Chapter 8 Acids and Bases Chemistry B11. Acids and Bases Acids: sour Bases: bitter or salty.
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Transcript of Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation...
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Chapter 16Acids and Bases
Q: Do you guys have equation solving calculators? Use for
quadratic formula?
John D. Bookstaver
St. Charles Community College
Cottleville, MO
Chemistry, The Central Science, 11th editionTheodore L. Brown, H. Eugene LeMay, Jr.,
and Bruce E. Bursten
![Page 2: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/2.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Some Definitions
• Arrhenius– An acid is a substance that, when
dissolved in water, increases the concentration of hydrogen ions (H+).
– A base is a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH-).
Ex: HCl, HC2H3O2
NaOH, NH3
NH3 + H2O NH4+ + OH-
HCl + H2O H3O+ + Cl-
![Page 3: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/3.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Some Definitions
• Brønsted-Lowry– An acid is a proton (H+) donor.– A base is a proton (H+) acceptor.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
A Brønsted-Lowry acid…
…must have a removable (acidic) proton.
A Brønsted-Lowry base…
…must have a pair of nonbonding electrons.
…where the H+, with no electrons for bonding, can attach itself.
![Page 5: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/5.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
If it can be either……it is amphiprotic.
(or ≈amphoteric)
HCO3-
HSO4-
H2O
![Page 6: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/6.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
If it can be either……it is amphiprotic.
(or ≈amphoteric)
HCO3-
HSO4-
H2O
+ H+ H2CO3 CO32- - H+
![Page 7: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/7.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
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.
![Page 8: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/8.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Conjugate Acids and Bases• The term conjugate comes from the Latin word
“conjugare,” meaning “to join together.”
• Reactions between acids and bases always yield their conjugate bases and acids.
• A conjugate base has one less H+ than its conjugate acid.
![Page 9: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/9.jpg)
AcidsandBases
Sample Exercise 16.1 Identifying Conjugate Acids and Bases
(a) What is the conjugate base of each of the following acids: HClO4, H2S, PH4
+, HCO3–? (b) What is the
conjugate acid of each of the following bases: CN–, SO42–,
H2O, HCO3– ?
Solution
Plan: The conjugate base of a substance is simply the parent substance minus one proton, and the conjugate acid of a substance is the parent substance plus one proton.
Solve: (a) HClO4 less one proton (H+) is ClO4–.
The other conjugate bases are HS–, PH3, and CO32–.
(b) CN– plus one proton (H+) is HCN. The other conjugate acids are HSO4
–, H3O+, and H2CO3. Notice that the hydrogen carbonate ion
(HCO3–) is amphiprotic. It can act as either an
acid or a base.
![Page 10: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/10.jpg)
AcidsandBases
• Write the formula for the conjugate acid of each of the following: HSO3
–, F– , PO43–, CO.
• Answers: H2SO3, HF, HPO42–, HCO+
© 2009, Prentice-Hall, Inc.
Practice Exercise
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AcidsandBases
© 2009, Prentice-Hall, Inc.
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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AcidsandBases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength
• Substances with negligible acidity do not dissociate in water.
• Their conjugate bases are exceedingly strong.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength• In any acid-base reaction, the equilibrium will
favor the reaction that moves the proton to the stronger base.
• *** (use table 16.4 from p672 or prior slide!)
HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)
• H2O is a much stronger base than Cl-, so H2O will get the proton, so equilibrium lies so far to the right that K is not measured (K>>1).
base base
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength• In any acid-base reaction, the equilibrium will
favor the reaction that moves the proton to the stronger base.
• Acetate is a stronger base than H2O, so acetate will get the proton, and the equilibrium favors the left side (K<1).
(confirm that this is true!)
HC2H3O2 (aq) + H2O (l) H3O+ (aq) + C2H3O2- (aq)
base base
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Autoionization of Water
• As we have seen, water is amphoteric.
• In pure water, a few molecules act as bases and a few act as acids.
• This is referred to as autoionization.
H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Ion-Product Constant• The equilibrium expression for this process is
Kc = [H3O+] [OH-](ignore the H2O reactants b/c they are liquid)
• This special equilibrium constant is referred to as the ion-product constant for water, Kw.
