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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
