acid and base equilibria what do you already know …courses.chem.psu.edu/chem112/fall/lecture...

1CH112 LRSVDS ACIDS BASES Part 2

Acid and Base EquilibriaChapter 16

Review Chapter 4.1 (Electrolytes)

Review Chapter 4.3 (Acid-Base Reactions)

Memorize 7 Strong Acids (Table 4.2)

Memorize the Strong Bases (Also Table

4.2)

Review Equilibrium (Ch. 15)

PRE-REQUISITES:


What do you already know

about ACIDS?

1) Acids Taste?

2) Acids dissolve active metals, usually

liberating ___________.

3) Acids are corrosive – they dissolve

compounds that are otherwise hard to

dissolve.

4) Acids turn litmus paper ______________.


What do you already know

about BASES?

1) Bases Taste?

2) Bases dissolve oil and grease.

3) Bases are slippery to the touch.

4) Bases react with many metal ions to form

precipitates.

5) Bases turn litmus paper ________________


Definitions:

Arrhenius ACID:

Any compound that releases H+

when dissolved in H2O.

Arrhenius BASE:

Any compound that releases OH!

when dissolved in H2O.

ARRHENIUS Acids and Bases


Hydronium ion

Need a new definition of acidsand bases based on

Proton Transfer


BrØnsted ACID:

Any compound capable of donating an

H+ ion.

BrØnsted BASE:

Any compound capable of accepting a

H+ ion.

BRØNSTED - LOWRYACIDS AND BASES


CONJUGATE ACID-BASE PAIRSCONJUGATE ACID-BASE PAIRS

Differ only by the presence or absenceof a proton (H+).

Conjugate Acid Conjugate Base + H+

Examples: Conjugate Acid Conjugate Base

Note:The stronger the acid, the weaker its

conjugate base.

The weaker the acid, the stronger itsconjugate base.


Is it weak, strong or negligible?

1. The conjugate of a strong acid is a______________. (Example: Cl! is theconjugate base of HCl). Spectatorshave negligible acidity/basicity.

2. The conjugate acid of OH! (strong

base) is _________ (negligible acidity).

Also true for strong bases H- and CH3-.

3. The conjugate of a weak acid is a

_________________.

Compound Conjugate

Strong Acid

Strong Base

Weak Base

9CH112 LRSVDS ACIDS BASES Part 2 10CH112 LRSVDS ACIDS BASES Part 2

Auto-ionization of water:

Water electrolyzes slightly to produce H+ and

OH! reversibly.

H2O(l) H+ (aq) + OH- (aq)

What is the equilibrium constant expression

for this process?

What is the [H+] of pure water at 25°C?


NOTE:

Kw is constant even when [H+] and [OH!]

are not equal.

Calculate [H+] in a 0.05 M Ca(OH)2 solution.

Kw is the Ion Product Constant

for water


1. What is the pH of pure water at 25°C?

2. What is the [H+] of an HCl solution

that has a pH of 2.34?

The pH Scale


Negative Log Scale is Useful

for Many Small Numbers:

pOH =

pKw =

How are pH and pOH Related?

Kw = Take log of all quantities:


pH and Concentration


Measuring pH

!pH Meter

!Indicators

H(Ind) Ind- + H+


Compare the effect of

concentration on pH

pH[H+]Solution

2.0 x 10-7 M2

1.0 x 10-7 M1

Many biological systems involve proton

transfer reactions; the rate of reaction

depends upon [H+]

What effect will a large change in pHhave on a biological system?


Relationships to Remember

Kw =

pH =

pOH=

pKw =

pH + pOH =


Which Bulbs Light Up?

Distilled H2O

Tap H2O

NaCl(aq)

1M HCl(aq)

1M CH3COOH

sugar(aq)

CH3OH

What is required for the bulb to light up?

CONDUCTIVITY: ability to conduct electricity

Bulb Wattage2.5 7.5 25 40


STRONG ACIDS

DISSOCIATE COMPLETELY to form:

Strong acids are strong electrolytes

[H+]final = [HA]initial = CHA

*** If the analytical concentration, CHA, is less

than 10!6 M, then the autoionization of water

must be considered.

Which one of the following is not a strong acid?

1. HNO3 5. HCl

2. HF 6. HBr

3. HClO3 7. HI

4. HClO4 8. H2SO4


PARTIALLY DISSOCIATE to form :

Weak acids are weak electrolytes

[H+]final < [HA]initial

Examples:

Concentrations are calculated using theequilibrium constant.

