1 CHAPTER 16 Acid – Base Equilibria. 2 Properties of Aqueous Solutions of Acids & Bases n Acidic...
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Transcript of 1 CHAPTER 16 Acid – Base Equilibria. 2 Properties of Aqueous Solutions of Acids & Bases n Acidic...
1
CHAPTER 16
Acid – Base EquilibriaAcid – Base Equilibria
2
Properties of Aqueous Solutions of Acids & Bases
Acidic propertiesAcidic properties taste sourtaste sour change the colors of indicatorschange the colors of indicators
turn litmus redturn litmus red
react with metals to generate Hreact with metals to generate H2(g)2(g)
react with metal oxides and hydroxides to form react with metal oxides and hydroxides to form salts and watersalts and water
aqueous solutions conduct electricityaqueous solutions conduct electricity
3
Properties of Aqueous Solutions of Acids & Bases
Basic propertiesBasic properties taste bittertaste bitter feel slipperyfeel slippery change colors of indicatorschange colors of indicators
turn litmus blueturn litmus blue react with acids to form salts and waterreact with acids to form salts and water aqueous solutions conduct electricityaqueous solutions conduct electricity
4
Strong Electrolytes Strong electrolytes Strong electrolytes ionizeionize or or dissociatedissociate
completelycompletely Three classes of strong electrolytesThree classes of strong electrolytes1 Strong AcidsStrong Acids
HNO H O H O NO
or
HNO H NO
3 2100%
3+
3
3 3
5
Strong Electrolytes2 Strong Soluble BasesStrong Soluble Bases
K OH K OHH O 100%2
6
Strong Electrolytes3 Most Soluble SaltsMost Soluble Salts
Ca(NO ) Ca 2 NO3 2 sH O 100% 2
32
7
Strong Electrolytes
Calculation of concentrations of ions in Calculation of concentrations of ions in solution of strong electrolytes is easysolution of strong electrolytes is easy
Example: Calculate the concentrations of ions Example: Calculate the concentrations of ions in 0.050 in 0.050 MM nitric acid, HNO nitric acid, HNO33..
8
Strong Electrolytes
Calculation of concentrations of ions in Calculation of concentrations of ions in solution of strong electrolytes is easysolution of strong electrolytes is easy
Example: Calculate the concentrations of ions Example: Calculate the concentrations of ions in 0.050 in 0.050 MM nitric acid, HNO nitric acid, HNO33..
HNO H O H O NO
0.050 0.050 3 2
100%3 3
0 050. M M M
9
Arrhenius Theory Svante Augustus Arrhenius - 1884Svante Augustus Arrhenius - 1884 acids generate Hacids generate H++ in aqueous solutions in aqueous solutions
-2322
- 32
HCO O H O HH HCO
Cl O H O H HCl
10
Arrhenius Theory bases generate OHbases generate OH-- in aqueous solutions in aqueous solutions
aqaq423(g)
-aqaq
OH NH O H NH
OH Na NaOH
11
Arrhenius Theory
neutralization - combination of Hneutralization - combination of H++ (or H (or H33OO++) )
with OHwith OH-- strong acids - ionize 100% in waterstrong acids - ionize 100% in water
HCl, HBr, HI, H2SO4, HNO3, HClO4, HClO3
strong bases - ionize 100% in waterstrong bases - ionize 100% in waterLiOH, NaOH, KOH, RbOH, CsOH,
Ca(OH)2, Sr(OH)2, Ba(OH)2
12
Arrhenius Theory
total ionic equation for strong acid with total ionic equation for strong acid with strong basestrong base
net ionic equation for strong acid with net ionic equation for strong acid with strong basestrong base
(l)O HCl Na OH Na Cl H 2-aqaq
-aqaqaqaq
(l)O H OH H 2-aqaq
13
Bronsted-Lowry Acid-Base Theory
acids - proton (Hacids - proton (H++) donor) donor
1221
-32
base acid base acid
Br O H O HHBr
14
Bronsted-Lowry Acid-Base Theory
bases - proton (Hbases - proton (H++) acceptor) acceptor
212 1
-423
base acid acid base