• At 25C, Kw = 1.0 10-14
![Page 17: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/17.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
pH
pH is defined as the negative base-10 logarithm of the concentration of hydronium ion.
pH = -log [H3O+]
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AcidsandBases
© 2009, Prentice-Hall, Inc.
pH• In pure water,
Kw = [H3O+] [OH-] = 1.0 10-14
• Since in pure water [H3O+] = [OH-],
[H3O+] = 1.0 10-14 = 1.0 10-7
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AcidsandBases
© 2009, Prentice-Hall, Inc.
pH• Therefore, in pure water,
pH = -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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AcidsandBases
Practice Exercise : (p 675)
• Indicate whether solutions with each of the following ion concentrations are neutral, acidic, or basic:
(a) [H+] = 4 x 10-9 M
(b) [OH-] = 1 x 10 -7 M
(c) [OH-] = 7 x 10-13 M
© 2009, Prentice-Hall, Inc.
![Page 21: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/21.jpg)
AcidsandBases
Sample 16.5: Calculating [H+] from [OH-] (p 675)
© 2009, Prentice-Hall, Inc.
Calculate the concentration of H+(aq) in (a)a solution in which [OH–] is 0.010 M, (b) a solution in which [OH–] is 1.8 ×10–9 M .
Note: In this problem and all that follow, we assume, unless stated otherwise, that the temperature is 25 ºC.
Analyze: We are asked to calculate the hydronium ion concentration in an aqueous solution where the hydroxide concentration is known.
Plan: We can use the equilibrium-constant expression for the autoionization of water and the value of Kw to solve for each unknown concentration.
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AcidsandBases
Sample 16.5: Solution, cont.
© 2009, Prentice-Hall, Inc.
Solve:a) Using Equation 16.16, we have:
This solution is basic because
(b) In this instance
This solution is acidic because
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AcidsandBases
© 2009, Prentice-Hall, Inc.
pH(p676)
These are the pH values for several common substances.
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AcidsandBases
Sample Ex 16.6 Calculating pH from [H+] (p 677)• Calculate the pH values for the two solutions described in Sample Ex 16.5.
Analyze: We are asked to determine the pH of aqueous solutions for which we have already calculated [H+].Plan: We can calculate pH using its defining equation, 16.17.Solve:(a) In the first instance we found [H+]. to be 1.0 ×10–12 M.
Because 1.0 ×10–12 has two significant figures, the pH has two decimal places, 12.00. (b) For the second solution, [H+] = 5.6 × 10–6 M. Before performing the calculation, it is helpful to estimate the pH. To do so, we note that [H+] lies between 1 × 10–6 and 1 × 10–5 Thus, we expect the pH to lie between 6.0 and 5.0. We use Equation 16.17 to calculate the pH.
![Page 25: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/25.jpg)
AcidsandBases
Check your work
• Check: After calculating a pH, it is useful to compare it to your prior estimate. In this case the pH, as we predicted, falls between 6 and 5. Had the calculated pH and the estimate not agreed, we should have reconsidered our calculation or estimate or both.
© 2009, Prentice-Hall, Inc.
![Page 26: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/26.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.7 Calculating [H+] from pHA sample of freshly pressed apple juice has a pH of 3.76. Calculate [H+].
Analyze: We need to calculate [H+] from pH.Plan: We will use Equation 16.17, pH = –log[H+], for the calculation.
Solve: From Equation 16.17, we have
Thus,
To find [H+] , we need to determine the antilog of –3.76. Scientific calculators have an antilog function (sometimes labeled INV log or 10x) that allows us to perform the calculation:
![Page 27: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/27.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Other “p” Scales• The “p” in pH tells us to take the negative base-
10 logarithm of the quantity (in this case, hydronium ions).
• Some similar examples are–pOH: -log [OH-]
–pKw: -log Kw
![Page 28: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/28.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
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
![Page 29: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/29.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
How Do We Measure pH?(p679)• For less accurate
measurements, one can use– Litmus paper
• “Red” paper turns blue above ~pH = 8
• “Blue” paper turns red below ~pH = 5
– Or an indicator.
![Page 30: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/30.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
How Do We Measure pH?
For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Strong Acids• You will recall that the seven strong acids are
HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.
• These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.
• For monoprotic strong acids,
[H3O+] = [acid].
![Page 32: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/32.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.8 Calculating the pH of a Strong Acid
What is the pH of a 0.040 M solution of HClO4?
Analyze and Plan: Because HClO4 is a strong acid, it is completely ionized, giving [H+] = [ClO4
-] = 0.040 MSolve: The pH of the solution is given by
pH = –log(0.040) = 1.40. Check: Because [H+] lies between 1 × 10–2 and 1 × 10–1, the pH will be between 2.0 and 1.0. Our calculated pH falls within the estimated range. Furthermore, because the concentration has two significant figures, the pH has two decimal places.