Keq= Ka =

WEAK ACIDS


Ka is the acid dissociation constant


How do you find [H+] for a weak acid?It is an equilibrium problem!

REVIEW:

General Approach to EquilibriumConstant Problems

1. Write the balanced reaction.

2. Write the general form for Keq.

3. Set up a data table (ICE).

4. Substitute equilibriumconcentrations into theexpression for Keq and solve.

!

x ="b ± b

2" 4ac

2a

!

ax2

+ bx + c = 0

Quadratic Equation:


What is the [H+] of 0.10 M HI?

What is the [H+] of 0.10 M acetic acid?

Ka = 1.8 x 10-5

Calculation of [H+] for acids


Calculations Continued

What is the pH of 0.10 M acetic acid?

What is the % dissociation?

Percent dissociation =

!

H+[ ]

HA+[ ]x100%


Capable of donating more than one

proton.

What happens to Ka with each successive

dissociation?

H2CO3(aq) H+ (aq) + HCO3!(aq)

Ka1 = 4.3 x 10!7

HCO3!(aq) H+ (aq) + CO3

2!(aq)

Ka2 = 5.6 x 10!11

H3PO4 (aq) H+(aq) + H2 PO4!(aq)

Ka1 = 7.5 x 10!3

H2PO4!(aq) H+ (aq) + HPO4

2!(aq)

Ka2 = 6.2 x 10!8

HPO42!(aq) H+(aq) + PO4

3!(aq)

Ka3 = 4.2 x 10!13

POLYPROTIC ACIDS


What are the concentrations of H+,

HCO3!, and CO3

2! in 1 x 10!3 M H2CO3?


In H2CO3(aq) solution, what are the

conjugate acids and bases present?

Rank them in order of:a) increasing acid strength.

b) increasing base strength.


Distribution curve for phosphoric

acid (H3PO4)

NaOH is added to a solution of H3PO4

until the pH reaches 12. Which species

are present in the solution?

1.00

0.50

0.00


STRONG BASES

Examples:

Strong Bases DISSOCIATE

COMPLETELY to form:

Strong bases are strong electrolytes.

KNOW:

! Group I and II hydroxides are

strong bases (EXCEPT Mg and Be).

! Arrhenius bases donate OH!.

! BrØnsted bases accept H+


WEAK BASES

PARTIALLY DISSOCIATE WATER to form :

Weak bases are weak electrolytes

[OH!]final < [B]initial

Base Hydrolysis:Bases react REVERSIBLY with water to form OH!

ions

What is the equilibrium constant for basehydrolysis?

Keq = Kb =


WEAK BASESBase Hydrolysis

" Weak bases can be neutral Example: NH3, amines (NR3)

NH3(aq) + H2O(l) NH4+(aq) + OH!(aq)

" Weak bases can be anions (anyion derived from a weak acid)

Example: F!, NO2!, CH3COO!

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


What is the pH of 0.1 M NH3?

Kb = 1.8 x 10-5

Base Hydrolysis


Things to KNOW

Acid Dissociation

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

Equilibrium constant = Ka

!

Ka

=[H

+][A

"]

[HA]

!

Kb

=[HB

+][OH

"]

[B]

Base Hydrolysis

B + H2O(l) HB+(aq) + OH!(aq)

Equilibrium constant = Kb


Acid: CH3COOH H+ + CH3COO!

Base: CH3COO! + H2O CH3COOH + OH-

-------------------------------------------------------

H2O H+ + OH-

Ka = Kb =

How is Ka related to Kb?

What is the Conjugate Acid/ Base pair for acetic acid?

Ka x Kb =

Ka x Kb =


When we add two reactions together, we

multiply their equilibrium constants.

For conjugate acid-base pairs:

Ka x Kb = Kw = 1 x 10!14

Larger Ka means smaller Kb

Stronger the acid, weaker the base


Conjugate acid/base

Strength

H-F + OH! F! + H2O

Stronger acid

6.9 x 10!4

Weaker acid

Ka = 10!14

Stronger

base Weaker base

Kb = 1.4 x 10!11

• The conjugate of a weak acid is a weak base

(and vice versa)

• The conjugate of a strong acid is a spectator ion

(example: Cl! is the conjugate base of HCl).

• The conjugate acid of OH! (strong base) is water.


What are the Spectator Ions?

