Johnston Chapter 17 Notes

8/8/2019 Johnston Chapter 17 Notes

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Chapter 17

Aqueous Acid-Base

Equilibria

17.1 Proton Transfers in Water

17.2 The pH Scale

17.3 Weak Acids and Bases

17.4 Recognizing Acids and Bases

17.5 Acidic and Basic Salts and

Oxides

17.6 Factors Affecting Acid Strength

17.7 Multiple Equilibria

17.1 Proton Transfers in Water

Brønsted-Lowry definition of acid-base reactions

Acid ± any substance that can donate a proton (H+) to another substance.

Can be neutral, cations or anions

HNO3, NH4+, H2PO4

Base ± any substance that can accept a proton (H+) from another

substance.

Can be neutral or anions

NH3, CO32-

Some critical thinking: Actually, it

could be a cation, too²but rarely!Which of the following could take

two or more protons?

A. NH2CH2CH2 NH2

B. N2H4

C. CH3 NH2

D. Two of A-C.

E. Three of A-C.

Types of acids and bases

Monoprotic acids: onlycapable of donating one

proton

HCl, HI, HS-, HPO4-,

Polyprotic acids: capable of donating two or more

protons

H2CO3, H3PO4,

Monoprotic bases: onlycapable of accepting one

proton

Polyprotic bases: capable of accepting two or more

protons

SO42-, CO3

2-, PO43-

Amphiprotic : molecules or ions whichcan behave either way (either an acid

or a base)

Types of acids and bases

Dissociation of Water Water: solvent for most acid/base reactions

Hydronium ion: hydrated hydrogen ion

2H2O (l) H3O+ (aq) + OH-(aq)

Referred to as autoionization ± self-ionization

D issociation of Water

Water and Equilibrium We already talked about the autoionization

of water

2H2O (l) H3O+ (aq) + OH- (aq)

So, what does the K for this autoionization

look like?

3w 0]][[K

v!! OH O H

An older²and hence better²way

of expressing this«

Some terminology for K w ...

K w would be expected to _____ with

increasing T.

A. decrease

B. increase

K w increases with increasing T since...

% (H > and (S < .

& (H < and (S < .

C. (H > and (S > .

D. (H < and (S > .

³ x´ moles of a weak acidneutralize fewer moles of a strong

base than would ³ x´ moles of a

strong acid .

A. True

B. False

Example 17 - 1Determine the ion concentration in a 5. x 1 -2

M aqueous solution of HClO4, a strong acid.

What are the ion concentrations in .5 L of an aqueous solution that contains 5. g of NaOH?

Example 17 - 2

17-1: 0.050M HClO4« This is a strong acid.

This means that it is a strong electrolyte.

Hence, it is 1 % dissociated and

H+ = . 5 M.

If you missed this one, think!!!

17-2: 5.00 g NaOH(aq) in 0.500L

solution«

N ote«

Having to calculate H+ for basic solutionsusually requires an extra step.

Basic solutions in general often require morework!

This is because we usually think in terms of [H+]² as we shall see when we get to pH.

17.2 The pH Scale Methods for expressing the hydronium and

hydroxide ion concentrations

pH = - log H3O+ and pOH = -log OH-

pH + pOH = 14

At a higher temperature, pK wwould be less than 14.00.

A. True

B. False

Features of the pH Scale1. pH = 7. defines a neutral solution. Acid

solutions have pH < 7 and basic solutions have

pH > 7. (Note: We assume 25rC here!)

2. The more acidic the solution, the lower its pH.

3. A change in pH of one unit reflects a tenfoldchange in the H+ ion concentration.

4. The immense range of concentrations from >1M to < 1 -14 M is compressed into a moreconvenient range, from ~ -1 to ~ 15

Example 17 - 3What are the concentrations of hydronium and

hydroxide ions in a beverage whose pH =

Thi s i s easy if you k now how to use your

calculator! Also, t hi s i s tr ivial grade-school mat h.

ant Equation...

« » !- ½

Alternate way for [OH

Other µp¶ scales Remember when we studied equilibrium

constants?