![Page 33: Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Q: Do you guys have equation solving calculators? Use for quadratic formula? John.](https://reader038.fdocuments.in/reader038/viewer/2022102604/56649e625503460f94b5e064/html5/thumbnails/33.jpg)
AcidsandBases
© 2009, Prentice-Hall, Inc.
Strong Bases• Strong bases are the soluble
hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).
• Again, these substances dissociate completely in aqueous solution.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.9 Calculating the pH of a Strong Base
What is the pH of (a) a 0.028 M solution of NaOH, (b) a 0.0011 M solution of Ca(OH)2?
Analyze: We are asked to calculate the pH of two solutions of strong bases.Plan: We can calculate each pH by either of two equivalent methods. First, we could use Equation 16.16 to calculate [H+] and then use Equation 16.17 to calculate the pH. Alternatively, we could use [OH–] to calculate pOH and then use Equation 16.20 to calculate the pH.
(b) Ca(OH)2 is a strong base that dissociates in water to give two OH– ions per formula unit. Thus, the concentration of OH–(aq) for the solution in part (b) is 2 × (0.0011 M) = 0.0022 M
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.9 Calculating the pH of a Strong Base
Solve:(a) NaOH dissociates in water to give one OH– ion per formula unit. Therefore, the OH– concentration for the solution in (a) equals the stated concentration of NaOH, namely 0.028 M.
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AcidsandBases
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(b) Ca(OH)2 is a strong base that dissociates in water to give two OH– ions per formula unit. Thus, the concentration of OH–(aq) for the solution in part (b) is 2 × (0.0011 M) = 0.0022 M
Sample Exercise 16.9 Calculating the pH of a Strong Base
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Dissociation Constants• For a generalized acid dissociation,
the equilibrium expression would be
• This equilibrium constant is called the acid-dissociation constant, Ka.
[H3O+] [A-][HA]
Kc =
HA (aq) + H2O (l) A- (aq) + H3O+ (aq)
= Ka
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Dissociation Constants (p682)The greater the value of Ka, the stronger is the acid.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
The pH of a 0.10 M solution of formic acid, HCOOH, at 25C is 2.38.
Calculate Ka for formic acid at this temperature.
We know that [H3O+] [COO-][HCOOH]
Ka=
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pHSOLUTION: The pH of a 0.10 M solution of formic acid, HCOOH, at 25C is 2.38. Calculate Ka for formic acid at this temperature.
To calculate Ka, we need the equilibrium concentrations of all three things.
We can find [H3O+], which is the same as [HCOO-], from the pH.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
pH = -log [H3O+]
2.38 = -log [H3O+]
-2.38 = log [H3O+]
10-2.38 = 10log [H3O+] = [H3O+]
4.2 10-3 = [H3O+] = [HCOO-]
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AcidsandBases
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Calculating Ka from pH
Now we can set up an ICE table…
[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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Calculating Ka from pH
[4.2 10-3] [4.2 10-3][0.10]
Ka =
= 1.8 10-4
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AcidsandBases
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Calculating Percent Ionization
• Percent Ionization = 100
• In this example
[H3O+]eq = 4.2 10-3 M
[HCOOH]initial = 0.10 M
[H3O+]eq
[HA]initial
Percent Ionization = 1004.2 10-3
0.10= 4.2%
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AcidsandBases
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Sample Exercise 16.11 Calculating Percent Ionization
A 0.10 M solution of formic acid (HCOOH) contains 4.2 × 10–3 M H+(aq) . Calculate the percentage of the acid that is ionized.
Analyze: We are given the molar concentration of an aqueous solution of weak acid and the equilibrium concentration of H+(aq) and asked to determine the percent ionization of the acid.
Plan: The percent ionization is given by Equation 16.27.Solve:
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Calculating pH from Ka
Calculate the pH of a 0.30 M solution of acetic acid, HC2H3O2, at 25C.
HC2H3O2 (aq) + H2O (l) H3O+ (aq) + C2H3O2- (aq)
Ka for acetic acid at 25C is 1.8 10-5.
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Calculating pH from Ka
The equilibrium constant expression is
[H3O+] [C2H3O2-]
[HC2H3O2]Ka =
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Calculating pH from Ka
We next set up a table…
[C2H3O2], M [H3O+], M [C2H3O2-], M
Initially 0.30 0 0
Change -x +x +x
At Equilibrium 0.30 - x 0.30 x x
We are assuming that x will be very small compared to 0.30 and can, therefore, be ignored.
