Transcript Chapter 18 “Acids, Bases and Salts”

Chapter 18 “Acids, Bases and
Salts”
Chapter 18 OBJECTIVES
• State and use the Arrhenius and Brønsted-Lowry
definitions of acids and bases.
• Identify common physical and chemical
properties of acids and bases.
• Describe dissociation constants and explain
what they indicate about acids and bases.
• Explain what most ‘acidic hydrogen atoms’ have
in common.
• Explain what most bases have in common.
• Describe nomenclature of acids and bases.
18-1 Defining Acids and Bases
• What are some properties of acids
and bases? (Let’s derive some.)
– Taste (Don’t do this at home!)
– Touch (Or this!)
– Reactions with metals
– Electrical conductivity
– Reactions with “Indicators”
– Neutralization
18-1 Defining Acids and Bases
• Acids = substances that ionize in
aqueous solution to form hydrogen
ions (H+)
• Bases = substances that accept H+
ions, producing OH-
The Arrhenius Definitions
• Acid – a substance that dissociates in
water to produce hydrogen ions (H+).
• Base – a substance that dissociates in
water to produce hydroxide ions (OH-).
• See Fig. 18-6 (page 599).
• Arrhenius acids and bases react
together (neutralize) to form a salt and
water.
• HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
The Brønsted-Lowry Definition
• Arrhenius definition is restrictive
– Applies only to water solutions.
– Does not explain why covalent
molecules are acids (HCl, HBr)
– Does not explain why certain
compounds like NH3 are bases.
• Brønsted-Lowry Definitions
– Acid: a proton (H+) donor.
– Base: a proton acceptor.
The Hydronium Ion
• Protons (H+) do not really exist in water
solutions in this way.
– H+ + H2O → H3O+
• Hydronium Ions (H3O+) are a better
approximation of what occurs.
• Molecules of different acids can ionize to
form a different # of H+ ions
– HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
– HCl and HNO3 are monoprotic acids – 1 H+
ion per mole of acid
– H2SO4 is diprotic acid – 2 H+ ions per mole of
acid
Conjugate Acid-Base Pairs
• The difference between an acid and a
base may be as simple as one H+ ion!
• To emphasize this relationship, chemists
use the terms ‘conjugate acid – conjugate
base’ pairs.
– The term “conjugate” means “joined together.”
• Conjugate Acid-Base Pair is two
compounds that differ by only one H+ ion.
• Examples (Fig. 18-12, page 603).
WARM UP
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What is a proton (H+) donor?
What is a proton acceptor?
What is the conjugate base to HCl?
What is the conjugate acid to OH-?
What is the conjugate base to NH4+?
What is the conjugate acid to
HSO4-?
18-2 Determining The Strengths of
Acids and Bases
• Strong and Weak Acids
– Strong acids easily lose H+ ions, so they are strong electrolytes
(high degree of dissociation).
– Weak acids do not dissociate very much.
• Strong and Weak Bases
– Strong bases (such as compounds with OH-) have high affinity
for H+ ions, and they are strong electrolytes.
– Weak bases react partially with water to form hydroxide ions.
• Use single arrows () to signify strong acids (~100%
dissociation). (HCl)
• Use double arrows (↔) to signify weak acids (low amount of
dissociation). (HC2H3O2)
• Strength of Conjugate Acid-Base Pairs
– The stronger the acid, the weaker its conjugate base.
– The stronger the base, the weaker its conjugate acid.
The Acid Dissociation Constant
• For the reaction HA (aq) + H2O (l) ↔
H3O+ (aq) + A- (aq) we may write an
equilibrium expression:
Keq = [H3O+][A-] / [HA][H2O]
or
Ka = [H3O+][A-] / [HA]
(Why?)
where Ka is the acid dissociation constant.
The larger the Ka, the stronger the acid.
Example
The Base Dissociation Constant
• For the reaction B (aq) + H2O (l) ↔ HB+
(aq) + OH- (aq) we may write an
equilibrium expression:
Keq = [HB+][OH-] / [B][H2O]
or
Kb = [HB+][OH-] / [B]
(Why?)
where Kb is the base dissociation constant.
The larger the Kb, the stronger the base.
Example
Calculating Dissociation Constants
• This is a very easy task once the
concentrations of ions are known.
• Sample problem (p612).
• LET’S TRY #1 AND #2 ON PAGE
613
Acid-Base Properties of Salts
• Salts are strong electrolytes, forming
cations and anions in water.
• Many of these ions are weak
Brønsted-Lowry acids or bases, so
they produce H+ or OH-.
• This is called a ‘salt hydrolysis
reaction.’
WARM UP
1. Determine the acid-base conjugate
pairs for the following reaction:
CO32-(aq) + H2O(l) → HCO3-(aq) + OH-(aq)
2. A weak monoprotic acid of 2.60M is
added to water. At equilibrium the
concentration of H3O+ is 0.34M. What is the
Ka for this acid?
Types of Salt Hydrolysis Reactions
• Salts of Strong Acids & Strong Bases
– Solution is neutral.
– NaOH(aq) + HCl(aq) → NaCl (aq) + H2O(l)
• Salts of Strong Acid & Weak Bases
– Solution is acidic because the NH4+ is a BrønstedLowry acid.
– NH3 (aq) + HCl (aq) ↔ NH4Cl
– NH4+(aq) + H2O(l) ↔ NH3 (aq) + H3O+ (aq)
• Salts of Weak Acids & Strong Bases
– Solution is basic (alkaline).
– 2NaOH (aq) + H2CO3 (aq) ↔ Na2CO3 + H2O
• Salts of Weak Acids & Weak Bases
– Not easily predicted due to the many complex
equilibrium involved.
18-3 Naming and Identifying Acids
and Bases
• Acids have “acidic hydrogens.”
– These have a slight positive charge
while still part of the molecule.
– Binary Acids: Contain hydrogen plus 1
other element Hydrochloric Acid (HCl)
– Oxy Acids: Contain hydrogen, oxygen
and one other element. Examples,
sulfuric acid (H2SO4); nitric acid (HNO3)
– Carboxylic Acids: Acids that are
organic acids and contain the carbon
atom. Example acetic acid (HC2H3O2)
18-3 Naming and Identifying Acids
and Bases
• Bases
– These always contain an unshared pair of
electrons.
– Anions: Many negatively charged ions
function as bases (OH-). Examples sodium
hyroxide (NaOH); calcium hyroxide
(Ca(OH)2).
– Amines: Compounds related to ammonia
and contain a nitrogen atom that has an
unpaired share of electrons.
• Nomenclature (See p619)
Chapter 18 OBJECTIVES
• State and use the Arrhenius and Brønsted-Lowry
definitions of acids and bases.
• Identify common physical and chemical
properties of acids and bases.
• Describe dissociation constants and explain
what they indicate about acids and bases.
• Use experimental data to determine dissociation
constants.
• Explain what most ‘acidic hydrogen atoms’ have
in common.
• Explain what most bases have in common.
• Describe nomenclature of acids and bases.