OH NH O H NH
15
Bronsted-Lowry Acid-Base Theory
acid-base reactions are proton transfer acid-base reactions are proton transfer reactionsreactions
– note that we are often making coordinate note that we are often making coordinate covalent bondscovalent bonds
1221
-43
base acid base acid
Cl NH NH HCl
16
Bronsted-Lowry Acid-Base Theory
conjugate acid-base pairsconjugate acid-base pairs– species that differ by a protonspecies that differ by a proton
HNOHNO3 3 + H+ H22O O H H33OO+ + + NO + NO33--
HNOHNO33 - acid - acid11 NO NO33- - - base- base11
HH22O - baseO - base2 2 H H33OO+ + - acid- acid22
HF + HHF + H22O O H H33OO+ + + F + F--
HF - acidHF - acid11 F F- - - base- base11
HH22O - baseO - base2 2 H H33OO+ + - acid- acid22
17
Bronsted-Lowry Acid-Base Theory
differences between Arrhenius & Bronsted-differences between Arrhenius & Bronsted-Lowry theoriesLowry theories
reaction does not have to occur in an reaction does not have to occur in an aqueous solutionaqueous solution
bases do not have to be hydroxidesbases do not have to be hydroxides for example- ammonia is not a hydroxidefor example- ammonia is not a hydroxide
base acid acid base
OH NH O H NH
212 1
-423
18
Bronsted-Lowry Acid-Base Theory
weak acids have strong conjugate weak acids have strong conjugate basesbases
weak bases have strong conjugate weak bases have strong conjugate acidsacids
primary reason they are weak acids or primary reason they are weak acids or basesbases
strong conjugates recombine to form the strong conjugates recombine to form the original speciesoriginal species
19
Bronsted-Lowry Acid-Base Theory
NHNH44++ must be a strong acid-it gives up H must be a strong acid-it gives up H++
to reform NHto reform NH33
NaOH NaOH Na Na++ (aq) + OH (aq) + OH--(aq)(aq)
NaNa++ must be a weak acid or it would must be a weak acid or it would recombine to form NaOHrecombine to form NaOH
remember NaOH ionizes 100%remember NaOH ionizes 100%
-423 OH NH O H NH
20
Bronsted-Lowry Acid-Base Theory
amines are weak bases that behave like amines are weak bases that behave like ammoniaammonia
amines have organic groups attached to amines have organic groups attached to -NH-NH22 group group
-33223
-423
OH NHCH O H NHCH
OH NH O H NH
21
Bronsted-Lowry Acid-Base Theory
water can be either an acid or base in water can be either an acid or base in Bronsted-Lowry theoryBronsted-Lowry theory
amphotericamphoteric - species that can be either - species that can be either an acid or basean acid or base
amphiprotic amphiprotic - proton transfer reactions - proton transfer reactions that species behave as either an acid that species behave as either an acid or baseor base
2121
-322
base acid acid base
OH O H O H OH
22
The Auto-Ionization of Water
Pure water ionizes Pure water ionizes very slightlyvery slightly– less than one-millionth molarless than one-millionth molar
H O + H O H O + OH2 2 3+ -
23
The Auto-Ionization of Water
Because the activity of pure water is 1, the Because the activity of pure water is 1, the equilibrium constant for this reaction isequilibrium constant for this reaction is
K H O OHc 3+
24
The Auto-Ionization of Water Experimental measurements have determined that Experimental measurements have determined that
the concentration of each ion is 1.0 x 10the concentration of each ion is 1.0 x 10-7-7 MM at 25 at 2500C C
K H O OH
1.0 x 10 1.0 x 10
1.0 x10
c 3+
-7 -7
14
25
The Auto-Ionization of Water This particular equilibrium constant is called theThis particular equilibrium constant is called the ion- ion-
product for water, Kproduct for water, Kww..