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AcidsandBases
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Calculating pH from Ka
Now,
(x)2
(0.30)1.8 10-5 =
(1.8 10-5) (0.30) = x2
5.4 10-6 = x2
2.3 10-3 = x
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Calculating pH from Ka
pH = -log [H3O+]pH = -log (2.3 10-3)pH = 2.64
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Sample Exercise 16.12 Using Ka to Calculate pH
Calculate the pH of a 0.20 M solution of HCN. (Refer to Table 16.2 or Appendix D for the value of Ka.)
Analyze: We are given the molarity of a weak acid and are asked for the pH. From Table 16.2, Ka for HCN is 4.9 × 10–10.
Plan: We proceed as in the example just worked in the text, writing the chemical equation and constructing a table of initial and equilibrium concentrations in which the equilibrium concentration of H+ is our unknown.
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AcidsandBases
Solve: Writing both the chemical equation for the ionization reaction that forms H+(aq) and theequilibrium-constant (Ka) expression for the reaction:
Next, we tabulate the concentration of the species involved in the equilibrium reaction, letting x = [H+] at equilibrium:
Substituting the equilibrium concentrationsfrom the table into the equilibrium-constant expression yields
Sample Exercise 16.12 Using Ka to Calculate pH
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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.
p689
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Sample Exercise 16.13 Using Ka to Calculate % Ionization
Calculate the percentage of HF molecules ionized in (a) a 0.10 M HF solution, (b) a 0.010 M HF solution.
Analyze: We are asked to calculate the percent ionization of two HF solutions of different concentration. From Appendix D, we find Ka = 6.8 × 10-4.
Plan: We approach this problem as we would previous equilibrium problems. We begin by writing the chemical equation for the equilibrium and tabulating the known and unknown concentrations of all species. We then substitute the equilibrium concentrations into the equilibrium-constant expression and solve for the unknown concentration, that of H+.
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Solve:(a) The equilibrium reaction and equilibrium concentrations are as follows:
The equilibrium-constant expression is
When we try solving this equation using the approximation 0.10 – x = 0.10 (that is, by neglecting the concentration of acid that ionizes in comparison with the initial concentration), we obtain
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AcidsandBases
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Sample Exercise 16.13 Using Ka to Calculate % IonizationBecause this value is greater than 5% of 0.10 M, we should work the problem without the approximation,using an equation-solving calculator or the quadratic formula. Rearranging our equation and writing it instandard quadratic form, we have
This equation can be solved using the standard quadratic formula.Substituting the appropriate numbers gives
Of the two solutions, only the one that gives a positive value for x is chemically reasonable. Thus,From our result, we can calculate the percent of molecules ionized:
(b) Proceeding similarly for the 0.010 M solution, we haveSolving the resultant quadratic expression,we obtainThe percentage of molecules ionized is
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This equation can be solved using the standard quadratic formula.Substituting the appropriate numbers gives
Of the two solutions, only the one that gives a positive value for x is chemically reasonable. Thus,From our result, we can calculate the percent of molecules ionized:
(b) Proceeding similarly for the 0.010 M solution, we haveSolving the resultant quadratic expression,we obtainThe percentage of molecules ionized is
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Weak BasesBases react with water to produce hydroxide ion.
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AcidsandBases
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Weak Bases
The equilibrium constant expression for this reaction is
[HB] [OH-][B-]
Kb =
where Kb is the base-dissociation constant.
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Weak Bases (p691)Kb can be used to find [OH-] and, through it, pH.
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AcidsandBases
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pH of Basic Solutions
What is the pH of a 0.15 M solution of NH3?
[NH4+] [OH-]
[NH3]Kb = = 1.8 10-5
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
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AcidsandBases
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pH of Basic Solutions
Tabulate the data.
[NH3], M [NH4+], M [OH-], M
Initially 0.15 0 0
At Equilibrium 0.15 - x 0.15 x x
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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 =
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AcidsandBases
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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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AcidsandBases
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Ka and Kb (p 694)
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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AcidsandBases
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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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AcidsandBases
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Reactions of Cations with Water
• Cations with acidic protons (like NH4
+) will lower the pH of a solution.
• Most metal cations that are hydrated in solution also lower the pH of the solution.
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AcidsandBases
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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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AcidsandBases
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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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AcidsandBases
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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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AcidsandBases
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Factors Affecting Acid Strength
• The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.
• So acidity increases from left to right across a row and from top to bottom down a group.
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AcidsandBases
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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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AcidsandBases
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Factors Affecting Acid Strength
For a series of oxyacids, acidity increases with the number of oxygens.
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AcidsandBases
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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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Lewis Acids
• Lewis acids are defined as electron-pair acceptors.
