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. Chapter 4: Arrhenius acid-base ideas:

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+. (Probs 14.1-2)

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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

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?

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2. For pictures, look at Conceptual Problem 14.3 & Table 14.1.

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3. You must become adept at this. Try Prob. 14.5Alternatively, if HA is a strong acid, is its conjugate basestrong or weak?

Prob 14.5

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III. Factors That Affect Acid Strength(comparative)

A. Bond strength (within hydrohaloacid series)

B. Electronegativity (within Period 2 elements)

C. For oxoacids1. Increasing electonegativity correlates with

increasing acid strength.2. Increasing oxidation number correlates with

increasing acid strength.

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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?

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

1. “Kc” =

2. Can we simplify this?

Kw =

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

4. In “pure” water at 25EC, [OH!] = ?

5. What is [H2O] = ?

6. Numerical value for Kw: Kw = [H3O+] [OH!] Prob. 14.8-10

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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. 14.11

2. Make sure you can also find [H3O+]. Prob. 14.12

B. Note that as [H3O+] increases, pH decreases because

pH is the negative log of [H3O+].

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C. What about rounding? Essentially the x /÷ rule plusone.

!log (1.0 x 10!9) = 9.00

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

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VI. Measuring pH

A. Indicators

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

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

(aq) acid base acid base Color 1 Color 2

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2. Example: Methyl orange (MeO)

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

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

(> pH 4.5)

3. pH paper has indicator linked to the paper

B. pH meters have H+ selective membranes/electrodes

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VII. pH in Solns. of Strong Acids & Bases

A. By definition, a strong acid or a strong basedissociates 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

Aside on kinetics: Generally, acid-base rxns. in aqueous systemshave extremely rapid kinetics. Does this mean it takes a long or ashort time to reach equilibrium?

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2. NaOH(aq) ÿ Na+(aq) + OH!(aq)

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

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

Try Prob. 14.13 (d), p. 566.

B. If you dissolve H2SO4 in water so [H2SO4]total = 0.5 M,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 foracid-base rxns. just as we did in Chapter 13.

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

[H3O+] [A!]

“Kc” = [HA] [H2O]

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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]

[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. 567.3. The acids that we previously described as “strong acids have

Ka >1. (Often >>>1) Try Prob. 14.15, p. 568.

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Conceptual Problem 14.15 (not 13), p. 568.

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.

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The other rxns. are subsidiary reactions, and we canworry about them later.

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.

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

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

B. Practice this thoroughly, so you will becomfortable doing it on an exam. Try Prob.14.17 (a), p. 573.

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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.)

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

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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. 573)

3. Do prob. 14.19, p. 573 on your own.

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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, p. 574, for summary of values.3. Try Prob. 14.20, p. 577 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 have donewith acids. (See Table 14.4, p. 577, for acomparison of Kb & Ka values for bases and theirconjugate acids.) Transition!

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XIII. Relationship Between Ka and Kb.

A. Sect II, we concluded that a strong acid musthave a weak conjugate base. Now, quant.

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 omitted to 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

3. The same result is obtained by multiplying the Kc

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expressions 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 Ka

and Kb must be inversely proportional. (If Ka isbig, 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. 14.25, p. 583.

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XV. 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.

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)

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FYI: Acid Rain and its Effects

A. Sulfur and nitrogen oxides + H2O form oxoacids.

B. SOx & NOx are formed in combustion processes.

C. Oxo acids can kill creatures like fish and dissolvemarble statues.

See prob. 14.31 & 32, p. 589.

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