chapter 14 - feb 21 - weebly

Acids & Bases

AP Chemistry

What is an Acid? Base?

• Arrhenius Model – Acid

donates an H+ in

aqueous solution – Base

donates OH- in aqueous

solution

• Brønsted-Lowry Model

– Acid is proton donor –

Base is proton acceptor

Formation of Hydronium Ion

Acids Dissolved in Water

• HA – represents a general acid

• Conjugate Acid-Base Pair – consists of two

substances related to each other by donating

and accepting of a single proton

• 2 Bases in Competition for a single proton

Acid

Base Conjugate

Acid

Conjugate

Base

Acid Dissociation Constant - Ka

Ka

=

H3O

+

A

−

H2O HA

Acid Dissociation Constant - Ka

• H2O – Does not exist in expression

• H3O+ and H+ – Both used – commonly

simplified to just H+

Ka

=H3O

+

A

−

HA =H

+

A

−

HA

Acid Dissociation Practice

Write the dissociation reaction and equilibrium

expression for each of the following acids (omit

water):

1. Acetic acid – CH3COOH

2. Ammonium ion

Acid Strength

Strong Acids

• equilibrium lies far to the right

• almost all of the HA dissociates

• yields a weak conjugate base (A-)

Weak Acids

• equilibrium lies far to the left

• almost all of the HA remains intact

• yields a strong conjugate base (A-)

Acid

Conjugate

Base

Acid Strength

Strong Acid ���� Weak Conjugate Base

Weak Acid ���� Strong Conjugate Base

Acid

Conjugate

Base

Acid Strength

Describing Acid Strength

Property Strong Acid Weak Acid

Ka Value** Ka is large Ka is small

** - Ka values are listed in Appendix 5 in your textbook

Describing Acid Strength

Property Strong Acid Weak Acid

Ka Value Ka is large Ka is small

Position of equilibrium Far to the right Far to the left

Describing Acid Strength

Property Strong Acid Weak Acid

Ka Value Ka is large Ka is small

Position of equilibrium Far to the right Far to the left

Equilibrium

concentration

comparing H+ and HA

[H+]≈[HA]0 [H+]<<<[HA]0

Describing Acid Strength

Property Strong Acid Weak Acid

Ka Value Ka is large Ka is small

Position of equilibrium Far to the right Far to the left

Equilibrium

concentration

comparing H+ and HA

[H+]≈[HA]0 [H+]<<<[HA]0

Strength of conjugate

base compared to H2O

A- much weaker

than H2O

A- much stronger

than H2O

Describing Acid Strength

Both Strong & Weak

Sulfuric Acid

• Diprotic Acid – 2 protons to donate

• 1st Proton – Strong Acid

• 2nd Proton – Weak Acid

H2SO

4(aq)→H

+(aq)+HSO4

−(aq)

Strong Acids / Bases

6 Strong Acids 6 Strong Bases

HClO4 –

HCl –

HBr –

HI –

HNO3 –

H2SO4 –

Strong Acids / Bases

6 Strong Acids 6 Strong Bases

HClO4 – perchloric acid

HCl – hydrochloric acid

HBr – hydrobromic acid

HI – hydroiodic acid

HNO3 – nitric acid

H2SO4 – sulfuric acid

Relative Strength of Bases

Acids listed in order of decreasing strength

HCl > HF > HNO2 > HCN > NH4+

List their conjugate bases in increasing strength:

Cl- < F- < NO2- < CN- < NH3

Is Water an Acid or Base?

Is Water an Acid or Base?

• Water is amphoteric – can behave as either an

acid or a base

• Water is able to autoionize

Is Water an Acid or Base?

Dissociation Constant for Water - Kw

• At 25˚C, pure water:

Kw

= H3O

+

OH

−

= H

+

OH

−

Kw

= 1.0×10−7( ) 1.0×10−7( )=1.0×10−14

H

+

= OH

−

=1.0×10−7

M

Importance of Kw

• No matter what is in the water, the product of

[H+] and [OH-] must equal 1.0 x10-14

• Neutral solution: [H+] = [OH-]

• Acidic solution: [H+] > [OH-]

• Basic solution: [H+] < [OH-]

Kw

= H+

OH

−

=1.0×10−14

Calculation of [H+] & [OH-] Practice

1. The concentration of OH- ions in a certain

household ammonia cleaning solution is

0.0025 M. Calculate the concentration of H+

ions. Is this solution an acid, base or neutral?

2. Calculate the concentration of OH- ions in a

HCl solution whose hydrogen ion

concentration is 1.3M.

pH Scale

• pH – the negative logarithm of the hydrogen ion concentration (in moles per liter)

• pOH – is analogous to the pH scale using the hydroxide ion concentration

• Since pH is a log scale based on 10, the pH changes by 1 for every power 10 change in [H+]

• pH + pOH = 14

pH = −log H

+

pH Scale

Acidic solutions: [H+]>1.0 x 10-7 M, pH < 7.00

Basic solutions: [H+]<1.0 x 10-7 M, pH > 7.00

Neutral solutions: [H+]=1.0 x 10-7 M, pH = 7.00

pH = −log H

+

pH Practice

1. The concentration of H+ ions in a bottle of

table wine was 3.2 x 10-4 M right after the

cork was remove. Only half of the wine was

consumed. The other half, after it had been

standing open to the air for a month, was

found to have a hydrogen ion concentration

equal to 1.0 x 10-3 M. Calculate the pH of the

wine on these two occasions.

pH Practice

2. The pH of rainwater collected in a certain

region of the northeastern United States on a

particular day was 4.82. Calculate the H+ ion

concentration of the rainwater.

3. The pH of a certain fruit juice is 3.33.

Calculate the H+ ion concentration.

Acids & Bases

AP Chemistry