K H O OH
1.0 x10
w 3+
14
26
The pH and pOH scales
A convenient way to express acidity and A convenient way to express acidity and basicitybasicity
pH is defined as pH is defined as
pH = -log H O3+
27
The pH and pOH scales
In general, a lower case p before a symbol is In general, a lower case p before a symbol is read as ‘negative logarithm of” the symbolread as ‘negative logarithm of” the symbol
pOH = -log OH
pAg = -log Ag
and so forth
-
+
28
The pH and pOH scales
If we know either [HIf we know either [H33OO++] or [OH] or [OH--], then we can calculate pH and pOH.], then we can calculate pH and pOH.
Example: Calculate the pH of a solution in which the [HExample: Calculate the pH of a solution in which the [H33OO++] =0.030] =0.030MM..
29
The pH and pOH scales
If we know either [HIf we know either [H33OO++] or [OH] or [OH--], then we can calculate pH and pOH.], then we can calculate pH and pOH.
Example: Calculate the pH of a solution in which the [HExample: Calculate the pH of a solution in which the [H33OO++] =0.030] =0.030MM..
52.1pH
100.3logpH
OH-log=pH2
+3
30
The pH and pOH scales
A convenient relationship between pH and A convenient relationship between pH and pOH may be derived for pOH may be derived for allall dilute aqueous dilute aqueous solutions at 25solutions at 2500C.C.
143 100.1]][OHO[H
31
The pH and pOH scales
Remember these two expressions!!Remember these two expressions!!
14.00pOHpH
100.1OHOH 143
32
The pH and pOH scales
The usual range for the pH scale isThe usual range for the pH scale is
and for pOH the scale isand for pOH the scale is
14.00pH 0pH
100.1OH down to 0.1OH 1433
MM
0pOH 00.14pOH
0.1OH toup 100.1OH 14
MM
33
The pH and pOH scales
34
Strengths of Acids
Binary AcidsBinary Acids acid strength increases with decreasing acid strength increases with decreasing
bond strengthbond strength hydrogen halideshydrogen halides bond strengthbond strength
HF>>HCl>HBr>HIHF>>HCl>HBr>HI acid strengthacid strength
HF<<HCl<HBr<HIHF<<HCl<HBr<HI
35
Strengths of Acids
36
Strengths of Acids
VIA hydridesVIA hydrides bond strengthbond strength
HH22O>> HO>> H22S> HS> H22Se> HSe> H22TeTe
acid strengthacid strengthHH22O<< HO<< H22S< HS< H22Se< HSe< H22TeTe
37
Strengths of Acids
for HBr in waterfor HBr in water
HBr + HHBr + H22O O H H33OO+ + + Br + Br--
essentially 100%essentially 100% can only distinguish acid strength can only distinguish acid strength
differences of strong acids in differences of strong acids in nonaqueous solutions like acetic acidnonaqueous solutions like acetic acid
38
Strengths of Acids AcidAcid Conjugate BaseConjugate Base Strongest acid Weakest baseStrongest acid Weakest base HClOHClO4 4 ClOClO44
--
HH++(H(H33OO++) -H) -H++ H H22OO
CHCH33COCO22H H CHCH33COCO22--
HH22O +HO +H++ OH OH--
NHNH33 NH NH22--
Weakest acid Strongest baseWeakest acid Strongest base
39
Strengths of Acids strongest acid in water is Hstrongest acid in water is H33OO++
HCl + HHCl + H22O O H H33OO+ + + Cl + Cl--
HCl is so strong it forces water to accept HHCl is so strong it forces water to accept H++
strongest base in water is OHstrongest base in water is OH--
NHNH22- - + H+ H22O O NH NH33 + OH + OH--
NHNH22- - is strong enough to remove His strong enough to remove H+ + from waterfrom water
because water is amphiproticbecause water is amphiprotic
40
Strengths of Bases
Strong Bases are strong electrolytes Strong Bases are strong electrolytes Dissociate completely in solutionDissociate completely in solution Bases do not need to contain OHBases do not need to contain OH-- ion ion
OO2-2-(aq) + H(aq) + H22O (l) O (l) 2OH 2OH--(aq)(aq)
HH--(aq) + H(aq) + H22O(l) O(l) H H22(g) + OH(g) + OH--(aq)(aq)
NN33-- (aq) + H (aq) + H22O(l) O(l) NH NH33(aq) + 3OH(aq) + 3OH--(aq)(aq)
41
Ionization Constants for Weak Monoprotic Acids and Bases
Let’s look at the dissolution of acetic acid, a Let’s look at the dissolution of acetic acid, a weak acid, in water as an example.weak acid, in water as an example.
