Acids and Bases Chapter 19 Acids and Bases. Acids and Bases Who Theory: Acid= When...
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Transcript of Acids and Bases Chapter 19 Acids and Bases. Acids and Bases Who Theory: Acid= When...
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Acidsand
Bases
Chapter 19Acids and Bases
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Acidsand
Bases
WhoTheory:
Acid=When
Arrhenius increases H+ 1880’s
Brønsted proton donor 1923
Lowry ditto 1923
LewisElectron-pair acceptor
1923
Three definitions of acid
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Acidsand
Bases
Johannes Nicolaus Brønsted (February 22, 1879-December 17, 1947)Danish physical chemist
Svante August Arrhenius (February 19, 1859 – October 2, 1927)
Swedish chemist; Nobel Prize in Chemistry, 1903
* Arrhenius equation (activation energy)
* Greenhouse effecthttp://en.wikipedia.org/wiki/Arrhenius
Thomas Martin Lowry (October 26, 1874–November 2, 1936)English organic chemist
Gilbert Newton Lewis (October 23, 1875-March 23, 1946)American physical chemist
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Acidsand
Bases
Some Definitions
• Arrhenius acids and basesAcid: Substance that, when dissolved
in water, increases the concentration of hydrogen ions (protons, H+).
Base: Substance that, when dissolved in water, increases the concentration of hydroxide ions.
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Acidsand
Bases
Some Definitions• Brønsted–Lowry: must have both
1. an Acid: Proton donor
and
2. a Base: Proton acceptor
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Acidsand
Bases
The Brønsted-Lowry acid donates a proton,
while the Brønsted-Lowry base accepts it.
Brønsted-Lowry acids and bases are always paired.
Which is the acid and which is the base in each of these rxns?
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Acidsand
Bases
A Brønsted–Lowry acid…
…must have a removable (acidic) proton.
HCl, H2O, H2SO4
A Brønsted–Lowry base…
…must have a pair of nonbonding electrons.
NH3, H2O
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Acidsand
Bases
If it can be either…
...it is amphiprotic.
HCO3–
HSO4 –
H2O
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Acidsand
Bases
What Happens When an Acid Dissolves in Water?
• Water acts as a Brønsted–Lowry base and abstracts a proton (H+) from the acid.
• As a result, the conjugate base of the acid and a hydronium ion are formed.
Movies…
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Acidsand
Bases
Conjugate Acids and Bases:• From the Latin word conjugare, meaning “to
join together.”• Reactions between acids and bases always
yield their conjugate bases and acids.
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Acidsand
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Acid and Base Strength
• Strong acids are completely dissociated in water.Their conjugate bases are
quite weak.
• Weak acids only dissociate partially in water.Their conjugate bases are
weak bases.
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Acidsand
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Acid and Base Strength
• Substances with negligible acidity do not dissociate in water.Their conjugate bases are
exceedingly strong.
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Acidsand
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Acid and Base Strength
In any acid-base reaction, the equilibrium favors the reaction that moves the proton to the stronger base.
HCl(aq) + H2O(l) H3O+(aq) + Cl–(aq)
H2O is a much stronger base than Cl–, so the equilibrium lies so far to the right K is not measured (K>>1).
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Acidsand
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Acid and Base Strength
Acetate is a stronger base than H2O, so the equilibrium favors the left side (K<1).
The stronger base “wins” the proton.
HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq)
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Acidsand
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Autoionization of WaterAs we have seen, water is amphoteric.• In pure water, a few molecules act as bases and a
few act as acids.
This process is called autoionization.
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Acidsand
Bases
Ion-Product Constant
• The equilibrium expression for this process is
Kc = [H3O+] [OH–]
• This special equilibrium constant is referred to as the ion-product constant for water, Kw.
• At 25°C, Kw = 1.0 10-14
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Acidsand
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pH
pH is defined as the negative base-10 logarithm of the hydronium ion concentration.
pH = –log [H3O+]
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Acidsand
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pH
• In pure water,
Kw = [H3O+] [OH–] = 1.0 10-14
• Because in pure water [H3O+] = [OH-],
[H3O+] = (1.0 10-14)1/2 = 1.0 10-7
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Acidsand
Bases
pH• Therefore, in pure water,
pH = –log [H3O+] = –log (1.0 10-7) = 7.00
• An acid has a higher [H3O+] than pure water, so its pH is <7
• A base has a lower [H3O+] than pure water, so its pH is >7.
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Acidsand
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pH
These are the pH values for several common substances.
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Acidsand
Bases
Other “p” Scales
• The “p” in pH tells us to take the negative log of the quantity (in this case, hydronium ions).
• Some similar examples arepOH –log [OH-]pKw –log Kw
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Acidsand
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Watch This!
Because
[H3O+] [OH−] = Kw = 1.0 10-14,
we know that
–log [H3O+] + – log [OH−] = – log Kw = 14.00
or, in other words,
pH + pOH = pKw = 14.00
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Acidsand
Bases
If you know one, you know them all:
[H+][OH-]pH
pOH
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Acidsand
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How Do We Measure pH?
