acid-base equilibria: acids and bases what makes an acid an acid? an acid possess a sour taste
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Acid-Base Equilibria: Acids and Bases What makes an Acid an Acid? An acid possess a sour taste An acid dissolves active metals magnesium An acid causes certain vegetable dyes to turncharacteristic colors What makes a Base a Base? A bases possess a bitter taste - PowerPoint PPT PresentationTRANSCRIPT
Acid-Base Equilibria: Acids and Bases
What makes an Acid an Acid?•An acid possess a sour taste•An acid dissolves active metals magnesium•An acid causes certain vegetable dyes to turn characteristic colors
What makes a Base a Base?•A bases possess a bitter taste•A base feels slippery to the touch •A base causes certain vegetable dues to turn a characteristic color
7 strong acids and 8 strong bases
• Acids - HI, HBr, HCl, HClO3, HClO4, H2SO4, HNO3
• Bases – LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2
The Arrhenius Definition of an Acid and a Base
An acidacid is a substance that produces H+ ions in water solutions
HCl H+ + Cl-
A basebase is a substance that produces OH- ions in a water solution
NaOH Na+ + OH-
acid dissociation equations
HC6H5O3 C6H5O31- + H+
Fe(H2O)63+ Fe(H2O)5(OH)2+ + H+
CH3CH2NH31+ CH3CH2NH2 + H+
The Proton in WaterThe Proton in WaterWhen HCl dissolves in water we write:
HCl(g) H+(aq) + Cl-(aq)
Reality for the Hydronium ionH5O2
+
H9O4+
Acidic solutions are formed by a chemical reaction in which
and acid transfers a proton (H+) to water, so we can write them
either way.HClHCl(aq)(aq) + H + H22OO(aq)(aq) H H33OO++
(aq)(aq) + Cl + Cl--(aq)(aq)
or or
HClHCl(aq)(aq) H H++(aq)(aq) + Cl + Cl--
(aq)(aq)
Nitrogen compounds are Bronsted acids when they are
protonatedprotonated.
NH4Cl
NH4+ NH3 + H+
CH3)2NH2+ (CH3)2NH + H+
The Bronsted-Lowry definition for Acids and Bases
Acids may be defined as a substance that is capable of donating protons
Bases may be defined as substance that accepts protons.
HCl + NH3 NH4+
+ Cl-
acid base conjugate conjugateacid base
Is Water an Acid?
NH3(aq) +H2O(aq) NH4+
(aq) + OH-(aq)
Is Water a Base?
HC2H3O2(aq) + H2O(aq) H3O+(aq) +C2H3O2
1-(aq)
The auto ionization of waterThe reaction occurs to a very small extent;
about 1 in 108 molecules is ionized at any given moment
HH
HH
OO:: ::
HH
HH
OO:: ::+ HH
HH
OO::HH
+
+ HHOO:....
Dissociation of Water, pH ScaleH2O(l) H+
(aq) + OH-(aq)
K =[H+] [OH-] [H2O]
since water is a liquid and its concentration is therefore constant, this
expression may be written as:
Kw = [H+] [OH-] = 1.0 x 10-14
[H+] = [OH-] = 1.0 x 10-7 M
Sample exercise: Indicate whether each of the following solutions is
neutral, acidic, or basic:
a) [H+] = 2 x 10-5 M
b) [OH-] = 0.010 M
c) [OH-] = 1.0 x 10-7 M
Calculate the concentration of H+(aq) in
(a)a solution in which the [OH-] is 0.020M
(b)a solution in which the [OH-] = 2.5 x 10-6 M.
Indicate whether the solution is acidic or basic
The pH Scale
pH = -log [H+]
If [H+] = 2. 5 x 105 the pH is?pH = -log [2. 5 x 10-5 ] = 4.6
If pH is 3.8 the H+ concentration isAntilog -3.8= 1.58 x 10-4 M
In a sample of lemon juice, [H+] = 3.8 x 10-4 M. What is the pH?
A commonly available window cleaner has a [H+] = 5.3 x 10-9 M. What is the pH?
