 
H2O (l) + Na+ (aq) + Cl‐ (aq)
Net ionic: H+ (aq) + OH‐ (aq)  H2O (l)
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The hydronium ion
 Due to polarity of water, unlikely that an H+ ion could exist in aqueous solution
q
 Would instead bond strongly with water (become “hydrated”)
H+(aq) + H2O (aq)  H3O+ (aq)
 H3O+ is called the hydronium ion
 Explains the formation of acidic solution as a reaction of the acid with water:
HCl(g) + H2O  H3O+ (aq) + Cl‐(aq)
Revised Arrhenius theory: An acid increases the amount of H3O+ in a solution.
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 A neutralization reaction is the reaction of H3O+ and OH‐:
Overall:
Overall: HCl(g) + H2O 
HCl(g) + H
O  H3O+ (aq) + Cl
(aq) + Cl‐(aq)
Total ionic: H3O+ (aq) + Cl‐(aq) + Na+ (aq)+ OH‐ (aq) 
2 H2O (l) + Na+ (aq) + Cl‐ (aq)
Net ionic: H3O+ (aq) + OH‐ (aq)2 H2O (l) Problems with Arrhenius theory
 Two problems:
 Salts
 Solvents
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Salt problem
 According to Arrhenius, only substances containing H+ or OH‐
g
can be acids or bases.
 All other ionic compounds should be neutral
 NOT THE CASE
 Na2CO3 (aq), NaHPO4 (aq), and NH3 (aq) are basic, but do not contain OH‐
 Al(NO3)3 is an acid, but contains no H+
Solvent problem
 According to Arrhenius, acidity/basicity is a p p y
property of the solute
 Therefore, acids should ALWAYS produce H+, and bases OH‐
 NOT THE CASE
 Some compounds will not ionize in organic (nonpolar) solvents (e.g., HCl)
→th
→the solvent plays a role in acid‐base properties
l t l
l i
id b
ti
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Brønsted‐Lowry theory
 Defines an acid as a substance that can donate H+
 A base as a substance that can accept H
p +
 Acid/base reactions are H+ exchange reactions (proton transfer)
Example 1:
 NaOH is a base by Arrhenius’ definition.
 NaOH is also a base by B‐L definition, since:
NaOH (s)  Na+ (aq) + OH‐ (aq), and OH‐ can act as a proton acceptor:
+
q)
q) 2 H2O
OH‐ ((aq) + H
3O ((aq) 
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Example 2:
 NH3 is not an Arrhenius base, but it is a B‐L base:
NH3 (g) + H2O (l) NH4+(aq) + OH‐(aq) According to B‐L theory, an acid can behave as an acid only in the presence of a base that is willing to accept an H+.
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Conjugate acid‐base pairs
 Every acid has a corresponding base, and vice versa.
 These corresponding acid‐base pairs are called p
g
p
conjugate acids and conjugate bases.
HCl (g) + H2O (l)  H3O+ (aq) + Cl‐ (aq)
NH3 (g) + H2O (l) OH‐(aq) + NH4+(aq)
 Explains why some salts are acidic/basic
 Example: Carbonate salts
Dissociation: Na2CO3 (s)  Na+ (aq) + CO32‐ (aq)
A/B reaction: CO32‐ (aq )+ H2O (l)  OH‐(aq) + HCO3‐(aq)
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 Some substances can act as B‐L acids in some reactions, and as B‐L bases in others
 i.e./ they can either lose a proton, or donate a proton
 “amphiprotic”
 Example: Bicarbonate ion, HCO3‐
 Another example: Water
Summary:
Theories of Acids and Bases
Theory
Definition
Arrheni s
Arrhenius
Brønsted Lo r
Brønsted‐Lowry
Acid
Base
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Strong and weak acids
 Strong acids will ionize completely when dissolved
 Weak acids will only partially ionize when dissolved
yp
y
 Representing an acid as HA, the dissociation reaction is:
HA (aq) + H2O (l)  H3O+ (aq) + A‐(aq)
Strong acids: Include hydrochloric acid, sulphuric acid, & nitric acid
Weak acids: Acetic acid
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Strong and weak bases
 All Group I hydroxides and some Group II hydroxides are strong bases
y
g
 They dissociate completely into their ions
 Weak bases dissociate partially.
 Example: Mg(OH)2
 Bronsted‐Lowry
Bronsted Lowry bases are also all weak bases.
bases are also all weak bases.
 e.g., NH3
Why is something an acid or base?
 Due to the electronegativities of the solute atoms and strengths of intramolecular bonds.
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