We also discussed equilibrium constants for acidsand bases.

These constants are usually quite small, so we canalso use the logarithmic function on K a and K b

p K a = - log K a

p K b = - log K b

Generalizatio n (for a general qua n tity, X)...

Example 17 - 4What is the pH of a .25M solution of NaOH?

Remember t hat we are deal ing wit h a base

and, t hus, t here i s a l ittle more wor k. W e

shall do t hi s two ways...

The quickest way (maybe)...

The other way...

Overview of pH and pOH

Given any one of these following: H3O , OH ,

pH or pOH, the other three can be determined.

pH = log H3O H3O

= 1 pH

pOH = log OH OH = 1 pOH

H3O OH = K w pH + pOH = pK w

At 25. °C:

H3O OH = 1. x 1

At 25. °C:

pH + pOH = 14.

P lease remember...

P lease remember that the following are the

same thing (but that one is simpler to

write)...

17.3 Weak Acids and Bases Think back to the table of strong acids

and bases. There aren¶t too manylisted.

What about the other acids and bases? They are referred to as weak acids and

bases.

They do not totally dissociate in

solution. ± Weak acids contain the acid and water as

major species.

± Weak bases contain the base and water asmajor species

Weak Acids Proton transfer to water is not quantitative

Therefore, there exits an equilibrium where

only a small fraction of the acid moleculeshave transferred their protons to water

In a solution, the major species are water

molecules and the acid, HA.

HA + H2O H3O+ + A- 3

AO H K

Percent Ionization

The hydronium ionconcentration is atequilibrium.

The weak acidconcentration is theinitial concentration .

initial

eq ionized

Example 17 - 5The pH of a .25 M aqueous HF solution is

1.92. Calculate the K a for this weak acid.

Thi s i s an e xam ple o f gett ing K g iven t he

concentrat ions at equil ibr ium. W e shall do

t hi s now b y just showing t he ste p s ( no comments)...

Is it usually true that, for a weak

acid (HA) dissolved in water,

H+ = A ?

A. Yes

This is because...A. of stoichiometry.

B. the solution must be neutral.

C. because I said so!!!

When would H+

not betrue for an acid, HA.

A. HA is a strong acid.

B. HA is extremely dilute (< 1 7 M).

C. HA is extremely weak ( p K a > 12).

D. B and C true; A false.

Weak Bases Proton transfer from water is not quantitative.

Therefore, there exists an equilibrium where only

a small fraction of the base molecules haveaccepted protons from water.

In a solution, the major species are water

molecules and the base, A.

A + H2O HA + OH

What about neutral bases?

Example 17 - 7Ammonia has K b = 1.8 x 1 -5. What is the pH

of .25 M aqueous ammonia?

Remember a base i s invol ved! S o t here may

be a l ittle e xtra wor k ! Since we have seen

t hi s sort o f t hing be f ore, we can ski p t he RICE ...

Recognizing Acids and Bases

Oxoacids ± an acid that contains an inner atom bonded to a variablenumber of oxygen atoms and acidic OH groups. HxEOy

S ome Weak Oxoacids

Carboxylic Acids

All carboxylic acids are weak acids.

Representative Polyprotic Acids

S ome Weak Acids

Example 17 - 8

Examine the following formulas. Decide if each represents a strong acid, a weak acid

or neither. Justify your conclusions.

(a) Cl3CCO2H

(b) CH3CH2CH2OH(c) HCN

(d) HClO4

rs...(a) Cl3CCO2H

A weak acid (but stronger because of the chlorines onthe terminal carbon).

(b) CH3CH2CH2OH Not an acid at all. This is a simple alcohol.

(c) HCNA weak acid (but don¶t breathe it!)

(d) HClO4

A strong acid. In fact, the strongest known in aqueoussolution!

How does water stack up as a weak

1. 1 @ 25

H OH K

« » « »- ½ - ½

Weak Bases

Water is a weak base

Another common weak base is NH3

Many other weak bases are derivatives of ammonia called amines.

