General Chemistry II 2302102

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General Chemistry II2302102
Acid and Base Equilibria
Lecture 2
i.fraser_at_rmit.edu.au Ian.Fraser_at_sci.monash.edu.a
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Acids and Bases - 3 Lectures
Outline - 5 Subtopics

• Autoionization of Water and pH (completed)
• Defining Acids and Bases (completed)
• Interaction of Acids and Bases with Water
• (part-completed)
• Buffer Solutions
• Acid-Base Titrations

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Acids and Bases
Objectives - Lecture 2

• By the end of this lecture AND completion of the
  set problems, you should
• Distinguish between strong weak bases
• Calculate equilibrium concentrations of acids
  bases using basicity constants (Kb) relate
  to Ka
• Determine the direction of acid-base reactions
• Understand the concept of buffer solutions,
  buffer action following dilution, following the
  addition of strong acid and following the
  addition of strong base.

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Acids Bases - 2 Lectures
Outline
Introduction to acids and bases Strong Weak
Acids Conjugate acid-base pairs Common Ion Effect
Lecture 1

• Bases
• Buffers
• Indicators
• Titrations
• Strong Acids
• Weak Acids

Lecture 2
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Acids Bases - Lecture 2
Objectives
By the end of this lecture AND completion of the
set problems, you should be able to

• Distinguish between strong weak bases
• Calculate equilibrium concentrations of acids
  bases using basicity constants (Kb) relate
  to Ka
• Determine the direction of acid-base reactions
• Describe and calculate buffering of weak acids
• Calculate concentrations from titration of a
  strong acid
• Calculate concentrations and pKa from titration
  of a weak acid

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• BASES

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BASES
Bases accept Protons and form acids.
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BASES
pKb -log10Kb Strong Bases Large Kb
equilibrium lies
to the right e.g. O2-(aq) Weak Bases
Small Kb , large pKb equilibrium lies to
left e.g. NH3(aq)
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BASES
Relation between Kb and Ka of the conjugate acid.
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WEAK BASES
Calculate OH-, H and pH when 0.1 mol of
NaNO2 is dissolved in sufficient water to produce
1.0 L of solution. Ka for HNO2 4.5 x 10-4.
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WEAK BASES
Assuming x ltlt 0.10, then 2.2 x 10-12 x2 thus, x
1.5 x 10-6 M OH- 1.5 x 10-6, H 1.0 x
10-14 / 1.5 x 10-6 so, H 6.7 x 10-9, pH
-log10(6.7 x 10-9) 8.17
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Direction of Acid-Base Reactions

• Acid-Base Reactions proceed spontaneously with
  the strongest acid and strongest base forming the
  weakest acid and the weakest base
• Returning to

NH4 is a stronger acid than H2O, and OH- is a
stronger base than NH3 So reaction proceeds
spontaneously to the left
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Relative Strengths of Acids (Bases)

• Pick the strongest acid (smallest pKa)

NH4 is a stronger acid than H2O, hence OH- is a
stronger base than NH3
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Relative Strengths of Acids (Bases)
For HNO2, Ka 4.5 x 10-4 so pKa 3.35 For
HCN, Ka 7.2 x 10-10 so pKa 9.14
HNO2 is a stronger acid than HCN, hence CN- is a
stronger base than NO2-. Reaction strongly
favours the right
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• BUFFER
• SOLUTIONS
• (BUFFERS)

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BUFFER SOLUTIONS
The presence of the conjugate base of an acid
inhibits the ionization of the acid. e.g. a
solution that contains 0.05M HOCl and 0.1M OCl-
(Ka 3.7 x 10-8)
HOCl 0.05 OCl- 0.1 H3O 1.9 x
10-8 pH 7.72 OH- 5.3 x 10-7
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BUFFER SOLUTIONS
The presence of the conjugate base of an acid
inhibits the ionisation of the acid. Compare
the dissociation of 0.05 M HOCl in water with its
dissociation in a solution containing 0.1 M OCl-
ion. in water
in 0.1 M OCl- HOCl 0.05
0.05 OCl- 4.3 x 10-5
0.1 H3O 4.3 x 10-5
1.9 x 10-8 OH-
2.3 x 10-10 5.3 x 10-7
pH 4.37
7.73
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BUFFER SOLUTIONS
BUFFER SOLUTION A mixture that contains a weak
acid and its conjugate base. Addition of small
amounts of acid or base result in only small
changes of pH. Compare the effect of adding 10-3
M H3O to a solution containing of 0.05 M HOCl in
water and 0.1M OCl- ion with the effect of adding
10-3 M H3O to pure water.
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Acids and Bases - End of Lecture 2
Objectives Covered in Lecture 2

• After studying this lecture and the set problems,
  
• you should be able to
• Distinguish between strong weak bases
• Calculate equilibrium concentrations of acids
  bases using basicity constants (Kb) relate
  to Ka
• Determine the direction of acid-base reactions
• Understand the concept of buffer solutions,
  buffer action following dilution, following the
  addition of strong acid and following the
  addition of strong base.