The equation for the ionization of acetic acid is:The equation for the ionization of acetic acid is:
COOCHOHOH COOHCH -3323
42
Ionization Constants for Weak Monoprotic Acids and Bases
The equilibrium constant for this ionization is The equilibrium constant for this ionization is expressed as:expressed as:
OHCOOHCH
COOCHOHK
23
33c
43
Ionization Constants for Weak Monoprotic Acids and Bases
The water concentration in dilute aqueous The water concentration in dilute aqueous solutions is very high.solutions is very high.
1 L of water contains 55.5 moles of water.1 L of water contains 55.5 moles of water. Thus in dilute aqueous solutions:Thus in dilute aqueous solutions:
M5.55OH2
44
Ionization Constants for Weak Monoprotic Acids and Bases
The water concentration is many orders of The water concentration is many orders of magnitude greater than the ion concentrations.magnitude greater than the ion concentrations.
Thus the water concentration is essentially Thus the water concentration is essentially constant.constant.
COOHCH
COOCHOHOHK
3
332c
45
Ionization Constants for Weak Monoprotic Acids and Bases
Since a constant multiplied by a constant is a Since a constant multiplied by a constant is a new constant - let’s give this new constant its new constant - let’s give this new constant its own name and symbolown name and symbol
KKaa = ionization constant = ionization constant
acid aceticfor
108.1COOHCH
COOCHOHK 5
3
33a
46
Ionization Constants for Weak Monoprotic Acids and Bases
In In simplified formsimplified form the equation and the equation and expression are written as:expression are written as:
5
3
3a
-33
108.1COOHCH
COOCHHK
COOCHHCOOHCH
47
Ionization Constants for Weak Monoprotic Acids and Bases
Values for several ionization constantsValues for several ionization constants
48
Ionization Constants for Weak Monoprotic Acids and Bases
From the above table we see that the order of From the above table we see that the order of increasing acid strength for these weak acids increasing acid strength for these weak acids is:is:
The larger KThe larger Kaa The stronger the acid The stronger the acid
If KIf Kaa >> 1 then the acid is completely ionized >> 1 then the acid is completely ionized
and the acid is a strong acid.and the acid is a strong acid.
HF > HNO > CH COOH > HClO > HCN3 3
49
Ionization Constants for Weak Monoprotic Acids and Bases
The order of increasing base strength of the The order of increasing base strength of the anions (conjugate bases) of these acids is:anions (conjugate bases) of these acids is:
F < NO < CH COO < ClO < CN-3-
3- - -
50
Ionization Constants for Weak Monoprotic Acids and Bases
Using Ka, the concentration of H+ (and hence the pH) can be calculated.– Write the balanced chemical equation clearly showing the
equilibrium.
– Write the equilibrium expression. Find the value for Ka.
– Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H+ is x.
Substitute into the equilibrium constant expression and solve. Remember to turn x into pH if necessary.
A sample problem can be found at end of the slides.
51
Ionization Constants for Weak Monoprotic Acids and Bases
Percent Ionization
HA(aq) + H2O(l) ↔ H3O+ (aq) + A-(aq)
% ionization = [H+]equ x 100
[HA]
Relates equilibrium H+ concentration to the initial HA concentration
52
Ionization Constants for Weak Monoprotic Acids and Bases
Percent Ionization
The higher the percent ionization
The stronger the acid
For weak acids
Percent ionization decreases as the molarity of the solution increases
acetic acid 0.05M 2.0% ionized
0.15 M 1.0% ionized
53
Ionization Constants for Weak Monoprotic Acids and Bases
54
Polyprotic Acids Many weak acids contain two or more acidic hydrogens.Many weak acids contain two or more acidic hydrogens.
