Introduction to
Acids and Bases
Chapter 19
What is and Acid?
• Arrhenius Acid
• Defined as any chemical that increases
the concentration of hydrogen ions (H+)in
solution. (usually by a dissociation
reaction
Examples
• Hydrochloric acid HCl  H+ + Cl• Sulfuric acid H2SO4  2H+ + SO42• Phosphoric acid H3PO4  3H+ + PO43-
Not all Hydrogens are acidic
• The hydrogen must be part of a polar
bond in order to dissociate.
• For example:
• HF is acidic, but CH4 is not.
• In CH4, the hydrogen is part of a non-polar
covalent bond and does not dissociate in
solution!
• HF is a polar bond and HF  H+ + F-
Bronsted – Lowry Acid
• Defined as a molecule or ion that is a hydrogen
ion donor.
• Also known as a proton donor because H+ is a
proton.
• The acid will donate its H+ ion to a base in an
acid base reaction.
H+ + OH-  H2O
Acid + Base
What is a Base?
• Arrhenius Base
• Defined as any chemical that increases
the hydroxide ions (OH-) concentration in
solution.
• Examples• NaOH  Na+ + OH• KOH  K+ + OH• Ca(OH)2  Ca2+ + 2OH-
Bronsted-Lowry Base
• Defined as a hydrogen ion acceptor.
• In an acid-base reaction the base
“accepts” the hydrogen ion from the acid.
NH3 + H+  NH4+
NH3 accepts the H+ from the acid.
Physical Properties of Acids
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Taste Sour
Feel Sticky
React with metals to produce hydrogen gas
Conduct electricity – because they produce
ions in solution
• Are corrosive (they can burn skin and other
materials)
• Are found in many naturally colored solutions
• Turn Litmus Indicator Red
Physical Properties of Bases
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Taste Bitter
Feel slippery or soapy
Also conduct electricity
Rarely found in colored solutions
Most do not have a smell (except
ammonia)
• Turn Litmus indicator Blue
• Turn Phenolphthalein Indicator Pink
Physical Properties of Neutral
Compounds
• Do not taste bitter or sour, but rather have
a sweet or oily taste, or no taste at all.
• Many feel like water, or feel greasy/ oily
• May or may not conduct electricity
• They have varied reactivities.
• Many have a strong chemical smell or no
smell at all.
Using pH to identify
Acids and Bases
The pH Scale
What is pH?
• pH = -log [H+]
pH= -log[H3O+]
• pH stands for the “power of the hydrogen
ion.”
• It is based on a logarithmic scale which
has a base power of 10.
– A pH of 1 differs from a pH of 2 by a factor of
10.
pH Scale - Continued
• Created to express acidity as a more
simple number than molarity.
• pH = - log [H+]
• If . . .
• [H+] = 1 x 10-7 then pH = 7 (neutral)
• [H+] = 1 x 10-3 then pH = 3 (acid)
• [H+] = 1 x 10-10 then pH = 10 (base)
Why is the pH scale 0 – 14?
The Self-Ionization of Water
H2O (l) ↔ H+ (aq) + OH-(aq)
• The equilibrium expression for this
reaction is
•
Kw = [H+][OH-] = 1 x 10-14
• This low equilibrium constant means very
few water molecules ionize.
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Water is considered neutral because in water
[H+] = [OH-] = 1 x 10-7 M
pH = -log (1 x 10-7 ) = 7
*All aqueous solutions have H+ and OH- ions
• Acids
HCl  H+ + Cl•
Acidic solutions increases the H+
concentration and [H+] > [OH-]
• Bases
NaOH  Na+ + OH•
Basic solutions increases the OHconcentration and [H+] < [OH-]
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pH scale ranges from 0-14
pH<7 indicates an acidic solution
pH=7 indicates a neutral solution
pH>7 indicates a basic solution
Strong Acids have pH of 0 or 1
Strong Bases have a pH of 13 or 14
Strength of Acids and Bases
“Strength” refers to how much an acid or
base ionizes in a solution.
STRONG
Ionize completely
(~100%)
Example:
HCl H+ + ClNaOH  Na+ + OH-
WEAK
Ionize partially (usually
<10%)
An equilibrium reaction!
Example:
HF ↔ H+ + FNH3 + H2O ↔ NH4+ + OH-
Strengths of Acids and Bases