• Atoms with an empty valence orbital can be Lewis acids.
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AcidsandBases
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Lewis Bases
• Lewis bases are defined as electron-pair donors.• Anything that could be a Brønsted-Lowry base is
a Lewis base.• Lewis bases can interact with things other than
protons, however.
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Buffers
• Buffers are solutions of a weak conjugate acid-base pair.
• They are particularly resistant to pH changes, even when strong acid or base is added.
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Buffers
If a small amount of hydroxide is added to an equimolar solution of HF in NaF, for example, the HF reacts with the OH− to make F− and water.
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Buffers
Similarly, if acid is added, the F− reacts with it to form HF and water.
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AcidsandBases
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Buffer Calculations
Consider the equilibrium constant expression for the dissociation of a generic acid, HA:
[H3O+] [A−][HA]
Ka =
HA + H2O H3O+ + A−
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Buffer Calculations
Rearranging slightly, this becomes
[A−][HA]
Ka = [H3O+]
Taking the negative log of both side, we get
[A−][HA]
−log Ka = −log [H3O+] + −log
pKa
pHacid
base
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Buffer Calculations
• SopKa = pH − log
[base][acid]
• Rearranging, this becomes
pH = pKa + log[base][acid]
• This is the Henderson–Hasselbalch equation.
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Henderson–Hasselbalch Equation
What is the pH of a buffer that is 0.12 M in lactic acid, CH3CH(OH)COOH, and 0.10 M in sodium lactate? Ka for lactic acid is 1.4 10−4.
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AcidsandBases
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Henderson–Hasselbalch Equation
pH = pKa + log[base][acid]
pH = −log (1.4 10−4) + log(0.10)(0.12)
pH
pH = 3.77
pH = 3.85 + (−0.08)
pH
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pH Range
• The pH range is the range of pH values over which a buffer system works effectively.
• It is best to choose an acid with a pKa close to the desired pH.
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When Strong Acids or Bases Are Added to a Buffer…
…it is safe to assume that all of the strong acid or base is consumed in the reaction.
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Addition of Strong Acid or Base to a Buffer
1. Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution.
2. Use the Henderson–Hasselbalch equation to determine the new pH of the solution.
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AcidsandBases
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Calculating pH Changes in Buffers
A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to make 1.00 L of solution. The pH of the buffer is 4.74. Calculate the pH of this solution after 0.020 mol of NaOH is added.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating pH Changes in Buffers
Before the reaction, since
mol HC2H3O2 = mol C2H3O2−
pH = pKa = −log (1.8 10−5) = 4.74
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating pH Changes in Buffers
The 0.020 mol NaOH will react with 0.020 mol of the acetic acid:
HC2H3O2(aq) + OH−(aq) C2H3O2−(aq) + H2O(l)
HC2H3O2 C2H3O2− OH−
Before reaction 0.300 mol 0.300 mol 0.020 mol
After reaction 0.280 mol 0.320 mol 0.000 mol
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Calculating pH Changes in Buffers
Now use the Henderson–Hasselbalch equation to calculate the new pH:
pH = 4.74 + log(0.320)(0.200)
pH = 4.74 + 0.06pH
pH = 4.80
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AcidsandBases
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Titration
In this technique a known concentration of base (or acid) is slowly added to a solution of acid (or base).
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AcidsandBases
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Titration
A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.
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AcidsandBases
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Titration of a Strong Acid with a Strong Base
Just before (and after) the equivalence point, the pH increases rapidly.
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AcidsandBases
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Titration of a Strong Acid with a Strong Base
From the start of the titration to near the equivalence point, the pH goes up slowly.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Titration of a Strong Acid with a Strong Base
At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Titration of a Strong Acid with a Strong Base
As more base is added, the increase in pH again levels off.
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AcidsandBases
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Titration of a Weak Acid with a Strong Base
• Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed.
• At the equivalence point the pH is >7.
• Phenolphthalein is commonly used as an indicator in these titrations.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Titration of a Weak Acid with a Strong Base
At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Titration of a Weak Acid with a Strong Base
With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.
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AcidsandBases
© 2009, Prentice-Hall, Inc.
Titration of a Weak Base with a Strong Acid
• The pH at the equivalence point in these titrations is < 7.
• Methyl red is the indicator of choice.
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AcidsandBases
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Titrations of Polyprotic Acids
When one titrates a polyprotic acid with a base there is an equivalence point for each dissociation.
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AcidsandBases
Objectives Met? You should be able to:
© 2009, Prentice-Hall, Inc.