CHAPTER 19
ACIDS & BASES
The pH scale
• The definition of pH is defined as the negative
logarithm of the hydrogen ion-concentration.
• Expressed mathematically, pH is shown as: pH=log[H+].
• The pH scale ranges from 0 to 14.
• A pH of 7 is considered neutral. Because the
number of hydrogen ions equal the number of
hydroxide ions.
• A solution with a pH of 0 is considered to be
highly acidic, and a solution with a pH of 14 is
highly basic or alkaline.
pH scale continued….
• Along with ranging from 0 to 14, the pH scale
also corresponds with it’s numbers by having
different colors for acids and bases.
• Acids usually range from red to light orange
while bases range from yellowish-green to
blue.
The Properties of Acids
• The properties of acids include: having a sour
taste, they change color of an acid-base
indicator, they react with compounds
containing hydroxide ions to form water and
salt, some react with metals to produce
hydrogen (H2) gas which can be useful test in
the lab, and they can be strong or weak
electrolytes in aqueous solution.
Strong Acids
• A strong acid is an acid that ionizes completely
in aqueous solutions.
• Such as: HCl(g)+H20(l)H3O+(aq)+ Cl-(aq)
• Strong acids are also a weak conjugate base.
• Because if an acid is good at donating a H+
ion, it has a difficult time getting it back.
• Some common strong acids include:
Hydrochloric acid, Nitric acid, and Sulfuric
acid.
Strong Bases
• Bases are considered strong bases when they
disassociate completely into metal ions and
hydroxide ions in aqueous solutions.
• Some common strong bases are Potassium
hydroxide, and sodium hydroxide.
Hydronium vs Hydroxide ion
• The Hydronium ions is formed when a water
molecule gains a hydrogen ion. Therefore, the
resulting charge of the new molecule is
positive. [H3O+]
• Inversely, the Hydroxide ion is formed when
water loses a hydrogen and becomes a
negatively charged ion. [OH-]
Arrenhius acids and bases
• Arrenhius defined an acid as hydrogencontaining compounds that ionize to yield
hydrogen ions [H+] in aqueous solution. He
also said that bases are compounds that ionize
to yield hydroxide ions [OH-] in aqueous
solution.
• Identify one of the following as an Arrenhius
base: LiOH, NH3, H2PO4, or CH3COOH
Acid vs Base on pH scale
• On the pH scale the pH of acids are lower than
the pH of bases
• Acids range from 0 to 6. While bases range from 8
to 14.
• Acids also turn an acid-base indicator from dark
red to orange depending on the strength of the
acid.
• Bases turn the color of an acid-base indicator
from green to blue, depending on how strong the
base is.
Lewis Acid vs. Lewis Base
• A Lewis acid is a substance that can accept a
pair of electrons to form a covalent bond.
• A Lewis base is a substance that can donate a
pair of electrons to form a covalent bond.
• An example of a Lewis base is in the reaction
H++O—HH—O—H.
• The hydroxide is a Lewis base in this reaction.
Bronsted-Lowry acid vs base
• The Bronsted-Lowry theory defines an acid as
a hydrogen-ion donor, and a base as a
hydrogen-ion acceptor.
• Consequently, when a Bronsted-Lowry acid
loses a hydrogen ion, it becomes a conjugate
base.
• Also, when a Bronsted-Lowry base accepts a
hydrogen ion, it forms the conjugate acid of
the base.
• Find the conjugate acid-base pair in the
following reaction: NH3+H2O  NH4+ + OH-
Finding [OH-] when given [H+]
• If the [H+] in a solution is 1.0 X 10-5M , is the
solution acidic, basic, or neutral? What is the
[OH-] of this solution?
• Given: [H+] 1.0x10-5M
• Find: [OH-] and acidic basic or neutral
• Use: Kw=[H+] X [OH-]=1.0 X 10-14
• Plug-in: [OH-]= 1.0 X 10-14/ 1.0 X 10-5
• Solve: [OH-]= 1.0 X 10-9 the solution is acidic
Now do one yourself….
• The [H+] in a solution is 2.4 X 10-7M, what is
the [OH-] of the solution?
Finding pH given [OH-]
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What is the pH of a solution if [OH-]= 4.0 X 10-11M
Given: [OH-]= 4.0 X 10-11
Find: pH
Use: pH= -log[H+] and Kw=[H+] X [OH-]=1.0 X 10-14
Plug-In: [H+]= 1.0 X 10-14/4.0 X 10-11
Solve: [H+]= 2.5 X 10-4M pH=-log[2.5 X 10-4M]
pH= 3.60
Now do one yourself…
• Calculate pH the if [OH-]=4.3 X 10-5M