acids
DESCRIPTION
ACIDS. 1) Sour taste: Lemon Juice – Citric acid. Vinegar – Acetic Acid. Stomach ulcers are aggravated by hydrochloric acid. HCl Dissolve active metals, usually liberating H 2 . 3)Corrosive – dissolve compounds that are otherwise hard to dissolve. Examples: - PowerPoint PPT PresentationTRANSCRIPT
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ACIDS1) Sour taste:
Lemon Juice – Citric acid.Vinegar – Acetic Acid. Stomach ulcers are aggravated by hydrochloric acid. HCl
2) Dissolve active metals, usually liberating H2.
3) Corrosive – dissolve compounds that are otherwise hard to dissolve.Examples:
Precious metals such as gold (Au) dissolve in HNO3 + HCl (aqua regia).
Hard water deposits dissolve in vinegar.
4) Turn litmus paper RED (low pH).
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BASES1) Bitter taste.
2) Dissolve oil and grease.Drano and lye soap contain NaOH.Breaks ester and amide bonds
3) Slippery to the touch – dissolves hair and skin. e.g., soap: Na+ -OOC(CH2)16CH3
4) React with many metal ions to form precipitates.
Mg2+ + 2OH- Mg(OH)2
Example: Hard water (=Ca2+, Mg2+) + soap White precipitate.
(bathtub rings)
5) Turn litmus paper BLUE (high pH)
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Arrhenius ACID:Any compound that releases H+ when dissolved in H2O.Example:
HCl(g) H+(aq) + Cl(aq)
Arrhenius BASE:Any compound that releases OH- when dissolved in H2O.Example:
KOH(s) + H2O (l) K+(aq) + OH(aq)
ARRHENIUS ACIDS AND BASES
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BrØnsted ACID:Any compound capable of donating a H+ ion.
Example:
HCl(g) H+(aq) + Cl(aq)
BrØnsted BASE:Any compound capable of accepting a H+ ion.
Example:
NH3(g) + H2O(l) NH4+(aq) + OH(aq)
BRØNSTED - LOWRYACIDS AND BASES
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WATERWater electrolyzes slightly to produce H+ and OH- reversibly.
H2O H+ + OH-
Autoionization of water
Kw = [H+][OH-] = 1.0 x 10-14 at 25oC
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For pure water, [H+] = [OH-] = 10-7, so pH =7
Kw is constant even when [H+] and [OH-] are not equal
Calculate [H+] in a 0.05 M Ca(OH)2 solution
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Most accurate method to measure pH is to use a pH meter.
However, certain dyes change color as pH changes. These are indicators.
HIn = H+ + In-
Indicators are less precise than pH meters.Many indicators do not have a sharp color change as a function of pH.
Measuring pH
pH = -log10[H+] (low pH = acidic)
pH + pOH = -log10[H+] + -log10[OH-] = 14
pH scale
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Which bulbs light up?
Solution Strong, weak, or non-electrolyte?
Distilled water
Tap water
NaCl(aq)
1 M HCl (aq)
1 M CH3COOH (aq)
Sugar
CH3OH
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STRONG ACIDSStrong Acids dissociate completely when dissolved in water to
form H+ and the corresponding BrØnsted base.
HA H+(aq) + A-(aq)
Strong acids are strong electrolytes:COMPLETE dissociation into ions
[H+]final = [HA]initial = CHA
(If the analytical concentration, CHA, is less than 10-6 M then the autoionization of water needs to be taken into account.)
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When dissolved in water weak acids only partially dissociate to form H+ and the corresponding base.
HA (aq) H+ (aq) + A- (aq)
Weak acids are weak electrolytes:PARTIAL dissociation into ions
[H+]final < [HA]initial
Examples: CH3CO2H HF H3PO4
Acid Dissociation Constant (Ka) <<1
WEAK ACIDS
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Ka = [H +][A−][HA]
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What is the [H+] of 0.10 M HI?
What is the [H+] of 0.10 M acetic acid? Ka = 1.8 x10-5
• 1.8 x 10-5 M• 4.2 x 10-3 M• 1.8 x 10-6 M• 1.3 x 10-5 M
What is the pH?