Strong Acids Conjugate Base

Conjugate Acids of Strong Bases (Arrhenius):


Salts dissociate completely in H2O

CH3COONa "

NH4Cl "

NaCl "

What is the pH of a solution formedby dissolving these salts in water?

Acid-Base Properties of AqueousSalt Solutions


Salt Solutions

When a salt is added to water will thesolution be acidic or basic?

1. Salts dissociate completely in H2O

2. Does the cation act as an acid?

(donate a proton)? If so what is Ka?

3. Does the anion act as a base?

(hydrolyze water?) If so what is Kb?

Hydrolysis: a cation or anion reactswith H2O to form H+(aq) or OH!(aq)


What is the pH of 0.12 M CH3COONa?

Ka (CH3COOH) = 1.8 x 10!5

Finding the pH of an aqueoussalt solution


Formation of Salt Solutions

Acid + Base " Salt + water

HCl + NaOH " NaCl + H2O

Na+ is the conj ACID of NaOH

Cl! is the conj BASE of HCl

CH3COOH +NaOH " CH3COONa + H2O

CH3COO! is the conj. BASE of CH3COOH

Na+ is the conj ACID of NaOH

HCl + NH3 " NH4Cl

NH4+ is the conj ACID of NH3

Cl! is the conj BASE of HCl


Will a salt be acidic or basic?

1. Salt derived from a strong acid and a

strong base

Neutral solution (pH = 7)

Example: NaCl (from NaOH and HCl)

2. Salt derived from a weak acid and a

strong base

Basic solution (pH > 7)

Examples:

NaClO (NaOH and HClO)

ClO! (aq) + H2O HClO (aq) + OH!(aq)

(CH3COO)2Ba (Ba(OH)2 and CH3COOH)

CH3COO!(aq) + H2O CH3COOH(aq) +OH!(aq)

Salt Solutions


3. Salt derived from a strong acid and a

weak base

Acidic solution (pH <7)

Example: NH4Cl (NH3 and HCl)

NH4+ + H2O NH3 + H3O

+

4. Salt derived from a weak acid and a

weak base

pH depends on acid/base involved

Example: NH4CN (NH4+ and CN!)

Salt Solutions


1. Is it a salt? Dissociates

completely in H2O

2. Does the cation act as an acid?

(donate a proton)? If so use Ka

3. Does the anion act as a base?

(hydrolyze water?) If so use Kb

# What is the pH of 0.02 M KN3?

Ka (HN3) = 1.9 x 10!5

Finding the pH of a salt solution


Summary: Salt Solutions

XY(s) + H2O(l) " X+(aq) + Y!(aq)

X+ yields a neutral pH solution if it is the

conjugate acid of a strong base.

Examples: Na+, K+

X+ yields an acid (low pH) if it is the

conjugate acid of a weak base.

Examples: NH4+

Y- yields a neutral pH solution if it is the

conjugate base of a strong acid.

Examples: Cl!, Br!, NO3!

Y- yields a basic (high pH) solution if it is

the conjugate base of a weak acid.

Examples: CH3COO!, OCl!, F!, CN!


Acid Strength

Larger Ka = Stronger Acid

What if you don’t know Ka?

H-XX = Cl-, CH3COO-, NO3

-, etc.

How easily does H+ break away from X-?

Weaker H-X bond = Stronger Acid

What does HX bond strength depend on?

H X

H X


• Remember the Order for

Electronegativity

For Our New Clambakes

Bring Internationally-Shipped

Canned Hams, Please


Correlation of Acid Strength withStructure: Polarization of H#X

Acid strength increases withincreasing polarization ofH#X bond


Draw the Lewis Structure

OxyacidsMany acids consist of a central atom withseveral attached oxygen atoms. These arecalled oxyacids.

Examples:

HOClO3 HOClO2

HOClO HOCl


Acid strength increases with:1) Increasing electronegativity of thecentral atom

HOCl > HOBr > HOI

2) Increasing oxidation state of thecentral atom

HOClO3 > HOClO2 > HOClO > HOCl

Acid Strength

General rule for uncharged oxyacids HxEOy:

If y-x > 2 then strong (H2SO4, HNO3,…)

If < 2 then weak (H2CO3, HBrO, HNO2,…)


Sample Problem

Of the following, __________

is the strongest acid.