– polyprotic acids ionize stepwisepolyprotic acids ionize stepwise– ionization constant for each stepionization constant for each step
Consider arsenic acid, HConsider arsenic acid, H33AsOAsO44, which has three ionization constants, which has three ionization constants
1 KK11=2.5 x 10=2.5 x 10-4-4
2 KK22=5.6 x 10=5.6 x 10-8-8
3 KK33=3.0 x 10=3.0 x 10-13-13
55
Polyprotic Acids
The first ionization step isThe first ionization step is
H AsO H H AsO
KH H AsO
H AsO
3 4 2 4
12 4
3 4
2 5 10 4.
56
Polyprotic Acids
The second ionization step isThe second ionization step is
81
43
242
2
24242
106.5AsOH
AsOHHK
AsOHHAsOH
57
Polyprotic Acids
The third ionization step isThe third ionization step is
HAsO H AsO
KH AsO
HAsO
42-
4
34
42-
3
3133 0 10.
58
Polyprotic Acids
Notice that the ionization constants vary in the following fashion:Notice that the ionization constants vary in the following fashion:
This is a general relationship. It is always easier to remove the first This is a general relationship. It is always easier to remove the first proton.proton.K K K1 2 3
59
Polyprotic Acids
60
Weak Bases
Remove protons from other substancesRemove protons from other substances Equilibrium establishedEquilibrium established
Weak base + HWeak base + H22O O ↔ Conjugate acid + OH↔ Conjugate acid + OH--
Calculate KCalculate Kbb base dissociation constant base dissociation constant
61
Weak Bases
62
Weak Bases
Generally have lone pair or negative Generally have lone pair or negative chargecharge
Neutral weak bases contain NNeutral weak bases contain N Anions of weak acids are weak basesAnions of weak acids are weak bases
63
Relationship between Ka and Kb
When two reactions are added to give a When two reactions are added to give a third, the equilibrium constant for the third, the equilibrium constant for the third reaction is the product of the third reaction is the product of the equilibrium constants for the first two.equilibrium constants for the first two.
Reaction 1 + Reaction 2 = Reaction 3Reaction 1 + Reaction 2 = Reaction 3
KK11 x K x K22 = K = K33
64
Relationship between Ka and Kb
For a conjugate acid base pair:For a conjugate acid base pair:
KKaa x K x Kbb = K = Kww
pKpKaa x pK x pKbb = pK = pKww
The larger the KThe larger the Kaa, the smaller the K, the smaller the Kbb
The stronger the acid, the weaker the The stronger the acid, the weaker the conjugate base.conjugate base.
65
Calculation of Ionization Constants
Example: In 0.12 Example: In 0.12 MM solution, a weak solution, a weak monoprotic acid, HY, is 5.0% ionized. monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak Calculate the ionization constant for the weak acid.acid.
66
Calculation of Ionization Constants
Example: In 0.12 Example: In 0.12 MM solution, a weak solution, a weak monoprotic acid, HY, is 5.0% ionized. monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak Calculate the ionization constant for the weak acid.acid.
HY H + Y
KH Y
HY
+ -
a
+ -
67
Calculation of Ionization Constants
Since the weak acid is 5.0% ionized, it is also Since the weak acid is 5.0% ionized, it is also 95% unionized.95% unionized.
Calculate the concentrations of all species in Calculate the concentrations of all species in solution.solution.
H Y
HY
+
0 05 012 0 0060
6 0 10
0 95 012 011
3
. ( . ) .
.
. ( . ) .
M M
M
M M
68
Calculation of Ionization Constants
Substitute into the ionization constant Substitute into the ionization constant expression to get the value of Kexpression to get the value of Kaa
KH Y
HYa
6 0 10 6 0 10
011
3 3 10
3 3
4
. .
