Transcript Chapter 15 Acids and Bases

RNA uses amino-acids to
build proteins/enzymes
Digestive Acids help to break down food
into reusable molecular fragments
Acids and
Bases
It is the acids in citrus fruits
that give them the sour taste
and allows the fruit to stay in a
state of preservation till
germination
Properties of Acids
 sour taste
 react with active metals (Al, Zn, Fe), but not Cu, Ag, or Au
2 Al(s) + 6 HCl(aq)  2 AlCl3(aq) + 3 H2(g)
 Corrosive
 react with carbonates, producing CO2
 marble, baking soda, chalk, limestone
CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + CO2(g) + H2O(l)
 change color of vegetable dyes
 blue litmus turns red
 react with bases to form ionic salts
HCl(aq) + NaOH(aq)NaCl(aq) + H2O(l)
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Common Acids
Chemical Name
Formula
Uses
Strength
Nitric Acid
HNO3
explosive, fertilizer, dye, glue
Strong
Sulfuric Acid
H2SO4
Hydrochloric Acid
HCl
Phosphoric Acid
H3PO4
Acetic Acid
HC2H3O2
explosive, fertilizer, dye, glue,
batteries
metal cleaning, food prep, ore
refining, stomach acid
fertilizer, plastics & rubber,
food preservation
plastics & rubber, food
preservation, Vinegar
Hydrofluoric Acid
HF
metal cleaning, glass etching
Weak
Carbonic Acid
H2CO3
soda water
Weak
Boric Acid
H3BO3
eye wash
Weak
Strong
Strong
Moderate
Weak
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Structure of Acids
 binary acids have acid hydrogens attached to a
nonmetal atom
 HCl, HF
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Structure of Acids
 oxy acids have acid hydrogens attached to an oxygen
atom
 H2SO4, HNO3
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Structure of Acids
 carboxylic acids have
COOH group
 HC2H3O2, H3C6H5O7
 only the first H in the
formula is acidic
 the H is on the COOH
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Properties of Bases
 also known as alkalis
 taste bitter
 alkaloids = plant product that is alkaline
 often poisonous
 solutions feel slippery
 change color of vegetable dyes
 different color than acid
 red litmus turns blue
 react with acids to form ionic salts
 Neutralization
HCl(aq) + NaOH(aq)NaCl(aq) + H2O(l)
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Common Bases
Chemical
Name
sodium
hydroxide
potassium
hydroxide
calcium
hydroxide
sodium
bicarbonate
magnesium
hydroxide
ammonium
hydroxide
Formula
NaOH
Common
Name
lye,
caustic soda
Uses
soap, plastic,
petrol refining
soap, cotton,
electroplating
Strength
Strong
KOH
caustic potash
Strong
Ca(OH)2
slaked lime
cement
Strong
NaHCO3
baking soda
cooking, antacid
Weak
Mg(OH)2
milk of
magnesia
antacid
Weak
NH4OH,
{NH3(aq)}
ammonia
water
detergent,
fertilizer,
explosives, fibers
Weak
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Structure of Bases
 most ionic bases contain OH- ions
 NaOH, Ca(OH)2
 some contain CO32- ions
 CaCO3 NaHCO3
 molecular bases contain structures that react with H+
 mostly amine groups
Amino acids have a base at one end and an acid at
the other, neighboring amino acids can neutralize to
form a polypeptide
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Indicators
 chemicals which change color depending on the acidity/basicity
 many vegetable dyes are indicators
 anthocyanins
 litmus
 from Spanish moss
 red in acid, blue in base
 phenolphthalein
 found in laxatives
 red in base, colorless in acid
Anthocyanins give these pansies their dark purple pigmentation and are
the pigment in red cabbage that is so sensitive to acidity
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acids and bases: Arrhenius Theory
 bases dissociate in water to produce OH- ions and cations
 ionic substances dissociate in water
NaOH(aq) → Na+(aq) + OH–(aq)
 acids ionize in water to produce H+ ions and anions
HCl(aq) → H+(aq) + Cl–(aq)
HC2H3O2(aq)
H+(aq) + C2H3O2–(aq)
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Arrhenius Theory
HCl ionizes in water
producing H+ and Cl– ions
NaOH dissociates in water
producing Na+ and OH– ions
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Hydronium Ion
 the H+ ions produced by the acid are so reactive they cannot exist in
water
 H+ ions are protons
 instead, they react with a water molecule(s) to produce complex ions,
mainly hydronium ion, H3O+
H+ + H2O  H3O+ ≅ H+(aq)
 there are also minor amounts of H+ with multiple water molecules,
H(H2O)n+
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Arrhenius Acid-Base Reactions
 the H+ from the acid combines with the OH- from the base to
make a molecule of H2O
 it is often helpful to think of H2O as H-OH
 the cation from the base combines with the anion from the acid
to make a salt
acid + base → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
H+(aq)+Cl-(aq)+Na+(aq)+OH-(aq)Na+(aq)+Cl-(aq)+H2O(l)
H+(aq) + OH-(aq) H2O(l)
 All acid base reactions have this same net ionic equation in the
Arrhenius idea of the acid and base
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Limitations of the Arrhenius Theory
 does not explain why molecular substances, like NH3, dissolve in water
to form basic solutions – even though they do not contain OH– ions
 does not explain how some ionic compounds, like Na2CO3 or Na2O,
dissolve in water to form basic solutions – even though they do not
contain OH– ions
 does not explain why molecular substances, like CO2, dissolve in water
to form acidic solutions – even though they do not contain H+ ions
 does not explain acid-base reactions that take place outside aqueous
solution
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Acids and bases: Brønsted-Lowry