Some of the N ± H bonds have been replaced with C ± H bonds.

R epresentative Organic Bases

How does water stack up as a weak

1. 1 @ 25

H OH K

« » « »- ½ - ½

Conjugate Acid ± Base Pairs

A pair of compounds which differ only by

one proton.

± After an acid donates a proton, the remaining

species turns into a conjugate base (CB).

± After a base accepts a proton, the remaining

species turns into a conjugate acid (CA).

HF (aq) + H2O (l) H3O+ (aq) + F- (aq)

Acid Base C on jugate

C on jugate

A + B C A + C B

Example 17 - 1

Write the chemical formula and the Lewisstructure and draw a molecular picture of

each of the following:(a) The conjugate acid of NH3

(b) The conjugate base of HCO2H

(c) The conjugate acid of HSO4

(W e¶ll show t he answers on t he ne xt two sl ides. )

The answers (formulas):

(a) The conjugate acid of NH3 is NH4+.

(b) The conjugate base of HCO2H is HCO2.

(c) The conjugate acid of HSO4 is H2SO4.

What is the conjugate acid of

water?A. H2O itself

B. H3O+

D. H2SO4

E. Lime Jello

What is the conjugate base of

H3O+?A. H2O

B. H3O+ itself

D. H2SO4

E. Luciferin

What is the conjugate base of

water?A. H2O itself

B. H3O+

D. H2SO4

E. Lime Jello

What is the conjugate acid of the

hydroxide ion?A. H2O

B. H3O+

D. There can be none!

E. A sweet, singing, Eskimo Pie.

What is the conjugate base of the

hydroxide ion?A. H2O

B. H3O+

D. The oxide anion

E. There can be none since hydroxide only

exists in aqueous solution.

The answers (formulas):

(a) The conjugate acid of NH3 is NH4+.

(b) The conjugate base of HCO2H is HCO2.

(c) The conjugate acid of HSO4 is H2SO4.

(Here, we are continuing from what we were

doing earlier!)

M ore details...

17.5 Acidic and Basic

Salts and Oxides An aqueous solution of a soluble salt

contains cations and anions.

The ions can often have acid-base properties.

Anions that are conjugate bases of weak acids make a solution basic.

Cations that are conjugate acids of weak bases make a solution acidic.

H2O (l) + F- (aq) HF (aq) + OH- (aq)

Notice the form for K b ...

This leads to an interesting relationshipwhen K w enters in...

and K b

and K w

How are the three related?

K a K b = K w

Where K a is the ionization constant for a weak acid and K b is the ionization constant for itsconjugate base.

p K a + p K b = 14. (25.

( N ote that we have already derived this!)

At 25rC, p K b = 4.35 for a given base. What is p K a at this

temperature?

Error tolerance: s0.01

At 25rC, K a = 4.7 v 107.Calculate p K b.

Error tolerance: s0.01

Chapt er 16

Important points about K a

and K b

Strength K a

Conjugate Base

Strength K b

Strong >1 Very Weak < 10-16

Weak 10-16 to 1 Weak 10-16 to 1

Very Weak < 10-16 Strong >1

Example 17 - 11

Sodium hypochlorite (NaOCl) is the active

ingredient in laundry bleach. Typically,

bleach contains 5.0% of this salt by mass,which is a 0.67 M solution. Determine the

concentrations of all species and compute

the pH of laundry bleach.

Steps for solution...

Write the equilibrium expressions.

Get the needed constants.

Set up and solve the equations. Determine the relevant species present and

get their concentrations.

W e shall do t hi s on t he ne xt sl ides wit hminimal comment ...

Relevant species...

Example 17 - 12

What are the important acid-base equilibria in

an aqueous solution of pyridinium chloride

(C5H5 NHCl)? What are the values of their equilibrium constants?

T hus you should a n sw er...(Note the three equation forms for the 1st equilibrium!)

17.6 Factors Affecting Acid Strength

Effect of charge ± affects the ability to

donate and accept protons (remember

opposites attract)

Structural Factors

In order to donate a proton, a molecule must break a H ± X bond.

