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Chap 14, Aqueous Equilibria,Acids & BasesThis chapter is an extension of the equilibrium chapter primarily to rxns. involving the transfer of

H+ions in aqueous soln.

I. Acid-Base Concepts: Brønsted-Lowry A. Arrhenius acid-base ideas (Review sect 7.5?):

1. Acids dissociate, produce H+ in H2O: HA(aq) W H+(aq) + A!

(aq)

2. Bases too, produce OH! in H2O: MOH(aq) W M+(aq) + OH!

(aq)

3. Arrhenius theory doesn’t always work (NH3 is basic)

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B. Brønsted-Lowry Theory

1. An acid is a substance that can donate a H+.

2. A base is a substance that can accept a H+.

3. A general expression of an acid-base rxn.:

HA + B W BH+ + A!

acid base acid base

4. Conjugates:a) The conjugate base is what is “leftover” after the acid has donated

its H+.b) The conjugate acid is what is “leftover” after the base has accepted

its H+. (Do parts of Probs 14.1-3)

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Make sure you can see the atoms:Key Concept Prob. 14.3

II. Acid Strength & Base Strength (context dependent!!!)

A. For now we take a qualitative approach to thissubject. Later, we will do a quantitative approach.

1. Think about H2O functioning as a base:

HA(aq) + H2O(l) W H3O+(aq) + A!

(aq)acid base acid base

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The 2 bases are competing for the H+ ion. Will the H+ ionspend most of its time associated with the weaker of the 2bases or the stronger?

2. For pictures, look at Key Concept 14.3 & Table 14.1.

3. You must become adept at this. Try Prob. 14.4-5Alternatively, if HA is a strong acid, is its conjugate basestrong or weak?

rob 14.5

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III. Hydrated Protons & Hydronium Ions A. When a H+ ion dissociates in water, it does not

usually exist as a free H+ ion. It binds to water:

1. H3O+ is called the hydronium ion.

2. H5O2+, H7O3

+, H9O4+, etc., likely exist as well.

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IV. Dissociation of Water A. Water is interesting. It can be both an acid & a base:

H2O + H2O W H3O+ + OH! Lewis acid base acid base dot?

B. Can we write a Kc expression for the above?

1. “Kc” = 2. Can we simplify this?

Kw =

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3. In “pure” water at 25°C, [H3O+] = 1.0 x 10!7 M.

4. In “pure” water at 25°C, [OH!] = ?

5. What is [H2O] = ?

6. Numerical value for Kw: Need to memorize?Kw = [H3O+] [OH!] Prob. 14.6-7.

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V. The pH Scale (power of Hydrogen)

There are a bunch of reasons for using the pH, asopposed to [H3O+]. Convenience in expressing thenumbers is probably most important to you.

A. Definition: pH = !log [H3O+], so: [H3O+] = 10!pH

1. Let’s start with an example of pH. Prob. 14.8 a)

2. Make sure you can also find [H3O+]. Prob. 14.9 a)

B. Note that as [H3O+] increases, pH decreases becausepH is the negative log of [H3O+].

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C. What about rounding? Essentially the x /÷ ruleplus one.

!log (1.0 x 10!9) = 9.00

!log (3.28 x 10!5) = 4.484 126 156

VI. Measuring pH A. Indicators

1. pH indicators have an acid form and a conjugate base form. The two forms are different colors:

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N

N N

S

O

O

OH

HInd(aq) + OH!(aq) W H2O(l) + Ind!

(aq) acid base acid base Color 1 Color 2

2. Example: Methyl orange (MeO)

HMeO(aq) + OH!(aq) W H2O(l) + MeO!

(aq) acid base acid basered (< pH 3) orange

(> pH 4.5)

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3. pH paper has indicator linked to the paper

B. pH meters have H+ selective membranes/electrodes

VII. pH in Solns. of Strong Acids & Bases A. By definition, a strong acid or a strong base

dissociates essentially completely in water:

1. HCl(aq) ÿ H3O+(aq) + Cl!(aq)

If [HCl]total = 0.5 M, at equilibrium:

[H3O+] = [Cl!] = 0.5 M, & [HCl].0 M

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Aside on kinetics: Generally, acid-base rxns. in aqueoussystems have extremely rapid kinetics. Does this mean it takes along or a short time to reach equilibrium?

2. NaOH(aq) ÿ Na+(aq) + OH!(aq)

If [NaOH]total = 0.5 M, at equilibrium:

[Na+] = [OH!] = M, & [NaOH]. M

Try Prob. 14.10 (d), p. 551.

B. If you dissolve H2SO4 in water so [H2SO4]total = 0.5M, can you predict [H2SO4],[HSO4

!], [SO42!], & [H3O+]

= ?

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VIII. Equilibria in Solutions of Weak Acids A. We can write an equilibrium constant relationship for

acid-base rxns. just as we did in Chapter 13.