in a Brønsted-Lowry Acid-Base reaction, an H+ is transferred
does not have to take place in aqueous solution
broader definition than Arrhenius
An acid is H+ donor, base is H+ acceptor
base structure must contain an atom with an unshared pair of
electrons
in an acid-base reaction, the acid molecule gives an H+ to the
base molecule
H–A + :B  :A– + H–B+
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Brønsted-Lowry Acids
 Brønsted-Lowry acids are H+ donors
 any material that has H can potentially be a Brønsted-Lowry acid
 because of the molecular structure, often one H in the molecule is easier
to transfer than others
 HCl(aq) is acidic because HCl transfers an H+ to H2O, forming H3O+
ions
 water acts as base, accepting H+
HCl(aq) + H2O(l) → Cl–(aq) + H3O+(aq)
Acid
base
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Brønsted-Lowry Bases
 Brønsted-Lowry bases are H+ acceptors
 any material that has atoms with lone pairs can potentially be a BrønstedLowry base
 because of the molecular structure, often one atom in the molecule is
more willing to accept H+ transfer than others
 NH3(aq) is basic because NH3 accepts an H+ from H2O, forming OH–
(aq)
 water acts as acid, donating H+
NH3(aq) + H2O(l)  NH4+(aq) + OH–(aq)
base
acid
Tro, Chemistry: A Molecular
Approach
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Amphoteric Substances
 amphoteric substances can act as either an acid or a base
 have both transferable H and atom with lone pair
Example
 water acts as base, accepting H+ from HCl
HCl(aq) + H2O(l) → Cl–(aq) + H3O+(aq)
 water acts as acid, donating H+ to NH3
NH3(aq) + H2O(l) → NH4+(aq) + OH–(aq)
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Brønsted-Lowry: Acid-Base Reactions
 one of the advantages of Brønsted-Lowry theory is that it allows
reactions to be reversible
H–A + :B
:A– + H–B+
 the original base has an extra H+ after the reaction – so it will act as an
acid in the reverse process
 and the original acid has a lone pair of electrons after the reaction – so
it will act as a base in the reverse process
:A– + H–B+
H–A + :B
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Conjugate Pairs
 In a Brønsted-Lowry Acid-Base reaction, the original base becomes an
acid in the reverse reaction, and the original acid becomes a base in
the reverse process
 each reactant and the product it becomes is called a conjugate pair
 the original base becomes the conjugate acid; and the original acid
becomes the conjugate base
NH3(aq) + H2O(l)
Base
Acid
NH4+(aq)
Conjugate Acid
+
OH–(aq)
Conjugate Base
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Brønsted-Lowry: Acid-Base Reactions
H–A
acid
+
HCHO2
acid
H 2O +
acid
+
:B
base
:A–
+
H–B+
conjugate
conjugate
base
acid
H2O
base
CHO2–
conjugate
base
NH3
base
HO–
+
conjugate
base
+
H 3 O+
conjugate
acid
NH4+
conjugate
acid
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Conjugate Pairs
In the reaction H2O + NH3
HO– + NH4+
H2O and HO– constitute an
Acid/Conjugate Base pair
NH3 and NH4+ constitute a
Base/Conjugate Acid pair
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Identify the Brønsted-Lowry Acids and Bases and
Their Conjugates in the Reaction
H2SO4
+
H2O
HSO4–
+
H3O+
When the H2SO4 becomes HSO4-, it lost an H+ so H2SO4 must be
the acid and HSO4- its conjugate base
When the H2O becomes H3O+, it accepted an H+ so H2O must be
the base and H3O+ its conjugate acid
H2SO4
acid
+
H2O
base
HSO4–
+
H3O+
conjugate
conjugate
base
acid
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Identify the Brønsted-Lowry Acids and Bases and
Their Conjugates in the Reaction
HCO3– +
H2O
H2CO3
+
HO–
When the HCO3 becomes H2CO3, it accepted an H+ so HCO3- must be
the base and H2CO3 its conjugate acid
When the H2O becomes OH-, it donated an H+ so H2O must be the
acid and OH- its conjugate base
HCO3– +
base
H2O
H2CO3
acid
conjugate
acid
+
HO–
conjugate
base
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Practice – Write the formula for the conjugate
acid of the following
H2O
NH3
CO32−
H2PO4−
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Practice – Write the formula for the conjugate
acid of the following
H2O
NH3
H3O+
NH4+
CO32−
HCO3−
H2PO41−
H3PO4
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Practice – Write the formula for the conjugate
base of the following
H2O
NH3
CO32−
H2PO4−
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Practice – Write the formula for the conjugate
base of the following
H2O
NH3
HO−
NH2−
CO32−
since CO32− does not have an H, it
cannot be an acid
H2PO41−
HPO42−
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Arrow Conventions
 chemists commonly use two kinds of arrows
in reactions to indicate the degree of
completion of the reactions
 a single arrow indicates all the reactant
molecules are converted to product
molecules at the end
 a double arrow indicates the reaction stops
when only some of the reactant molecules
have been converted into products
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Strong or Weak
 a strong acid is a strong electrolyte
 practically all the acid molecules ionize, →
 a strong base is a strong electrolyte
 practically all the base molecules form OH– ions, either through dissociation
or reaction with water, →
 a weak acid is a weak electrolyte
 only a small percentage of the molecules ionize,
 a weak base is a weak electrolyte
 only a small percentage of the base molecules form OH– ions, either
through dissociation or reaction with water,
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Strong Acids