.
.
69
Calculations Based on Ionization Constants
Example: Calculate the concentrations of the various Example: Calculate the concentrations of the various species in 0.15 species in 0.15 MM acetic acid, CH acetic acid, CH33COOH, solution.COOH, solution.
It is always a good idea to write down the ionization It is always a good idea to write down the ionization reaction and the ionization constant expression.reaction and the ionization constant expression.
5
3
-33
a
-3323
108.1COOHCH
COOCHOHK
COOCHOH OHCOOHCH
70
Calculations Based on Ionization Constants
Next we combine the basic chemical Next we combine the basic chemical concepts with some algebra to solve the concepts with some algebra to solve the problemproblem
M0.15 [] Initial
COOCH OH OHCOOHCH -3323
71
Calculations Based on Ionization Constants
Next we combine the basic chemical Next we combine the basic chemical concepts with some algebra to solve the concepts with some algebra to solve the problemproblem
xMxMxM
M
- Change
0.15 [] Initial
COOCH OH OHCOOHCH -3323
72
Calculations Based on Ionization Constants
Next we combine the basic chemical Next we combine the basic chemical concepts with some algebra to solve the concepts with some algebra to solve the problemproblem
xMxM-x)M.(
xMxMxM
M
150 [] mEquilibriu
- Change
0.15 [] Initial
COOCH OH OHCOOHCH -3323
73
Substitute these algebraic quantities into the Substitute these algebraic quantities into the ionization expression.ionization expression.
5
3
33a
108.115.0
COOHCH
COOCHOHK
x
xx
Calculations Based on Ionization Constants
74
Solve the algebraic equation, using simplifying Solve the algebraic equation, using simplifying assumption.assumption.
Calculations Based on Ionization Constants
52
52
108.115.0
108.115.0
xx
x
x
75
Solve the algebraic equation, using simplifying Solve the algebraic equation, using simplifying assumption.assumption.
Calculations Based on Ionization Constants
52
3a
52
52
108.115.0
assumption thismake then 10K If
108.115.0
108.115.0
x
xx
x
x
76
Complete the algebra and solve for concentrations.Complete the algebra and solve for concentrations.
Calculations Based on Ionization Constants
MM
Mx
x
15.0106.115.0COOHCH
COOCHOH106.1
107.2
33
333
62
77
Note that the properly applied simplifying assumption gives Note that the properly applied simplifying assumption gives the same result as solving the quadratic equation does. the same result as solving the quadratic equation does.
Calculations Based on Ionization Constants
2a
4acbb
c b a
0107.2108.1
108.115.0
2
652
5
x
xx
X
xx
78
Calculations Based on Ionization Constants
3-3
6255
101.6- and 106.1
12
107.214108.1108.1
x
x
79
Synthesis Question One method of increasing the solubility One method of increasing the solubility
and the absorption of a medication is to and the absorption of a medication is to convert weakly acidic drugs into sodium convert weakly acidic drugs into sodium salts before making the pills that will be salts before making the pills that will be ingested. How does this preparation ingested. How does this preparation method enhance the drug’s solubility in method enhance the drug’s solubility in the stomach? the stomach?
80
Synthesis Question
The sodium salt of a weakly acidic The sodium salt of a weakly acidic compound is a strong conjugate base. compound is a strong conjugate base. In the presence of stomach fluids, 1.0 M In the presence of stomach fluids, 1.0 M HCl, the conjugate base readily reacts HCl, the conjugate base readily reacts with the HCl generating the active and with the HCl generating the active and soluble form of the medication. soluble form of the medication.
81
Group Question
Medicines that are weakly basic are not Medicines that are weakly basic are not absorbed well into the bloodstream. absorbed well into the bloodstream. One method to increase their absorption One method to increase their absorption is to take an antacid at the same time is to take an antacid at the same time that the medicine is administered. How that the medicine is administered. How does this method increase the does this method increase the absorption? absorption?