Acid -Base Chemistry An acid is a H + (proton) donor . General formula of acid = H-A .
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Transcript of Acid -Base Chemistry An acid is a H + (proton) donor . General formula of acid = H-A .
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AcidAcid-Base -Base ChemistryAn acidacid is a HH++ (proton) donor (proton) donor.General formula of acid = H-Aacid = H-A.
H-A HH-A H+ + + A + A--
acid proton conjugate base
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22When H-A dissociates to H+ and A– in aqueous (water) solution, the “hydronium ion” or H3O+ forms:H-AH-A + H+ H22O HO H33OO++ + A + A––
H-AH-A + H-O-H H-O-H + AA––
HH
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meansmeans
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33A BaseBase is a HH++ (proton) acceptoracceptor.A basebase has an unshared pair of unshared pair of electrons electrons to share with H+.we can represent a basebase by the general formula :B :B or :Base.:Base.:Base + :Base + HH++ H H-Base-Base++ from an conjugate acid acid
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44When a Base accepts protons from water, it increases the concentration of HO– (hydroxide ions) in solution::B :B + H-O-H H-BB++ + – O-HOR:B + H2O HB+ + –OH
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55ExamplesHHNO3 + H2O H3O+ + NO3
–
nitric acid nitrate ion acidacid conjugate baseconjugate base
HHCl + H2O H3O+ + Cl–hydrochloric acid chloride ion acidacid conjugate conjugate basebase
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:NH:NH33 + H2O +NH4 + – OHammonia ammonium
ion basebase conjugate acidconjugate acid
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77Strengths of Acids & BasesA strong acid HA completely strong acid HA completely dissociates dissociates to H+ (= H3O+) and A– in H2O:
H-AH-A + H2O H3O+ + A– .1 mole 1 mole to start to start gives 1 mole H1 mole H33OO+ +
+ 1 mole A+ 1 mole A––
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88The common strong acids are:HClHCl hydrochloric acidhydrochloric acidHBr hydrobromic acidHIHI hydroiodic acidhydroiodic acidHNO3 nitric acidHH22SOSO44 sulfuric acidsulfuric acid[HClO4 perchloric acid] don’t
memorize [ ]
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99A strong base strong base is one which completely dissociates to provide HO– in water.These bases are ionic metal hydroxides:Metal cation + hydroxide anionMetal cation + hydroxide anion MOH M+ + –OH1 mole 1 mole + 1 mole
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1010There are 4 common strong There are 4 common strong bases:bases:LiOH lithium hydroxide
NaOHNaOH sodium hydroxidesodium hydroxide
KOHKOH potassium hydroxidepotassium hydroxide
Ba(OH)Ba(OH)2 2 barium hydroxidebarium hydroxide
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1111
Some Advice...Some Advice...It is best to memorizememorize the
listslists of strong acids & strong acids & strong strong
bases --bases --assume all others are
weak.
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1212But Also,LEARN HOWLEARN HOW
to write the equationsfor the reaction of each
chemical as astrong acid strong acid
or strong base.strong base.
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1313What about WEAK acids & bases?
A weakweak acid or weak base is incompletely ionized incompletely ionized or
dissociated in water solution.The ionization reaction of a The ionization reaction of a weak acid or weak base is a weak acid or weak base is a
reversible equilibrium!!reversible equilibrium!!
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1414Example
CH3CO-H + H2O CH3CO– + H3O+
O O
acetic acid acetateacetic acid acetatebefore equilibrium:before equilibrium:1 mole 0 0
after equilibrium:after equilibrium:0.98 mole 0.02 0.02
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The position of equilibrium is different for each weak acid
and weak base.
The equilibrium constant describes how much
ionization has occurred.
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1616
CH3CO-H + H2O CH3CO– + H3O+
O OFor acetic acid:For acetic acid:
Keq = [CH3CO2–]•[H3O+]
[CH3CO2H]•[H2O]
OROR
Keq•[H2O] = K Kaa = [CH[CH33COCO22––]•[H]•[H33OO++]]
[CH[CH33COCO22H]H]
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1717We use Ka as a measure of the strength of an acid because
[H2O] is very large and does not effectively change as a result of
the ionization reaction.Typical Ka values for weak acids
are in the range of1 x 10-1 to 1 x 10-10
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1818Sample KSample Kaa values: values: do NOT memorize!!!!do NOT memorize!!!!WhatWhat KKaa==CH3CO2H acetic acid 1.8x10-5
H2O water 1 x 10-16
CH3CH2OH alcohol 1 x 10-17
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1919What do these small Ka values mean?* Solutions of weak acids and bases at equilibrium contain both the unionized (H-A) and ionized forms (H+ and A–). * The largest species present is H-A (unionized acid.)
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2020And...* Any compound having Ka smaller than H2O is considered “neutral” =
“not acidic in water”.
Thus, alcohols are not acidic compounds. They are neutral substances.
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2121MonoproticMonoprotic acids are those
which can release one Hrelease one H++ per per molecule molecule of acid.
PolyproticPolyprotic acids can release release more than one Hmore than one H++ per molecule
of acid.
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2222 Examples Monoprotic HMonoprotic HCl Hydrochloric Acid DiproticDiprotic HH22SO4 Sulfuric acid
TriproticTriprotic HH33PO4 Phosphoric Acid
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2323Amphiprotic (Amphoteric) Amphiprotic (Amphoteric) Compounds Compounds can act as acids or bases.ExamplesExamplesHCO3
– bicarbonate ion
as acidas acid HCO3– CO3
-2 + HH++
as base as base HCO3– + H+ HH2CO3
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2424Dihydrogen phosphate HDihydrogen phosphate H22POPO44
–– is another important amphotericamphoteric ion.as acidas acid HH22PO4
–– HPO4-2 + HH++
as baseas base H2PO4
–– + H+ HH3PO4
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2525Water is the most important amphoteric compound of all.H-O-HH + H-O-H HO– + H3O+
“ “acid” acid” “base”“base”Keq = [HO–]•[H3O+] OROR
[H2O]•[H2O]
Keq•[H2O]2 = K Kww = [HO = [HO––]•[H]•[H33OO++] ]
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2626
In pure H2O at room temperature, Kw has a value
of 1 x 10-14.Since Kw = [HO–]•[H3O+], this means[HO–]=[H3O+] = 1x10-7
moles/literin neutral water.
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2727pH is defined aspH = -log ([HpH = -log ([H33OO++]) ]) of a solution.A logarithm tells what power of 10 = a certain number.That is, if y = 10y = 10xx, then log(y) = log(y) = xx.
Thus, 100 = 102 , so log(100) = 2.
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2828We saw that for pure H2O,[H3O+] = 1 x 10-7.
Since -log (1 x 10-7) = 7,we say neutral water has pH = neutral water has pH =
77.
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2929Recall: [HO–] and [H3O+] are related through the equilibrium 2 H2O HO– + H3O+ .AcidsAcids are HH++ donors donors and cause an increaseincrease in [H[H33OO++]].
So: pH decreases. Also: [HO–] decreases.
Acidic solutions have pH <7. Acidic solutions have pH <7. (in the range 0 to 6.99)
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3030Again: 2 H2O HO– + H3O+ .A BaseBase increases [HO-] or
decreases [Hdecreases [H33OO++]].So pH increases:
a basicbasic or “alkaline” solution solution has has a pH >7 a pH >7
(in the range 7+ to 14).
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313114
0
7 Neutral [HO–] = [H3O+]pH
Acidic [HO–] < [H3O+]
Basic [HO–] > [H3O+]