What is the % dissociation?
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% Dissociation of CH3CO2H
CHA(M) [H+](M) % Dissoc.
10 0.013 0.13
1 0.004 0.4
0.1 0.0013 1.3
0.01 0.0004 4.0
0.001 0.00013 13.4
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OXYACIDSMany Brønsted acids consist of a central atom with several attached oxygen atoms. These are called oxyacids.
Acid strength increases with increasing oxidation number of the central atom:
HOClO3 > HOClO2 > HOClO > HOCl
General rule for uncharged oxyacids HxEOy:
If y-x > 2 then strong (H2SO4, HNO3,…)
If < 2 then weak (H2CO3, HBrO, HNO2,…)
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Increasing electronegativity of the central atom increases acid strength
HOCl > HOBr > HOI
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Polyprotic acids are capable of donating more than one proton.
Contain more than one ionizable proton.
The Ka always gets smaller with each ionization
Examples:H2CO3(aq) H+ (aq) + HCO3
-(aq) Ka = 4.3 x 10-7
HCO3-(aq) H+ (aq) + CO3
2- (aq) Ka = 5.6 x 10-11
H3PO4 (aq) H+(aq) + H2 PO4- (aq) Ka = 7.5 x 10-3
H2PO4-(aq) H+ (aq) + HPO4
2- (aq) Ka = 6.2 x 10-8
HPO42-(aq) H+(aq) + PO4
3-(aq) Ka = 4.2 x 10-13
POLYPROTIC ACIDS
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What are the concentrations of H+, HCO3-, CO3
2- in 1 x 10-3 M H2CO3?
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Which one of the following are not strong acids?
1. HNO3 5. HOBr2. HF 6. HBr3. HClO3 7. HPO4
2-
4. HClO4 8. H2SO3
Strong Acids
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STRONG BASESGroup I and II hydroxides (except Mg and Be).Arrhenius bases donate OH-.Brønsted bases accept H+
Examples:
NaOH, KOH, Ca(OH)2
KOH + H2O K+ (aq) + OH- (aq)
Strong bases are strong electrolytes.
[OH-] = Cbase
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WEAK BASESWhen dissolved in water weak bases only partially react to form OH and the corresponding BrØnsted acid.
B + H2O HB+(aq) + OH(aq)
Weak bases are weak electrolytes: [OH-] < Cbase
Weak bases can be neutralExample: NH3, amines
NH3 + H2O = NH4+(aq) + OH(aq)
Or Anions (any ion derived from a weak base) Example: F, NO2,
CH3COO
F(aq) + H2O = HF(aq) + OH(aq)
Base Dissociation Constant Kb << 1
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Kb = [HB+][OH−][B]
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What is the pH of 0.1 M NH3?
Kb = 1.8 x 10-5
1. 2.872. 4.743. 7.004. 9.255. 11.1
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CONJUGATE ACID BASE PAIRSCONJUGATE ACID BASE PAIRSDiffer only by the presence or absence of a proton (H+).
Conjugate Acid = Conjugate Base + H+
Examples:H3O+ / H2O H2O / OH
NH4+ / NH3
HCl / Cl
Ka x Kb = constant = 1 x 10-14
• The conjugate of a weak acid is a weak base (and vice versa)• The conjugate of a strong acid is a spectator ion (example: Cl is the
conjugate base of HCl).• The conjugate acid of OH (strong base) is water.
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When we add two reactions together, we multiply their equilibrium constants.
