CHEM 1405

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CHEM 1405

• Class Meeting 24

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Assignments and Reminders

• Reading Assignment
• Chapter 12 by Thursday April 20th
• Homework Problems due Thursday Apr 20th
• Chapter 10 even numbered Problems 2-8 and
  1830, 36, 38, 42-50
• Class website
  http//iws.ccccd.edu/jstankus/

Please use only one side of the page when
submitting Homework
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Chemistry Help Resources

• My office hours
• Tuesdays after class in Lecture room
• Thursdays 1-2 in Math Lab
• Free Tutoring through college
• Students must submit a tutor request form in
  order to receive detailed information about the
  available tutoring services.  The form is
  available on Collin's website and in the
  following offices
• CPC room A108 (ask for Sonia Castillo)
• PRC room F109 (ask for Shontel Penny or Mary
  Eldridge)
• SCC rooms G200 and G141
• There are group tutoring services available for
  the following courses (SUBJECT TO CHANGE!)
• CHEM 1405, 1411, 1412, 2423,
•  
• Also available will be online tutoring in the
  following courses (SUBJECT TO CHANGE!)
• CHEM 1412 below

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Objectives

• What is meant by the ionization constant for a
  weak acid or weak base?
• How can we calculate the concentration of
  hydrogen ion from an ionization constant and a
  given concentration of a weak acid or weak base?
• What is a buffer? How does a buffer work?
• How does the body maintain the pH of blood and
  other fluids?
• How can we use the Henderson-Hasselbalch equation
  to calculate the pH of a buffer?

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Equilibrium Calculations

• The ionization of a weak acid or base is a
  reversible reaction
• Typically reaches equilibrium when only a small
  percentage of molecules have ionized.

We will now treat these equilibria in a more
quantitative fashion
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Equilibrium Constant Expressions
Coefficients
a A(g) b B(g)      c C(g) d D(g)
Products
Reactants
Equilibrium Constant
Remember C is molar concentration of C
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Equilibrium Constant Example
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Equilibrium Constant Example
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Ionization of Weak Acids
At equilibrium
Ka
This is the Acid Ionization constant, Ka
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Acid Ionization Constant Example

• Calculate the H in a 0.10 M solution of acetic
  acid

1.8 x 10-5
From table 10.1
From the balanced equation We know that H
CH3COO- x Therefore CH3COOH 0.10
M - x
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Acid Ionization Constant Example (cont)
1.8 x 10-5
Substituting into the equilibrium constant
expression
Since only a very small amount of acetic acid is
ionized 0.10-x 0.10
x2 1.8 x 10-6
Solving for x
H
x 1.3 x 10-3
Need to check assumptions
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Acid Ionization Constant Example (cont)
Checking assumptions
We assumed that 0.10 x 0.10 with
that assumption we calculated x to be
0.0013 0.10 0.0013 0.10 to two significant
figures Our assumption is good
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Ionization of Weak Bases
At equilibrium
Since only a small amount of water reacts its
concentration is assumed constant
This is the base Ionization constant, Kb
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Base Ionization Constant Example

• Calculate the OH- in a 0.010 M solution of
  aniline

4.2 x 10-10
From table 10.2
From the balanced equation We know that OH-
C6H5NH3 x Therefore C6H5NH3
0.010 M x 0.010 M
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Base Ionization Constant Example
Substituting into the base ionization expression
4.2 x 10-10
x2 4.2 x 10-12
x 2.0 x 10-6 M OH-
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Salts in Water

• Recall that an acid reacting with a base forming
  a salt is called a neutralization reaction
• Not all salt solutions are neutral
• The equilibria will determine if the solution is
  acidic, basic or neutral

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Salts in Water Acidic, Basic or Neutral
Prediction rules

• The salt of a strong acid and a strong base forms
  a neutral solution.
• The salt of a strong acid and a weak base forms
  an acidic solution.
• The salt of a weak acid and a strong base forms a
  basic solution.
• The salt of a weak acid and a weak base may form
  an acidic, basic, or (by chance) neutral solution.

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Acids and Bases added to pure Water

• pH changes on addition

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Buffers Control of pH

• A buffer solution is one in which the pH remains
  nearly constant even if acid or base is added.

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Buffer solutions

• A buffer solution is a solution containing a weak
  acid and its salt, or a weak base and its salt.
• Small quantities of added acid are neutralized by
  one buffer component and small quantities of
  added base by the other.
• As a result, the solution pH is maintained nearly
  constant.

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How Buffers work

• Application of Le Chateliers Principle

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Common Buffer Solutions
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Common Ion Effect
Adding Acetate Ions Shifts equilibrium to left

• The common ion effect refers to the ability of
  ions from a strong electrolyte to repress the
  ionization of a weak acid or weak base or the
  solubility of a slightly soluble substance

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Common Ion Effect Example

• What is the H in a solution that is 0.10 M
  acetic acid and 0.10 M  sodium acetate?

Let x H Sodium acetate is totally
ionized CH3COO- 0.10 x CH3COOH
0.10 - x
Assume x ltlt 0.10
x H 1.8 x 10-5
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pH of Buffer Solutions
Manipulating the equilibrium expression
Where pKa -log(Ka)
This is the Henderson-Hasselbalch equation
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Henderson-Hasselbalch Equation Example

• What is the pH of a solution that is 0.20 M H2S
  and 0.20 M HS-? 

The Ka 1 x 10-7 for H2S
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Henderson-Hasselbalch Equation Example

• What is the pH of a solution that is 0.10 M HCN
  and 0.50 M NaCN? 

The Ka 6.2 x 10-10 for HCN
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Buffer Capacity

• There are limits to how much acid or base a
  buffer can handle
• Generally related to the concentration of the
  buffer components
• Rule of thumb
• Effective pH range of a buffer is about
• pH pKa1

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Buffers in Blood

• Blood Plasma normally varies from 7.35 to 7.45 in
  pH
• Blood pH gt 7.8 or lt 6.8 will do irreversible
  damage to the brain
• Blood pH lt 7.35 is acidosis
• Blood pH gt 7.45 is alkalosis
• Body has at least three buffer systems to
  maintain the proper pH

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Buffers in Blood
Bicarbonate/carbonic Acid system
Dihydrogen Phosphate/Monohydrogen Phosphate system
Protein system
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Lactic Acid in Muscles

• Muscle contractions produce lactic acid
• This can lower blood pH
• Blood Buffers
• Work hard enough can overload buffers