In a sample of freshly pressed apple juice has a pH of 3.76. Calculate the [H+]
What if we took the –log of the Kw expression
Kw = [H+] [OH-] = 1.0 x 10-14
pKw = pH + pOH = 14
What is the pH, [H+], [OH-], of a solution with a pOH of 2.5?
Is the solution acidic or basic?
Major speciesHCl(aq) + H2O(aq) H3O+
(aq) + Cl-(aq)
or
HCl(aq) H+(aq) + Cl-
(aq)
HC2H3O2(aq) + H2O(aq) H3O+(aq) +C2H3O2
1-(aq)
Pb(NO3)2 + NaCl →NaNO3 + PbCl2
Indicators
What is the pH of 0.010 M solution of HCl?
If it ionizes completely which is what strongstrong means then take the negative log of the concentration.
HCl(aq) H+(aq) + Cl-
(aq)
.01M .01M .01M
pH = 2
What is the pH of a solution made from 20mL of 2.0M HCl
and 35mL of 3.2M HNO3?
What about H2SO4
H2SO4 → H+ + HSO41- (Strong)
HSO41- → H+ + SO4
2- (Weak)
What about weak acids
HX(aq) H+(aq) + X-
(aq), then Ka = [H+][X-]
[HX]The smaller the value of the acid
dissociation constant Ka, the weaker the acid
What is the Ka of a 0.10 M solution of formic acid (HCHO2) which has a
pH = 2.38?
HCHO2 H+ + CHO21-
I 0.10 0 0 C .00417 .00417 .00417 E .0958 .00417 .00417
Ka = (.00417)2 = 1.8 x 10-4
.0958
What is the concentration of H+ ions in a 0.10 M solution of HC2H3O2 (Ka
= 1.8 x 10-5)? pH? % ionization? HC2H3O2 H+ + C2H3O2
1-
I .10 0 0
C -X +X +X
E .10 – X X X
1.8 x 10-5 = X2 X = 1.3 x 10-3
.10 – X pH = 2.87
percent dissociation
1.3 X 10-3 x 100 = 1.3%
.10
What is the pH and percent ionization of a 0.20 M solution of
HCN? Ka = 4.9 x 10-10
Acid-Base Equilibria: Strong BasesAcid-Base Equilibria: Strong BasesThe most common soluble strong Bases are the hydroxides
of group IA and Ca, Ba and Sr
What is the pH of a 0.010 M solution of Ba(OH)2?
AminesAnions of Weak Acids
NH3 + H2O NH4+ + OH-
ICE
Dealing with Weak BasesDealing with Weak Bases
Weak base + H2O conjugate acid + OH-
NH3 (aq) + H2O (l) NH4 +
(aq) + OH-(aq)
Kb =[NH4
+] [OH-][NH3]
The base dissociation constant Kb refers to the equilibrium in which a base reacts with H2O to
form the conjugate acid and OH-
Calculate the [OH-] in a 0.15 M solution of NH3.
Polyprotic Acids
H2SO3(aq) H+(aq) + HSO3
-(aq)
Ka1 = 1.7 x 10-2
HSO3-(aq) H+
(aq) + SO32-
(aq) Ka2 = 6.4 x 10-8
Calculate the pH of a .1M solution
Anions of Weak AcidsAnions of Weak Acids
HCHC22HH33OO2(aq)2(aq) + H + H22OO(aq)(aq) H H33OO++(aq)(aq) + + CC22HH33OO22
-- (aq)(aq)
Bronsted base
Bronsted acid
Conjugateacid
Conjugatebase
A second class of weak base is composed of the anions of weak acids Anions of weak acids can be incorporated into salts.