1. HA(aq) + H2O(l) W H3O+(aq) + A!(aq)

[H3O+] [A!] “Kc” = [HA] [H2O]

2. [H2O] is so large compared to the acid component, it isessentially constant. Therefore:

[H3O+] [A!][H2O] x “Kc” = If [H2O] x “Kc” = Ka [HA]

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[H3O+] [A!] Ka = [HA] B. The value of Ka gives us an absolute (not relative)

measure of acid strength.1. Is it clear that stronger acids have larger Ka’s?2. Refer to Table 14.2, p. 552.3. The acids that we previously described as “strong acids

have Ka >1. (Often >>>1) Try Prob. 14.12-13.

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IX. Equilibrium Calculations: Weak Acid Solns.

A. We will now use the same approach (0.1 M !x)for calculating concentrations of components atequilibrium that we used in Chapter 13. (Summary in Fig. 14.7.)

1. List the system components.

2. Write equations & examine Ka values for the rxns.

3. Identify the principal reaction by its larger Ka value. The other rxns. are subsidiary reactions, and we canworry about them later.

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4. Make a table like we did in Chap. 13:

[Reactant] [H3O+] [A!] [Initial] M 0.050 .0 0

[Change] M !x +x +x [Equilibrium] M 0.050 !x x x

5. Now shift to your algebra mode:a) Substitute the bottom row into the Ka expression.b) Solve for x.

6. Use the value of x to solve for all [equilibrium].

7. If you wish, check your assumption re. Item #3.

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B. Practice this thoroughly, so you will becomfortable doing it on an exam. Prob. 14.14 b)

X. % Dissociation in Solns. of Weak Acids.

A. This is another way of comparing [A!]/[HA]. By definition:

[A!] % dissociation = x 100%[HA]total

B. If you compare equal [HA]total’s of a strong acid& a weak one, strong acid will obviously have ahigher % dissociation. (Check this on your own.)

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C. What happens when you look at % dissociationas a function of [HA]total?

1. The % dissociation increases as you dilute the acid.

2. This should seem logical if you consider collisiontheory and if you examine the forward and backwardrate laws. (See Fig. 14.8, p. 558)

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XI. Polyprotic Acids

A. Acids that donate > 1 H+ are polyprotic. Ex.:H2SO4, H3PO4, H2CO3, & citric acid

B. If the 2 (or more) acidic H are near each other,Ka1 ~104!106 > than Ka2. (Electrostatics.)

1. Let’s look at the dissociation of H2CO3.

1st dissociation Ka1 =

2nd dissociation Ka2 =

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2. Look at Table 14.3 for summary of values.3. Try Prob. 14.17, p. 562 on your own.

XII. Equilibria in Solutions of Weak Bases.

A. Bases accept H+: H2O + B W BH+ + OH!

[BH+] [OH!] Kb = [B]

B. This is directly analogous to what we havedone with acids. (See Table 14.4 for acomparison of Kb & Ka values for bases and theirconjugate acids.) Transition!

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XIII. Relationship Between Ka and Kb.

A. In Section II, we reached the qualitativeconclusion that a strong acid must have a weakconjugate base (& vice versa). We now needto look at this quantitatively.

B. Consider the loss of H+ by the acid HF, andthe gain of H+ by the base F!:

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1. Reactions:

Acid rxn. HF(aq) + H2O(l) W H3O+(aq) + F!(aq)Base rxn. F!(aq) + H2O(l) W HF(aq) + OH!(aq)

Sum: HF + H2O + F! + H2O W H3O+ + F! + HF + OH!

(states omittedto save space)

2. A Kc expression for the summed rxn. would be:

[H3O+] [F!] [HF] [OH!]“Kc” = [HF] [H2O] [F!] [H2O]

which simplifies to: Kc = [H3O+] [OH!] = Kw

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3. The same result is obtained by multiplying the Kcexpressions for the individual rxns.:

[H3O+] [F!] [HF] [OH!]Ka x Kb = x = [H3O+][OH!] = Kw [HF] [F!]

Aside: We have just proven for this case something that is truegenerally. When 2 or more rxns are added together, the Kc for thesummed rxn. = the product of the Kc values for the individual rxns.

C. Finally, if Ka x Kb = Kw, that means that Kaand Kb must be inversely proportional. (If Kais big, Kb must be small.) This supports ourintuitive comments on the strength of acids &their conjugate bases earlier in Section II.

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XIV. Acid-Base Properties of Salts

A. Ionic compounds are also called salts. 1. One way to make salt is: acid + base ÿ salt + water.2. Understand salt’s acid-base properties by looking at

the strengths of the acid & base used to prepare it.

B. Some guidelines:1. If both acid & base were strong, salt will be neutral.2. If acid was strong & base weak, salt will be acidic.3. If base was strong & acid weak, salt will be basic.4. If both acid and base were weak, the outcome is

decided by which is less weak (compare Ka & Kb). Try prob. 14.22, p. 568.

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XV. Factors That Affect Acid Strength(comparative)

A. Bond strength (within hydrohaloacid series)

B. Electronegativity (within Period 2 elements)

XVI. Lewis Acids and Bases

A. By definition:1. A Lewis acid is an e! pair acceptor.2. A Lewis base is an e! pair donor.

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B. Lewis acid-base concepts are extremely usefulfor analyzing some types of chemicalrxns./processes.1. Organic chemistry2. Metal ion-ligand interactions (hemoglobin?)3. Some branches of inorganic chemistry (boron)