Litmus paper• “Red” paper turns
blue above ~pH = 8• “Blue” paper turns
red below ~pH = 5An indicator
• Compound that changes color in solution.
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Acidsand
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How Do We Measure pH?
pH meters
measure the voltage in the solution
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Acidsand
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Strong Acids• You will recall that the
seven strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.
• These are strong electrolytes and exist totally as ions in aqueous solution.
• For the monoprotic strong acids,
[H3O+] = [acid].
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Acidsand
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Strong Bases
• Strong bases are the soluble hydroxides, which are the alkali metal (NaOH, KOH)and heavier alkaline earth metal hydroxides (Ca(OH)2, Sr(OH)2, and Ba(OH)2).
• Again, these substances dissociate completely in aqueous solution.
[OH-] = [hydroxide added].
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Acidsand
Bases
Dissociation Constants• For a generalized acid dissociation,
the equilibrium expression is
• This equilibrium constant is called the acid-dissociation constant, Ka.
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Acidsand
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Dissociation Constants
The greater the value of Ka, the stronger the acid.
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Acidsand
Bases
Calculating Ka from the pH
• The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate Ka for formic acid at this temperature.
• We know that
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Acidsand
Bases
Calculating Ka from the pH
The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate Ka for formic acid at this temperature.
To calculate Ka, we need all equilibrium concentrations.
We can find [H3O+], which is the same as [HCOO−], from the pH.
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Acidsand
Bases
Calculating Ka from the pH
pH = –log [H3O+]
– 2.38 = log [H3O+]
10-2.38 = 10log [H3O+] = [H3O+]
4.2 10-3 = [H3O+] = [HCOO–]
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Acidsand
Bases
Calculating Ka from pH
In table form:
[HCOOH], M [H3O+], M [HCOO−], M
Initially 0.10 0 0
Change –4.2 10-3 +4.2 10-
3
+4.2 10-3
At Equilibrium
0.10 – 4.2 10-3
= 0.0958 = 0.10
4.2 10-3 4.2 10 - 3
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Acidsand
Bases
Calculating Ka from pH
[4.2 10-3] [4.2 10-3][0.10]
Ka =
= 1.8 10-4
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Acidsand
Bases
Calculating Percent Ionization
In the example:
[A-]eq = [H3O+]eq = 4.2 10-3 M
[A-]eq + [HCOOH]eq = [HCOOH]initial = 0.10 M
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Acidsand
Bases
Calculating Percent Ionization
Percent Ionization = 1004.2 10-3
0.10
= 4.2%
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Acidsand
Bases
Calculating pH from Ka
Calculate the pH of a 0.30 M solution of acetic acid, C2H3O2H, at 25°C.
Ka for acetic acid at 25°C is 1.8 10-5.Is acetic acid more or less ionized than
formic acid (Ka=1.8 x 10-4)?
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Acidsand
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Calculating pH from Ka
The equilibrium constant expression is:
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Acidsand
Bases
Calculating pH from Ka
Use the ICE table:
[C2H3O2], M [H3O+], M [C2H3O2−], M
Initial 0.30 0 0
Change –x +x +x
Equilibrium 0.30 – x x x
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Acidsand
Bases
Calculating pH from Ka
Use the ICE table:
[C2H3O2], M [H3O+], M [C2H3O2−], M
Initial 0.30 0 0
Change –x +x +x
Equilibrium 0.30 – x x x
Simplify: how big is x relative to 0.30?
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Acidsand
Bases
Calculating pH from Ka
Use the ICE table:
[C2H3O2], M [H3O+], M [C2H3O2−], M
Initial 0.30 0 0
Change –x +x +x
Equilibrium 0.30 – x ≈ 0.30 x x
Simplify: how big is x relative to 0.30?
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Acidsand
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Calculating pH from Ka
Now,
(1.8 10-5) (0.30) = x2
5.4 10-6 = x2
2.3 10-3 = x
Check: is approximation ok?
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Acidsand
Bases
Calculating pH from Ka
pH = –log [H3O+]pH = – log (2.3 10−3)pH = 2.64
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Acidsand
Bases
Polyprotic Acids
Have more than one acidic proton.
If the difference between the Ka for the first dissociation and subsequent Ka values is 103 or more, the pH generally depends only on the first dissociation.
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Acidsand
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Weak Bases
Bases react with water to produce hydroxide ion.
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Acidsand
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Weak Bases
The equilibrium constant expression for this reaction is
where Kb is the base-dissociation constant.
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Acidsand
Bases
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Acidsand
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Weak Bases
Kb can be used to find [OH–] and, through it, pH.
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Acidsand
Bases
pH of Basic Solutions
What is the pH of a 0.15 M solution of NH3?