This becomes easier as bond strengths decrease,

and therefore the acids become stronger.

It is also affect by polarity.

Example 17 - 13

Oxalic acid, HO2C ± CO2H, has K a1 = 1.3 x

10-2 an K a2 = 1.4 x 10-4. Formic acid

HCO2H, has K a = 1.8 x 10-4. Explain whythe first proton of oxalic acid is

substantially more acidic than the proton of

formic acid, but the second proton is less

acidic.

A little reasoning...

The more polar a bond is,

± the less covalent it is!

The less covalent it is, ± the weaker it is! (That is, the electron density

between the atoms decreases.)

Thus polar bonds

± break more easily in polar media! (That is, polar solvents.)

T hus, the solutio n com eth to

mynde... The COOH group is quite electron

withdrawing.

Thus, the first proton in HOOC-COOH ismore easily removed than is the proton inHCOOH! (It¶s OH bond is more polar!)

The second proton in oxalic acid is more

difficult to remove since we now have anegatively charged species, HOOC-COO.

N ecessary point...

The simple arguments used here and in the

book presuppose that K 1a >> K 2a.

Usually, if they are three orders of

magnitude apart (or more), the simple ideas

hold!But, if they are closer together, all bets are off!

C oncentrations of species in an

aqueous solution of a diprotic acid:

Example 17 - 14

Carbonated water contains carbonic acid, a diproticacid that forms when carbon dioxide dissolves inwater.

CO2 (g) + H2O (l) H2CO3 (aq)

A typical carbonate beverage contains 0.050 M

H2CO3. Determine the concentrations of the ions present in this solution.

(W e sol ve t hi s wit h minimum comments... )

The solution...

Example 17 - 16

Potassium sulfite is commonly used as a food

preservative, because the sulfite anion undergoes

reactions that release sulfur dioxide, an effective preservative. Determine the concentrations of the

ionic species present in a solution of potassium

sulfite that is 0.075 M.

(When you sol ve f or O H

, t hings pretty much parallel what we just d id wit h H + f or a d i prot ic

acid!)

Some points here...

We shall look at a general case first.

Again, we get a simple solution if the K -

values are separated by several orders (3 atleast²4 or more better) of magnitude.

The derivation is quite straightforward and

will be done with no comment on the slides.

The Derivation

T he Deri vatio n ...

F or this example...

The solution (no comments)

The solution (no comments)...

C omment...

There is a ³symmetry´ between solving for

H+ in acidic solution and OH] in basic

solution. In the latter case, K b¶s replace K a¶s and we

solve for OH] directly instead of H+].

Nothing pHurther need be said unless youenjoy suffering!

Some exact solutions...

You have been given some methods that

work quite well in many cases.

However, at times, you will encounter errors with very dilute solutions or very

small K a (with H+) or very small K b (with

) values.

Strategy with exact solutions...

Write out all equilibrium constants.

Impose conservation of mass.

Impose conservation of electrical charge.

Combine the equations, and solve for H+]

in acidic solutions (or OH] in basic

solutions; we shall not discuss this latter case).

Q uestio n

This problem is mean, sneaky, anddeceptive!

± Thus, I love it! Here it is:

± What is the pH of a 1.0 x 108 M aqueoussolution of HClO4?

± If you said ³ pH = 8.0,´ please put on your dunce cap! (Please remember that water itself already has H+] = 1.0 v 107 M.)

The exact solution

The exact solution...

F or 1.0 v 10-8 M HClO4...

A ³monster case´...

We shall end this discussion with an exactsolution for a triprotic acid .

Why? ± This is about the most complicated case you

will ever encounter (e. g ., H3PO4).

± Having this, the diprotic and monoprotic cases

are easily derived. Johnston has programmed hexaprotic acids

in his calculator. He is, of course, ashowoff...

The defining restraints on the system...

Algebra pHun, part I:

Diprotic acid...

Just set K 3 = 0 and go from there...

Monoprotic acid

M onoprotic acid...

Just set K 2 = 0, leaving only K 1 and K w...

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