For conjugate acid-base pairs:Ka x Kb = Kw = 1 x 1014
Larger Ka means smaller Kb
The stronger the acid, weaker its conjugate base
pKa = log Ka
pKb = log Kb
log ( Ka x Kb ) = log Kw = 14
log Ka log Kb = 14
pKa + pKb = 14
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Weaker acid stronger conjugate base
H-F + OH- F- + H2O
Stronger acid6.9 x 10-4
Weaker acidKa = 10-14
Stronger base Weaker baseKb = 1.4 x 10-11
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ACETIC ACID
Acid: CH3COOH H+ + CH3COO
Base: CH3COO + H2O CH3COOH + OH-
----------------------------------------------- H2O H+ + OH-
Ka = Kb =
Kw = [H+][OH-] = Ka x Kb = 1 x 10-14
pKa + pKb = 14
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Hydrolysis: when a cation or anion reacts with H2O to form H+(aq) or OH(aq)
Will a salt be acidic or basic?1. Salt derived from a strong acid and a strong base
Neutral solution (pH = 7)
Example: NaCl (from NaOH and HCl) • Salt derived form a weak acid and a strong base
Basic solution (pH > 7)
Examples: NaClO (NaOH and HClO)
ClO (aq) + H2O HClO (aq) + OH(aq)
(CH3COO)2Ba (Ba(OH)2 and CH3COOH)
CH3COO(aq) + H2O CH3COOH(aq) +OH(aq)
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3. Salt derived from a strong acid and a weak base
Acidic solution (pH <7)
Example: NH4Cl (NH3 and HCl)
NH4+ + H2O NH3 + H3O+
• Salt derived form a weak acid and a weak base
pH depends on acid/base involved
Example: NH4CN (NH4+ and CN)
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What is the pH of 0.02 M KN3
Ka (HN3) = 1.9 x 10-5
1. 3.212. 5.493. 7.004. 8.515. 10.8
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LEWIS ACIDSAny substance that can accept a pair of electrons.
• Small cations• Molecules with unfilled octets
e.g. H+, BF3
Examples of Lewis Acids: Highly charged transition metal cations, e.g. Fe3+, Fe2+, Co3+ Group III cations (Al3+, Ga3+) and compounds (AlCl3)Smaller group II cations: Be2+ and Mg2+
LEWIS BASESAny substance that can donate a pair of electrons.
• Has lone pair electrons• May be neutral or anionic.Examples: NH3, OH-, Brønsted bases, H2O, Cl-
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LEWIS CATIONSTo compare acidity of Lewis acids, first compare charge. If
charge is the same then compare size.
Charge/Size Ratios Metal Ion Charge/Ionic radius (Å)
Na+ 1.0Li+ 1.5Ca2+ 2.1Mg2+ 3.1Zn2+ 2.7Cu2+ 2.8Al3+ 6.7Cr3+ 4.8Fe3+ 4.7
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HYDRATIONMetal ions attract the lone pairs on the oxygen in water molecules. This is a Lewis acid – Lewis base reaction.
Hydrated metal ions are acidic. Acidity increases with increasing charge/size ratio of the metal ions.
Hydrolysis is a reaction that dissociates water:
M(H2O)nz+ M(H2O)n-1(OH)(z-1)+ + H+
Fe(H2O)63+ Fe(H2O)5(OH)2+ + H+ (Ka=6.7 x 10-3)
Mz+ OH
H
:
:-
+
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ACIDS AND BASES SO FARACIDS AND BASES SO FAR1) Arrhenius, Brønsted, and Lewis definitions2) pH, pOH
3) Acid and Base Dissociation Constants – Ka and Kb
4) [H+] [OH-] = 1 x 10-14 = Ka x Kb
1) pH and % ionization calcn for strong and weak acids/bases
2) Conjugate Acid-Base Pairs:ArrheniusBronsted-LowryLewis
7) Salts – Hydrolysis8) Structure Related to Acid-Base Properties (Oxyacids)
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YOU SHOULD KNOW
GIVEN FIND
pH [H+], [OH-], pOH[H+] or [OH-] pHList of acids Weaker /StrongerList of pKa’s or Ka’s Weaker /StrongerKa or pKa and [HX] pH, [H+], [OH-]pH and [HX] Ka
Recall that a small Ka means high pKa, and both mean weak acid and not much dissociation.
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Acid/Base SALTS Review1) Which one of the following salts would have a
basic aqueous solution?
1. KF 3. NaI 2. Al(NO3)3 4. NH4Br
2) Arrange the following in the order of increasing base strength:
N3- NO3
- HPO42- CN-
3) Which of the following cannot act as a Lewis base?
1. Cl- 4. NH3 2. OH- 5. H+
3. CN-