The Nature of Acids and Bases

• Arrhenius Definition• Bronsted-Lowry Definition

HA + B A- + HB+

Conjugate acid-base pair

Equilibrium Expressions

• Acid dissociation constant is for the equilibrium that exists as an acid spontaneously dissociates.

• Works the same as the other equilibrium expressions we’ve been writing.

Acid Strength

Types of acids

• Diprotic Acids- two hydrogen atoms, dissociate with different strengths.

• Oxyacids- H is attached to an oxygen.• Organic Acids- contain a carboxyl group.

• Hydrohalic Acids- H attached to a halogen.

R OH

O

Write the equilibrium expression for the autoionization of water.

Autoionization of Water

• Water is amphoteric (properties of acid and base)

Do Now

The pH Scale

• pH = -log[H+]• Related by exponents of 10. As [H+]

increases, the pH will decreases.

log Kw = log [H+] + log [OH-]pKw = pH + pOH14 = pH + pOH

The number of

S.F. in the concentration

value equals the

number of decimal

places in the pH value.

Try Me Problems!!

• Calculate the pH or pOH as required for each of the following solutions at 25oC, and state whether the solution is neutral, acidic, or basic.a) 1.0 x 10-5 M OH-

b) 1.0 x 10-7 M OH-

c) 1.0 M H+

More Try Me!

• Calcualte pH and pOH for each of the solutions from the problem before.

Even More Try Me!

• The pH of a sample of human blood was measured to be 7.41 at 25oC. Calculate the pOH, [H+], and [OH-] for the sample.

Acid Strength Matters

• Identify the MAJOR SPECIES present in your sample.

• Determine if the autoionization of water will impact the concentration of [H+] or not (compare concentrations)

Strong Acid Example

Lets pretend I have 1.0 M HCl.major species: H+ , Cl-, H2O

H+ from HCl will be 1.0 M H+ from H2O will be 1.0 x 10-7

M H+ from H2O is negligible.

We can now base our calculations on the H+ values from our major species.

In the Event of a Weak Acid…

1. Write out major species.2. Determine if H20’s H+ are

significant.3. Write Ka for dominating species

4. Use an ICE chart

Weak Acid Example

Lets pretend I have 1.0 M HF, solve for [H+]

major species: HF, H2O

HF H+ + F- Ka = 7.2 x 10-4

H2O H+ + OH- Kw= 1.0 x 10-

14

Dominating source of H+: HF

ICE it up!

HF H+ + F-I 1.0 0 0C -x +x +xE 1-x x x

Plug into Ka expression, BUT DO NOT SOLVE YET

5% rule

• If x is small, cross out subtracted/added x’s• How small? 5% of original concentration.

• Similarly, percent dissociation/ionization has formula:

100xionizationpercent ionconcentratinitialddissociateamount 100xionizationpercent ionconcentratinitialddissociateamount

%5100][ xoHA

x %5100][ xoHA

x

Try Me!

• Calculate the pH of a solution that contains 1.0 M HCN (Ka = 6.2 x 10-

10) and 5.0 M HNO2 (Ka = 4.0 x 10-

4). Also calculate the concentration of cyanide ion (CN-) in this solution at equilibrium.

Try Me Trickier!

Lactic acid (HC3H5O3) is a waste product that accumulates in muscle tissue during exertion, leading to pain and fatigue.

In a 0.10 M aqueous solution, lactic acid is 3.7 % dissociated. Calculate Ka for the acid.

Bases

• Strong bases have/cause complete ionization, just like acids.

• KOH and NaOH are the most common group I soluble strong bases

• Group II hydroxides are insoluble strong bases

• Not all bases need to have OH- (Brønsted Lowry)

H3CNH2(aq) + H2O(l) H3CNH3+

(aq) + OH-(aq)

Common Base Structures

AmmoniaAmmonia MethylamineMethylamine DimethylamineDimethylamine

PyridinePyridine EthylamineEthylamine EphedrineEphedrine

Useful Application

• Amines can form soluble acid salts

Pseudoephedrine + HCl ↔ pseudophedrineHCl

Strong Base Try Me

Calculate the pH of a 5.0 x 10-2 M NaOH solution.

Weak Base Try Me

Calculate the pH of a 15 M solution of ammonia (Kb = 1.8 x 10-5)

Polyprotic Acids

• Dissociate stepwise

• For weak acids, Ka1 > Ka2 > Ka3

• Sulfuric acid is a freak because it is strong for hydrogen 1 and a weak acid for hydrogen 2. Only impacts normal process if we are using dilute solutions.

Try ME! Concentrated H2SO4

Calculate the pH of a 1.0 M solution of H2SO4.

Try Me 2! Dilute H2SO4

• Calculate the pH of 1.0 x 10-2 M H2SO4

Regular Type Polyprotics

Calculate the pH, and the concentration of each ion formed by the complete dissociation of hydrogen in a 5.0 M solution of phosphoric acid.

Once upon a time there was a salt…

• What is a salt?

• What power does it have in solution?

Solution is Basic Solution is Acidic

salt of astrong acid

and astrongbase

Solution is Neutral

salt of a

weak acid

and a

strong

base

salt of astrong acid

and the C.A.

of a weakbase

salt

of th

e C

.B.

of a

wea

k ac

idan

d th

e C

.A.

of a

wea

kba

se

salt with a

highly charged

metal cation

varies

Try This On!

Calculate the pH of a 0.10 M NH4Cl solution, given that Kb for NH3 is 1.8 x 10-5

Structural Effects

• Hydrohalic Acids• Bond Strength• Bond Polarity

• Oxyacids• Number of Oxygen atoms

H-F H-Cl H-IH-Br

OXIDES

Covalent oxides in water form acids• CO2 + H2O H2CO3

• Ionic oxides in water form bases• K2O + H2O 2KOH

If X has high EN,the oxide will act as acid

If X has low EN,OH stays together

and oxide actsas a base.

Lewis Definition

• Lewis Acids• Accepts e- pair

• Lewis Bases• Dontates e- pair