CHM 116 Spring 2012

Lecture 13

Section 18.9, 18.4

Next Lecture

Read Sec. 19.1, 19.2

In Silberberg

Lab: LeChatelier’s Principle (Ch 5)

Perform test procedures on 3 systems:

Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq)

CH3CO2H(aq) + H2O(l) ⇌ CH3CO2-(aq) + H3O+(aq)

HBrm + H2O ⇌ Brm- + H3O+

determine the direction of the equilibrium shiftidentify substances that increased and decreased

F

B

F F

H

N

H H

+

F

B

F F

H

N

H H

acid base adduct

A Lewis acid is an electron-pair acceptor.

A Lewis base is an electron-pair donor.

M2+H2O(l)

M(H2O)42+(aq)

adduct

Molecules as Lewis Acids

3 electron pairs, 1

available orbital

1 unshared electron

pair

Protons act as Lewis acids in that they accept an electron pair in all reactions: B: + H + B H +

New covalent

bond

Reactions of Acids and BasesChemical Reactivity

Lewis Bases(Nucleophile: electron-rich

species that attacks an electron deficient center)

1. Lone pair of electrons

e.g. H3N:, H2O:

2. Anion

e.g. OH-, Cl-

3. electrons

e.g. H2C=CH2

Lewis Acids(Electrophile: electron-deficient species attracted to a source of

electron density)

1. Partial (+)

2. Carbocations

3. H+

4. Empty “p” orbital

e.g. H3B, H3Al

. .

Acid/Base Equilibrium Calculations

Calculations are the same as before:

Find K Find missing [concentration] Find x and [concentration]eq

1. Assume x is small (weak acids!)2. Assume reaction goes to completion

then calculate x and [ ]eq

3. Use some method of reaching an exact solution (e.g., quadratic equation)

We will see some new assumptions, however.

A study of phenylacetic acid (C6H5CH2COOH, simplified here as HPAc) shows that the pH of 0.12 M HPAc is 2.62. What is the Ka of phenylacetic acid?

Find the Ka from the pH and [HA]i

HPAc(aq) + H2O(l) H3O+(aq) + PAc-(aq)

pH = 2.62 [H3O+] = 10-pH = 10-2.62 = 2.4 x 10-3 M

[PAc] = [H3O+] = 2.4 x 10-3 M

Ka = 4.8 x 10-5

Concentration(M) HPAc(aq) + H2O(l) H3O+(aq) + PAc-(aq)

Initial 0.12 - ~1x10-7 0

Change --x +x [H3O+] +x

What simplifying assumptions have we made?

Equilibrium -0.12 - x x[H3O+] +(<1x10-7)

1. We can neglect H3O+ from water

Find the Ka from the pH and [HA]i

Concentration(M) HPAc(aq) + H2O(l) H3O+(aq) + PAc-(aq)

So Ka =(2.4x10-3) (2.4x10-3)

0.12= 4.8 x 10-5

Checking for % error. = 4x10-3 %

x 100[HPAc]dissn;

2.4x10-3M0.12M

[H3O+]from water; 1x10-7M

2.4x10-3Mx 100

= 2.0 %

Initial 0.12 - ~1x10-7 0

Change --x +x [H3O+] +x

Equilibrium -0.12 - x x[H3O+] +(<1x10-7)

Find the Ka from the pH and [HA]i

What simplifying assumptions have we made?

Determining Concentrations from Ka and initial [HA]

Propanoic acid (CH3CH2COOH, which we simplify as HPr) is an organic acid. What is the equilibrium pH of 0.10 M HPr? (Ka = 1.3 x 10-5)?

HPr(aq) + H2O(l) H3O+(aq) + Pr-(aq) Ka = 1.3 x 10-5

initial concentration

Doccam

Equilibria Involving A Weak Base

What is the pH of 0.010 M NH3?

NH3 + H2O NH4+ + OH-

Kb = 1.8 x 10-5

Step 1. Define equilibrium concs. in ICE table

[NH3] [NH4+] + [OH-]

initial

change

equil.

You have 0.010 M NH3. Calculate the pH.

NH3 + H2O NH4+ + OH- Kb = 1.8 x 10-5

Step 2. Solve the equilibrium expression.

Assume x is small, so

Equilibria Involving A Weak Base

You have 0.010 M NH3. Calculate the pH.

NH3 + H2O NH4+ + OH- Kb = 1.8 x 10-5

Step 3. Calculate pH

Equilibria Involving A Weak Base

Stronger acid weaker conjugate base

Stronger base weaker conjugate acid

HF + H2O F- + H3O+

F- + H2O HF + OH-

Add equations

2 H2O H3O+ + OH-

Ka x Kb = [H3O+][OH-] = Kw

Remember: When we add equations, we can multiply K’s!

For a conjugate acid/base pair:

Consider HF acid:

And its conjugate base:

Determining the pH of a Solution of a Conjugate Base of a Weak Acid

What is the pH of 0.25M sodium acetate (CH3COONa, or NaAc for this problem).

Ka of acetic acid (HAc) is 1.8x10-5.

Plan: 1. Sodium salts are soluble in water so [Ac-] = 0.25M.

2. Write the association equation for acetic acid; use the Ka to find the Kb.

Doccam

Calculate the pH of a 0.10 M solution of Na2CO3.

Ka = 4.7 x 10-11 for dissociation of HCO3-

CO32- + H2O HCO3

- + OH-

base acid acid base

Determining the pH of a Solution of a Conjugate Base of a Weak Acid