NaCNaC22HH33OO22 Na Na++(aq) + (aq) + CC22HH33OO22-- (aq)(aq)
C2H3O2- + H2O HC2H3O2 + OH- Kb = 5.6 x 1010
NaOHNaOH(aq)(aq)+HC+HC22HH33OO2(aq2(aq→H→H22O+O+NaCNaC22HH33OO22
NaCNaC22HH33OO22 Na Na++(aq)(aq) + C + C22HH33OO22
--(aq)(aq)
NaNa++(aq)(aq) + H + H22O O ← NaOH← NaOH(aq) (aq) + H+ H++
(aq)(aq)
CC22HH33OO22--(aq)(aq)+H+H22O O HCHC22HH33OO2 (aq)2 (aq)+OH+OH- -
(aq)(aq)
NH4Cl → NH4+ + Cl-
NH4+ NH3 + H+
NH4Cl
NaC2H3O2
NH4C2H3O2
ClO- + H2O HClO + OH-
I
C
E
Calculate the pH of a 0.01 M solution of sodium hypochlorite (NaClO)
Anions of Weak AcidsAnions of Weak Acids
Now it’s you turn: the Kb for BrO- is 5.0 x 10-6. Calculate the pH of a 0.050 M solution of NaBrO
KKaa and K and Kbb
NH4+
(aq) NH3(aq)+ H+(aq)
NH3(aq)+ H2O NH4+
(aq)+ OH-(aq)
KKaa = =[H[H++][NH][NH33]]
[NH[NH44++]]
KKbb = =[NH[NH44][OH][OH-- ] ]
[NH[NH33]]
NH4+(aq) NH3(aq) + H+ (aq)
NHNH33(aq) + H(aq) + H22O(l)O(l) NH NH44++(aq) + OH(aq) + OH-- (aq) (aq)
HH22O O H H++(aq) + OH(aq) + OH--(aq)(aq)When two reactions are added to give a third reaction, the equilibrium constant for the third reaction reaction is given by the product of the equilibrium constants for the two added reactions
KKaa x K x Kbb = K = KwwpKa + pKb = pKw
Calculate the (a) base-dissociation constant, Kb, for the fluoride ion, is the pKa of HF = 3.17
pKa = -log Ka
3.17 = -log Ka
Antilog -3.17 = 6.76 x 10-4
Since Ka x Kb = Kw
(6.76 x 10-4)x Kb = 1.0 x 10-14
Kb = 1.0 x 10-14/ 6.76 x 10-4 = 1.5 x 10-11
Now it’s your turn
Calculate the pKb for carbonic acid (Ka = 4.3 x 10-7)
Acid-Base Properties of Salt Solutions•Anions of weak acids, HX, are basic and will react with
H2O to produce OH-
X- (aq) + H2O (l) HX(aq) + OH-(aq)
•Anions of strong acids, such as NO3-, exhibit no basicitiy,
these ions do not react with water and consequently do not influence the pH
•Anions of polyprotic acids, such as HCO3-, that still have
ionizable protons are capable of acting as either proton donors or acceptors depending upon the magnitudes of the
Ka or Kb
•Anions of polyprotic acids, such as HCO3-, that still have
ionizable protons are capable of acting as either proton donors or acceptors depending upon the magnitudes of the Ka or Kb
Predict whether the salt Na2HPO4 will form an acidic or basic solution on dissolvingin water.
Na2HPO4 2Na+ (aq) + HPO4-
HPO4-(aq) + H2O H3O+ + PO4
3-(aq)
HPO4- (aq) + H2O H2PO4
2-(aq) + OH-(aq)
K3 = 4.2 x 10-13HPO4
- acting like an acid
HPO4- acting like an base
So HPO- is the conjugate base of H2PO4-.
Since the K2 of H2PO4- = 6.2 x 10-8 then:
Kb =Ka
=Kw
1.0 x 10-14
6.2 x 10-8
= 1.6 x 10-7
Since Kb is larger than Ka, HPO4- will act like a base
•Salt derived from a strong base and a strong acid will have a pH of 7
•Salt derived from a strong base and a weak acid will have a pH above 7
•Salt derived from a weak acid and a weak base depends upon whether the dissolved ion acts as an acid or a base as determined by the size of
the Ka or Kb
Acid-Base Character and Chemical Structure
HF > HCl > HBr > HI(most polar least) Based on electronegativity difference HF is the most polar but a weak acid because the bond is so strong
two things to consider polarity difference and strength of the bond
Acid strength of oxyacids
The more oxygen's the stronger the acid because of the oxygen pulling the electrons towards
themselves.
HClO4> HClO3> HClO2> HOCl