Transcript Chapter 2 Atoms and Elements

Chapter 14 Acids and Bases
14.4
Dissociation Constants
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Strong Acids
In water, a strong acid
• dissociates into ions
• produces high
concentrations of H3O+
and an anion A-
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Weak Acids
In a weak acid,
• equilibrium favors the
undissociated (molecular)
form of the acid
• the concentrations of H3O+
and anion A- are low
HA(aq) + H2O(l)
H3O(aq) + A−(aq)
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Characteristics of Acids and
Bases
4
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Acid Dissociation Constant
A weak acid has
• an equation for dissociation written
HA(aq) + H2O(l)
H3O+(aq) + A(aq)
• an equilibrium expression of
Keq = [H3O+][A]
[HA][H2O]
• a Ka expression written without [H2O], which is a
liquid and is a constant
Ka = [H3O+][A]
[HA]
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Acid and Base Dissociation
Constants
6
Basic Chemistry
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Writing a Ka for a Weak Acid
Write the Ka for H2S.
1. Write the equation for the dissociation of H2S.
H2S(aq) + H2O(l)
H3O+(aq) + HS−(aq)
2. Set up the Ka expression
Ka
=
[H3O+][HS-]
[H2S]
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Learning Check
Write the Ka for the weak acid HCN.
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Solution
HCN (aq)+ H2O(l)
H3O+ (aq) + CN− (aq)
Set up the Ka expression
Ka
=
[H3O+][CN−]
[HCN]
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Learning Check
Write the Kb for the weak base CH3CH2NH2.
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Solution
CH3CH2NH2(aq)+ H2O(l)
CH3CH2NH3+ (aq) + OH(aq)
Set up the Kb expression
Kb =
[CH3CH2NH3+ ] [OH]
[CH3CH2NH2]
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Basic Chemistry
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