Acid-Base ChemistryUnit 8

L U N DQUI ST – 20 17

Acid-Base basicsU N I T 8 .1

The two parts of water

• Water can ionize

• Form positively charges hydronium (H3O+)

• Form negatively charged hydroxide (OH-)

• Happens in ALL water at very small amounts

• K 298K = 1 x 10-14

What is an acid or a base?

A R R H E NIUS :

• Acids – produce more hydronium (H3O+)

• Bases – produce more hydroxide (OH-)

BUT why is NH3 a base?

What is an acid or a base?

B R Ø N S TED-LOWR Y

• Acids – proton (H+) donors

• Bases – proton (H+) acceptors

What is an acid or a base?

CO N J UG ATE ACI D

• Add a hydrogen

CO N J UG ATE B A S E

• Remove a hydrogen

What is an acid or a base?

H2O

Practice

Exercise 1

Practice

Exercise 1

What is an acid or a base?

L E W I S

• Acid – electron acceptor

• Base – electron donor

• Classic examples NX3 is a base, BX3is an acid

• Form coordinate covalent bond , a bond where one species provides ALL the electrons in the bond

Practice: Do p. 15

More Definitions

- - P R OTIC

Acids can only donate 1 H at a time (1 H per collision)

• Monoprotic – have only 1 H to donate (HCl, HI, CH3COOH…)

• Diprotic – have only 2 H to donate (H2C2O4, H2CO3, H2Cr2O7…)

• polyprotic – have many H to donate (H3PO4…)

• Polyprotic base – can accept more than 1 H (anions with -2, -3

charges)

NOT A THING: immediately decomposes to H2O + CO2

More Definitions

A MP H I PROTIC / A MP H OT ERI C

• A substance that can be either an acid or a base

• Ex: water!

D I S S OCIATIO N R E ACT IONS

• Separating the donated hydrogen in a reaction

• INGORING water

Practice

Exercise 2

Practice: Do p. 16 ONLY write the dissociation

Strength of Acids / BasesU N I T 8 .2

Based on dissociation

• \

Complete dissociation:Every hydrogen (H+) separates from the anion (A-)

The six strong acids: HNO3, H2SO4, HClO4, HCl, HI, & HBr

S T R O NG

Why is HClO4 strong, but HClO3 not?

• Oxygen is an “electron hog”

• Pulls electron away from hydrogen

• Makes H very positive and easily attracted to other molecules

Electronegative

Based on dissociation

• \

S T R O NG

Complete dissociation:Every hydrogen (H+) separates from the anion (A-)

ONLY group I, and II metal hydroxides are strong: Ca(OH)2, KOH

MOH M+ OH-

Excludes

Mg, & Be

Based on dissociation

Partial dissociation:Few hydrogen (H+) separates from the anion (A-)

EVERYTHING ELSE

W E A K

Strength and conjugates

• The stronger the acid the weaker it’s conjugate base

• REMEMBER YOUR EQUILIBRIUM

HCl + H2O ⇄H3O+ + Cl-

Weak acid (base) equilibrium

• Since they are partially soluble they establish and equilibrium

• Have a K value

• Ka – acid

• Kb – base

• VERY similar just Kb is for OH-, while Ka is for H3O+/ H+

• REMEMBER: water (l) is NOT part of the K expression

Practice:CH3COOH

Exercise 3

What Ka/Kb mean:

Practice

Exercise 4

Acid Ka

H2O 1x10-7

HF 7.2x10-4

HCl 1600000

HNO2 4.0 x10-4

HCN 5.2 x 10-10

Practice: Do p. 16

The pH scaleU N I T 8 .3

The self Ionization of water

Discovered by Kohlrausch (~1900)

• Water can ionize

• Form positively charges hydronium (H3O+)

• Form negatively charged hydroxide (OH-)

• Happens in ALL water at very small amounts

• K w = [H3O+][OH-]

= (1 x 10-7) (1 x 10-7)

1 x 10-7 1 x 10-7Kw = 1 x 10-14

What does it mean

[OH-] = [H3O+]

• neutral (water)

[OH-] > [H3O+]

• Basic / alkaline

[OH-] < [H3O+]

• acidic

At 25oC Kw = 1 x 10-14

Kw = Ka (Kb)

Practice:

Exercise 5

Practice:

Exercise 6

The pH scale• Discusses the [H+]/[H3O+]