The stronger the acid, the more willing it is
to donate H
HCl(aq)  H+(aq) + Cl-(aq)
HCl(aq) + H2O(l)  H3O+(aq)+ Cl-(aq)
 use water as the standard base

strong acids donate practically all their H’s
 100% ionized in water
 strong electrolyte

[H3O+] = [strong acid]
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Weak Acids

weak acids donate a small fraction of their
H’s
HF(aq)
H+(aq) + F-(aq)
HF(aq) + H2O(l)
H3O+(aq) + F-(aq)
 most of the weak acid molecules do
not donate H to water
 much less than 1% ionized in water

[H3O+] << [weak acid]
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Polyprotic Acids
 often acid molecules have more than one ionizable H – these are called
polyprotic acids
 the ionizable H’s may have different acid strengths or be equal
 1 H = monoprotic, 2 H = diprotic, 3 H = triprotic
 HCl = monoprotic, H2SO4 = diprotic, H3PO4 = triprotic
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Polyprotic Acids
 polyprotic acids ionize in steps
 each ionizable H removed sequentially
 removing of the first H automatically makes removal of the second H
harder
 H2SO4 is a stronger acid than HSO4-
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HClO4
Conjugate
Bases
ClO4-1
H2SO4
HI
HBr
HCl
HNO3
H3O+1
HSO4-1
H2SO3
H3PO4
HNO2
HF
HC2H3O2
H2CO3
H 2S
NH4+1
HCN
HCO3-1
HS-1
H 2O
CH3-C(O)-CH3
NH3
CH4
OH-1
HSO4-1
I-1
Br-1
Cl-1
NO3-1
H 2O
SO4-2
HSO3-1
H2PO4-1
NO2-1
F-1
C2H3O2-1
HCO3-1
HS-1
NH3
CN-1
CO3-2
S-2
OH-1
CH3-C(O)-CH2-1
NH2-1
CH3-1
O-2
Increasing Basicity
Increasing Acidity
Acids
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Strengths : Acids and Bases
 commonly, Acid or Base strength is measured by determining the
equilibrium constant of a substance’s reaction with water
HA + H2O
B: + H2O
A-1 + H3O+1
HB+1 + OH-1
 the farther the equilibrium position lies to the products, the stronger the
acid or base
 the position of equilibrium depends on the strength of attraction between
the base form and the H+
 stronger attraction means stronger base or weaker acid
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