[NH4+] [OH−]
[NH3]Kb = = 1.8 10-5
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Acidsand
Bases
pH of Basic Solutions
Tabulate the data.
[NH3], M [NH4+], M [OH−], M
Initial 0.15 0 0
Equilibrium 0.15 - x 0.15 x x
Simplify: how big is x relative to 0.15?
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Acidsand
Bases
pH of Basic Solutions
(1.8 10-5) (0.15) = x2
2.7 10-6 = x2
1.6 10-3 = x2
(x)2
(0.15)1.8 10-5 =
Check: is approximation ok?
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Acidsand
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pH of Basic Solutions
Therefore,
[OH–] = 1.6 10-3 M
pOH = –log (1.6 10-3)
pOH = 2.80
pH = 14.00 – 2.80
pH = 11.20
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Acidsand
Bases
Ka and Kb are linked:
Combined reaction = ?
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Acidsand
Bases
Ka and Kb are linked:
Combined reaction = ?
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Acidsand
Bases
Ka and Kb
Ka and Kb are related in this way:
Ka Kb = Kw
Therefore, if you know one of them, you can calculate the other.
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Acidsand
Bases
A 0.020 M solution of niacin has a pH of 3.26. (a) What percentage of the acid is ionized in this solution? (b) What is the acid-dissociation constant, Ka, for niacin?
PRACTICE EXERCISES1. Niacin, one of the B vitamins, has the following molecular structure:
2. What is the pH of (a) a 0.028 M solution of NaOH, (b) a 0.0011 M solution of Ca(OH)2? What percentage of
the bases are ionized?
3. Calculate the percentage of HF molecules ionized in (a) a 0.10 M HF solution, (b) a 0.010 M HF solution. Ka for HF is 6.8 x10-4.
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Acidsand
Bases
Reactions of Anions with Water
• Anions are bases.
• As such, they can react with water in a hydrolysis reaction to form OH– and the conjugate acid:
X–(aq) + H2O(l) HX(aq) + OH–(aq)
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Acidsand
Bases
Reactions of Cations with Water• Cations with acidic protons
(like NH4+) lower the pH of a
solution by releasing H+.
• Most metal cations (like Al3+) that are hydrated in solution also lower the pH of the solution; they act by associating with H2O and making it release H+.
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Acidsand
Bases
Reactions of Cations with Water
• Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water.
• This makes the O-H bond more polar and the water more acidic.
• Greater charge and smaller size make a cation more acidic.
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Acidsand
Bases
Effect of Cations and Anions
1. An anion that is the conjugate base of a strong acid will not affect the pH.
2. An anion that is the conjugate base of a weak acid will increase the pH.
3. A cation that is the conjugate acid of a weak base will decrease the pH.
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Acidsand
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Effect of Cations and Anions
4. Cations of the strong Arrhenius bases will not affect the pH.
5. Other metal ions will cause a decrease in pH.
6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Ka and Kb values.
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Acidsand
Bases
What effect on pH? Why?An anion that is the conjugate base of a strong acid does not affect pH.
= very weak base
An anion that is the conjugate base of a weak acid increases pH. = strong base
A cation that is the conjugate acid of a weak base decreases pH. = strong acid
Cations of the strong Arrhenius bases (Na+, Ca2+) do not affect pH.
= very weak acid(not really acidic at all)
Other metal ions cause a decrease
in pH.= moderate bases(cations)
Weak acid + weak base Depends on Ka and Kb
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Acidsand
Bases
Factors Affecting Acid Strength
• The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.
• Acidity increases from left to right across a row and from top to bottom down a group.
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Acidsand
Bases
Factors Affecting Acid Strength
In oxyacids, in which an OH is bonded to another atom, Y,
the more electronegative Y is, the more acidic the acid.
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Acidsand
Bases
Factors Affecting Acid Strength
For a series of oxyacids, acidity increases with the number of oxygens.
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Acidsand
Bases
Factors Affecting Acid Strength
Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.
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Acidsand
Bases
Lewis Acids
• Lewis acids are defined as electron-pair acceptors.
• Atoms with an empty valence orbital can be Lewis acids.
• A compound with no H’s can be a Lewis acid.
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Acidsand
Bases
Lewis Bases
• Lewis bases are defined as electron-pair donors.• Anything that is a Brønsted–Lowry base is also a
Lewis base. (B-L bases also have a lone pair.)• Lewis bases can interact with things other than
protons.
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Acidsand
Bases
A 0.020 M solution of niacin has a pH of 3.26. (a) What percentage of the acid is ionized in this solution? (b) What is the acid-dissociation constant, Ka, for niacin?
PRACTICE EXERCISES1. Niacin, one of the B vitamins, has the following molecular structure:
2. Calculate the percentage of HF molecules ionized in (a) a 0.10 M HF solution, (b) a 0.010 M HF solution. Ka for HF is 6.8 x10-4.
3. A solution of acetic acid is 2% ionized at 25°C. Ka=1.8x10-5. What was the original concentration of the acid?