• Usually very small

• Based on powers of 10 so logs• Log (1 x 10-3) = -3• Log (1 x 108) = 8

Mental logs:

• Ages• 1 � Log (1) = 0 • 5 � Log (5) = • 10 � Log (10) = 1• 17 � Log (17) = • 31 � Log (31) = • 50 � Log (50) = • 100 � Log (100) = 2

0.7

1.23

1.49

1.7

As you approach the next power of 10, your

coefficient increases

The pH scale

• More mental logs

Concentration log[H+] -pH

• [H+] = 1.0 Log (1.0) = 1

• [H+] = 0.01 Log (0.01) = -2

• [H+] = 0.02 Log (0.02) = -1.7

• [H+] = 0.03 Log (0.03) = -1.5

• [H+] = 0.001 Log (0.001) = -3

• [H+] = 0.005 Log (0.005) = 2.3

• [H+] = 9 x 10-3 Log (9 x 10-3) = 2.04

The pH scale

p__ = -log [__]

• pH = -log [H+]

• pOH = -log [OH-]

• pKa = -log (Ka)

• pKb = -log (Ka)

pH + pOH = 14

pKa + pKb = 14

The pH scale

Finding concentration from pH

• [H+]=10-pH

• [OH-]=10-pOH

• Ka =10-pKa

• Kb =10-pkb

[H][OH] = 1 x 10-14

At 298K (25oC)

KaKb = 1 x 10-14

At 298K (25oC)

Practice:

Exercise 7

pH of strong Acids

• Since strong acids/bases dissociate fully they’re fairly easy equations to find [H+]/[OH-]

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Exercise 8

pH of strong Acids

• Since strong acids/bases dissociate fully they’re fairly easy equations to find [H+]/[OH-]

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Exercise 9

Practice: do p. 17

Weak acid/base pH

• Weak acids/bases establish an equilibrium

• RICE TABLES!!!!

• Always follows

HA ⇄H+ + A- Ka < 1

MOH ⇄M+ + OH- Kb < 1

Lets dive in!

Exercise 10

When can I neglect X

• Short answer:

• ALWAYS!! (YAY!)

• Long Answer

• When 100 K < [HAi]

• AP test does not have ANY quadratic equations on it so if you end up needing it you probably messed up �

Practice

Exercise 11

Practice

Exercise 12

Practice

Exercise 13

Mixtures of acids

• Only the acid with the highest Ka will dissociate

• Anything after that is common ion, and won’t contribute much

• Set up a second rice table

Practice

Exercise 14

Percent dissociation

• What portion of the acid dissociates

• Since weak HA/ A- can only give up 1 H+ or OH- at a time, it is simply what fraction of the acid broke up

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����� 100

Practice

Exercise 15

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����� 100

Practice: Do p. 18 to 21

HydrolysisU N I T 8 .4

Water Splitting

S A LT S

• Any metal and nonmetal

• Formed from the neutralization of an acid by a base

• Salts are rarely neutral

• Cause water equilibrium to favor either H3O+ or OH-

Water Splitting

H Y D R OLY SIS

• Salts separate water into H+

and OH-

• Based on what made them

• Neutral salt – Strong w/ strong

• Acid salt – strong HA, weak A-

• Basic salt – weak HA, strong A-

Water splitting: write the hydrolysis of each salt and determine if they are acid or basic

ACI D IC S A LT

• CuNO3

• BeCl2

B A S I C S A LT

• NaClO

• CaCr2O7

Exercise 16

Practice

Exercise 17

Practice: Do p. 22

Quiz Next Class• Write a reaction from Ka

• Find pH from concentration of a weak acid

• Write a base and acid net ionic equation

• Write a hydrolysis

• What makes a weak acid weak?

• Find concentration of a weak acid from pH

• Find pH of a mixture of acids

Titration CurvesU N I T 8 .5

neutralization

mol HA = mol A-= mol MA

• When acid = base = salt

• Reaction H++OH-�H2O

(H+)(Ma)(Va) = (OH-)(Mb)(Vb)

• Determines an unknown concentration of acid or base

Practice

• If 50.0 mL of 0.50 M HCl is used to completely neutralize 25.0 mL KOH, what is the molarity of the base?

• If 50.0 mL of 0.50 M H2SO4 is used to completely neutralize 25.0 mL KOH, what is the molarity of the base?

