definitions of acids and bases green damjii chapter 8 section 1 copyright the mcgraw-hill...
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4.3 An Arrhenius acid is a substance that produces H + (H 3 O + ) in water Complete these reactions: Nitric acid plus water yields Acetic acid plus water yieldsTRANSCRIPT
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Definitions ofAcids and Bases
Green & Damjii – Chapter 8 – Section 1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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4.3
There are multiple definitions for the terms ‘acid’ and ‘base’.
You will need to be familiar with three (3) of them:• Arrhenius acids and bases• Bronsted-Lowry acids and bases• Lewis acids and bases
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4.3
An Arrhenius acid is a substance that produces H+ (H3O+) in water
Complete these reactions:
• Nitric acid plus water yields
• Acetic acid plus water yields
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An Arrhenius base is a substance that produces OH- in water
4.3
Complete these reactions:• sodium hydroxide plus water yields
• ammonia plus water yields
• baking soda plus water yields
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Why didn’t chemists just stick with the Arrhenius definitions of acids and bases?
• not all acid base reactions occur in an aqueous solvent
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A Brønsted-Lowry acid is a proton donor• H atom attached by a bond that is easily broken
A Brønsted-Lowry base is a proton acceptor• contains a non-bonding electron pair – usually the
lone pair on an O or N atom
acidbase acid base
15.1
acid conjugate basebase conjugate
acid
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15.4
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Conjugate acid-base pairs:• The conjugate base of a strong acid is weak (has no measurable strength).
ex: HCl (strong acid): Cl− (weak conjugate base)• The conjugate acid of a strong base is weak (has no measurable strength)..
ex: NaOH (strong base): H2O (weak conjugate acid)
• The conjugate base of a weak acid is strong.
ex: CH3COOH (weak acid): CH3COO − (strong conjugate base)
• The conjugate acid of a weak base is strong.
ex: NH3 (weak base): NH4 + (strong conjugate acid)
15.4
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O
H
H + O
H
H O
H
H H OH-+[ ] +
Acid-Base Properties of Water
H2O (l) H+ (aq) + OH- (aq)
H2O + H2O H3O+ + OH-
acid conjugate base
base conjugate acid
15.2
referred to as the autoionization of water
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Conjugate acid-base pairs:
• H3O+ is the strongest acid that can exist in aqueous solution.
ex: H2O (weak base): H3O+ (strong conjugate acid)
• The OH- ion is the strongest base that can exist in aqueous solution.
ex: H2O (weak acid): OH- (strong conjugate base)
NOTE: Substances – like water – that can both accept and donate a proton are called amphiprotic.
15.4
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Identify the Bronsted-Lowry conjugate acid base pairs in each reaction:
CH3COOH(aq) + H2O (l) ⇌ H3O+(aq) + CH3COO−
(aq)
NH4
+ + NH2− 2 NH3 (l)
H2SO4 (aq) H+(aq) + HSO4
− (aq) ⇌ 2 H+(aq) + SO4
2− (aq)
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Why didn’t chemists just stick with the Bronsted-Lowry definitions of acids and bases?
Lewis recognized the special nature of the bond between a base and the proton it accepts…
• dative bond (aka coordinate covalent bond)• both of the electrons come from the base • proton is not contributing any electrons to the bond
(it doesn’t have any!)• these bonds are indicated by an arrow rather than a line
arrow points in the direction the electrons are donated• these covalent bonds are identical to other covalent bonds
So – acid base reactions now include reactions that involve the formation of a dative covalent bond.
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A Lewis acid is a substance that can accept a pair of electrons
A Lewis base is a substance that can donate a pair of electrons
H+ H O H••••
+ OH-••••
••acid base
N H••
H
H
H+ +
acid base 15.12
N H
H
H
H+
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Lewis acids include all Bronsted-Lowry acids (which include all Arrhenius acids)… but the term is typically used just for acids that are not already included in the other definitions.
EXs:
• boron trifluoride (see next slide)
• aluminum chloride
• any species that can accept an electron pair into its incomplete valence shell …
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Lewis Acids and Bases
N H••
H
H
acid base
F B
F
F
+ F B
F
F
N H
H
H
No protons donated or accepted – just electron pairs !
Identify the dative bond ! Mark it with an arrow!
15.12
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Lewis bases are any species with a non-bonding electron pair
EXs:
• molecules that are NOT hydrides of group 3 &/or 4 elements – generally containing an element with lone pair electrons
• anions
• includes ligands that combine with transition metal ions to form complex ions (we learned about this in Unit 2)
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There are several metal hydroxides that are amphoteric – • they behave as both acids and bases.
Show how zinc hydroxide and/or aluminum hydroxide
• can act as a base (what type? Lewis)
• can act as an acid (what type? Lewis )