A. F3C-COOH

B. Cl3C-COOH

C. Br3C-COOH

D. Br2ClC-COOH

E. H3C-COOH


Inductive effect stabilizes

the conjugate base

Ka

CH3COOH 1.8 x 10!5

CH2ClCOOH 1.4 x 10!3

CHCl2COOH 3.3 x 10!2

CCl3COOH 2.0 x 10!1


LEWIS ACIDSAny substance that can accept

_________________________

•Have positive charge

•Need more electron density

Examples of Lewis Acids:

Highly charged transition metal cations:

Group III cations:

and compounds:

Smaller group II cations:

Compounds with Incomplete Octet:

LEWIS BASES

Any substance that can donate

_______________________

•Have lone pair electrons

•May be neutral or anionic

Examples of Lewis Bases:

Broader Definition of Acids and Bases:


Ni

H3N

H3N NH3

NH3

NH3

NH3

2+

Ni2+(aq) + 6NH3(aq) " Ni(NH3)62+(aq)

Lewis Bases Coordinate to

Metal Cations


LEWIS CATIONS

To compare acidity of Lewis acids, first

compare charge. If charge is the same then

compare size.

Charge/Size Ratios

Metal Ion Charge/Ionic radiusNa+ 1.0

Li+ 1.5

Ca2+ 2.1

Mg2+ 3.1

Zn2+ 2.7

Cu2+ 2.8

Al3+ 6.7

Cr3+ 4.8

Fe3+ 4.7



Hydration

Metal ions attract the lone pairs on the oxygen in

water molecules. This is a Lewis acid – Lewisbase reaction – the metal ion is the Lewis acid.

Hydration increases with increasing charge/sizeratio of the metal ions.

Hydrated metal ions have acidic character whichincreases with increasing charge/size ratio ofthe metal ions.

Mz+ OH

H

:

:$-

$+


HYDRATION

WHY do metal ions behave like acids?

Hydrolysis is a reaction that dissociates water.

M(H2O)nz+ M(H2O)n-1(OH)(z-1)+ + H+

Fe(H2O)63+ Fe(H2O)5(OH)2+ + H+

The hydration of the metal ion provides a

source of protons (this is an example of

hydrolysis):

Al(H2O)63+ Ka=1.2x 10-5

Fe(H2O)63+ Ka=6.7x 10-3

Zn(H2O)42+ Ka=3.3x 10-10

Ag(H2O)2+ Ka=1.2x 10-12


Sample problem

What is the concentration ofphosphate ions in a 2.5 Msolution of phosphoric acid? (ThepH of the solution is 0.87.)

The acid-dissociation constants ofphosphoric acid (H3PO4) at 25oCareKa1 = 7.5 x 10!3

Ka2 = 6.2 x 10!8

Ka3 = 4.2 x 10!13

A. 1.9 x 10!19 M

B. 1.0 x 10!6 M

C. 0.13 M

D. 6.2 x 10!8 M

E. 4.2 x 10!13 M


Sample Problem

Which of the following salts will

give the most basic solution

when dissolved in water?

1. KBrO4

2. KBrO3

3. KBrO2

4. KBrO

5. KClO4


Acid/Base SALTS Review

1) Which one of the following saltswould have a basic aqueoussolution?

1. KF 3. NaI

2. Al(NO3)3 4. NH4Br

2) Arrange the following in the orderof increasing base strength:

NO2! NO3

! PO43! HSO4

!

3) Which of the following cannot actas a Lewis base?

1. Cl!

2. OH!

3. CN!

4. NH3

5. H+


ACIDS

React with water to form H3O+ ions.

2 types

Molecule containing ionizable protons

CH3COOH, HF, HNO3

Cations/Lewis Acids.

Non-metal cations ex. NH4+

Metal cations (Lewis acids)

BASESReact with water to form OH!

Molecules that contain OH!

NaOH, Ba(OH)2

Anions/Lewis Bases

Hydrolyze water to form OH!

Example: CH3COO!, F!

Molecules with extra lone pairs(LewisBases)

Examples: H2O, NH3


SALTS

MA Mn+ + An- in water

• If Mn+ has high charge/size ratio then acidic

• If A! is conjugate base of weak acid then basic

• If A! is conjugate base of strong acid then

neutral


YOU SHOULD KNOW

GIVEN FIND

pH [H+], [OH-],pOH[H+] or [OH-] pHList of acids Weaker /StrongerList of pKa’s or Ka’s Weaker /StrongerKa or pKa and [HX] pH, [H+], [OH-]pH and [HX] Ka

Recall that a small Ka " high pKa, and

both mean weak acid and not much

dissociation.

acid and base equilibria what do you already know …courses.chem.psu.edu/chem112/fall/lecture...

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