Exercise 18

Titrations

• The gradual mixing of an acid and base to determine the concentration of and unknown

• An indicator changes color at a set [H+]

Indicator Acid base

Phenolphthalein Pink Colorless

Methyl Orange Orange Yellow

Methyl blue Blue Yellow

Litmus Pink/red blue

Practice: do p. 23

Titration Curves

• Graphing the pH of a solution as acid/base is added

• pH vs vol. added

• Equivalency point

• When the solution is neutralized, mol HA = mol A- = mol MA

• Strong + strong = pH ~7

• Weak + weak = pH ~7

Titrations of weak acid/base

• Produces a buffer

• Resist changes in pH

• Formed from a weak acid/base and its conjugate salt

• Causes a “tail”

• Starts a mid-pH

• Takes a LONG time

• Equivalency point over 7 (under for weak base + strong acid)

Titrations of weak acid/base

Cool stuff with titrations

• Equivalence point

• Mole acid = mole base = mole salt

• Choose an indicator with a pKa near the expected pH of the equivalence point

• Half-equivalence point

• Mathematical halfway point (vol) of the equivalence point

• pH = pKa

Cool Stuff with Titrations

Buffer zone

• When a solution is resistant to changes in pH

• Greatest resistance at half the buffer zone

• Can absorb the most acid/base

Titration of polyprotic acids

• Acids can only donate 1 proton/H+ at a time

• Causes a multitier titration curve

• Multiple equivalence points

• Half equivalence point of each is the pKa of that acid derivative

Choosing an indicator

• Your ideal indicator has a Ka

near the equivalence point

• Where its pKa = pH at the equivalence point

Which be best for the 1st ionization?a) Crystal violet Ka = 2.9 x 10-1

b) Phenolphthalein Ka = 4.5 x 10-9

c) Methyl orange Ka = 9.1 x 10-4

d) Bromphenol blue Ka = 2.2 x 10-4

BuffersU N I T 8 .6

Buffers

• Solutions that resist changes in pH

• Have a high capacity to maintain a set pH

• A solution of a weak acid / base with it’s salt (conjugate)

• Ex: NaC2H3O2 + HC2H3O2

NH3 + NH4Cl

• Contains both acid and base so it can neutralize either

Buffers

• Remember

• ��� � � �– ��"� � ���

• pH = -log [H]

• Ka X kb= Kw

Base & conjugate acid

Acid & conjugate base

pH of Buffers

• MEMORIZE!

• Easiest way to do buffers

• DO NOT READ THE BOOK!!!

• If acid is added

• Add mol HA to numerator

• Subtract mol HA from denominator

• If base is added

• Subtract mol A- from numerator

• Add mol A- to denominator

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Practice: do p. 24 # 1b

Practice:

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Exercise 19

Ka

= 1.8x104

Preparing Buffers

• Keep concentration high (0.1 to 1.0 M)

• Choose an acid/base with a Ka / Kb near the [H+] you need

• pKa near pH

• pKb near pOH

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Exercise 21(a little out of order)

Practice:

Exercise 20

Practice:

Exercise 22

More Titrationsfive points of interest on a titration curve

1. The pH before the titration begins

Do just like normal pH

Exercise 23

More Titrationsfive points of interest on a titration curve

2. The pH on the way to the equivalence point

Use the buffer formulaadd acid/base, subtract opp.

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Exercise 23

More Titrationsfive points of interest on a titration curve

3. The pH at the midpoint of the titration (half equivalence point)

pH = pK since at this point ½ the base has neutralized half the acid the ������

�+��,�� 1 so [H+]=Ka

(d) If propanoic acid is titrated with 0.3 M NaOH, what is the pH at the midpoint of the titration?

Exercise 23

More Titrationsfive points of interest on a titration curve

4. The pH at the equivalence point

Since all the acid and base are neutralized (H++OH-→H2O) this is just the pH of the salt. Write the hydrolysis of the salt and use RICE to find pH.

(e ) If propanoic acid is titrated with 0.3 M NaOH, what is the pH at the equivalence point for the titration of 50mL 0.265M propanoic acid, and 0.3M NaOH?

Exercise 23

More Titrationsfive points of interest on a titration curve

5. The pH AFTER the equivalence point

pH and limiting reagent. Determine the amount of excess and find pH

(f ) If propanoic acid is titrated with 0.3 M NaOH, what is the pH when 50mL of 0.3M NaOH is added to 50mL 0.265M propanoic acid?

Exercise 23

Practice: Finish p. 24 & 25

Test SOON!!

• Lab next class

• Test class after that!

• Do your personal progress checks!