Acids and Bases

The Chemistry of
Acids and Bases

3 definitions of Acids/Bases
nThe theory of acids/bases
has changed over time due to
research and experimentation
n1) Arrehnius (oldest)
n2) Bronstead-Lowrey (newer)
n3) Lewis (newest)

1.Arrhenius Definition - 1887
n Acids produce hydrogen cations (H+)

or hydronium ions (H3O +) when

dissolved in water (HCl → H+ + Cl-)
n Bases produce hydroxide anions
(OH-) when dissolved in water.
(NaOH → Na + + OH-)

n Only in aqueous solutions.
n Aqueous solutions are liquids that
have moleculess dissolved in
WATER

Svante Arrhenius
n He was a Swedish chemist
(1859-1927), and a Nobel
prize winner in chemistry
(1903)
n One of the first chemists to
explain the chemical theory of
the behavior of acids and
bases

2. Brønsted-Lowry - 1923
n A broader definition than Arrhenius
n Acid is hydrogen-ion donor (H+ or proton);
base is hydrogen-ion acceptor.
n Acids and bases always come in pairs.
n HCl is an acid.
–When it dissolves in water, it gives it’s
proton to water.
HCl(g) + H2O(l) ↔ H3O+(aq) + Cl-(aq)
n Water is a base; makes hydronium ion.

Johannes Brønsted Thomas Lowry
(1879-1947) (1874-1936)
Denmark England

Why Ammonia is a Base
n Ammonia can be explained as a
base by using Brønsted-Lowry:
NH3(aq) + H2O(l) ↔ NH4 (aq) + OH (aq)
1+ 1-

Ammonia is the hydrogen ion
acceptor (base), and water is the
hydrogen ion donor (acid).
This causes the OH1- concentration to
be greater than in pure water, and
the ammonia solution is basic

Acids and bases come in pairs
n A “conjugate base” is the charged
particle of the original acid, after it
donates it’s hydrogen ion
n A “conjugate acid” is the charged
particle formed when the original base
gains a hydrogen ion
n Thus, a conjugate acid-base pair is
related by the loss or gain of a single
hydrogen ion.

Acids and bases come in pairs
n General equation is:
HA(aq) + H2O(l) ↔ H3O+(aq) + A-(aq)
n Acid + Base ↔ Conjugate acid + Conjugate base
n NH3 + H2O ↔ NH4 1+ + OH 1-

base acid c.a. c.b.
n HCl + H2O ↔ H3O1+ + Cl1-

acid base c.a. c.b.
n Amphoteric – a substance that can act as
both an acid and base- as water shows

3. Lewis Acids and Bases
n GilbertLewis focused on the donation
or acceptance of a pair of electrons
during a reaction
n Lewis Acid - electron pair acceptor
n Lewis Base - electron pair donor
n Most general of all 3 definitions; acids
don’t even need hydrogen!

What Happens When an Acid
Dissolves in Water?
• Water acts as a base
and abstracts a proton
(H+) from the acid.
• As a result, the
conjugate base (Cl-) of
the acid and a
hydronium ion (H3O+) are
formed.

Acids
and
Bases

Arrhenius Acid
Any substance that releases H+ ions as the
only positive ion in the aqueous solution.

1+ 1-

+ +

HCl H 2O H3O+ Cl-

hydrogen chloride water hydronium ion chloride ion
(an Arrhenius acid)

Definitions

Arrhenius - In aqueous solution…
•Acids form hydronium ions (H3O+)

HCl + H2O → H3O + Cl + –

H
H + –
O O Cl
Cl
H H H H

acid
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Definitions

Arrhenius - In aqueous solution…
•Bases form hydroxide ions (OH-)

NH3 + H2O → NH4+
+ OH -

H H
+ –
N O N O
H H H H
H H H H

base

Neutralization
Neutralization is a chemical reaction between an acid
and a base to produce a salt (an ionic compound) and
water.
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
base acid salt water

Some neutralization reactions:
H2SO4(aq) + 2 NaOH(aq) Na2SO4 + 2 H2O(l)
sulfuric acid sodium hydroxide sodium sulfate water

2 HC2H3O2(aq) + Ca(OH)2(aq) Ca(C2H3O2)2 + 2 H2O(l)
acetic acid calcium hydroxide calcium acetate water

Salts
NaCl

Salts - Ionic compounds containing a positive ion other than
the hydrogen ion and a negative ion other than the hydroxide
ion. i.e., a metal and a non-metal
NaCl(s) + H2O(l) Na1+(aq) + Cl1-(aq)

Formulas and names of common salts
SALT FORMULA Common Name

sodium chloride NaCl (table) salt

sodium nitrate NaNO3 Chile saltpeter

sodium bicarbonate NaHCO3 baking soda

potassium carbonate K2CO3 potash

ammonium chloride NH4Cl sal ammoniac

Neutralization

ACID + BASE → SALT + WATER
HCl + NaOH → NaCl + H2O
strong strong neutral

HC2H3O2 + NaOH → NaC2H3O2 + H2O
weak strong basic

• Salts can be neutral, acidic, or basic.
• Neutralization does not mean pH = 7.

ACID + BASE → SALT + WATER

HCl + NaOH → NaCl + H2O
strong strong neutral

HC2H3O2 + NaOH → NaC2H3O2 + H2O
weak strong basic

• Salts can be neutral, acidic, or basic.
• Neutralization does not mean pH = 7.

pH scale: measures acidity/basicity

ACID BASE

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
NEUTRAL

Acids have a pH between 0 and 7

Bases have a pH between 7 and 14

“Neutral” pH is 7

Properties of Acids
n They taste sour (don’t try this at home).
n They can conduct electricity.
–Can be strong or weak electrolytes in
aqueous solution
n React with metals to form H2 gas.
n Change the color of indicators (for
example: blue litmus turns to red).
n React with bases (metallic hydroxides)
to form water and a salt.

Properties of Acids
n They have a pH of less than 7 (more on
this concept of pH in a later lesson)
n They react with carbonates and
bicarbonates to produce a salt, water,
and carbon dioxide gas
n How do you know if a chemical is an
acid?
–It usually starts with Hydrogen.
–HCl, H2SO4, HNO3, etc. (but not water!)

Acids Affect Indicators, by
changing their color

Blue litmus paper turns red in
contact with an acid (and red paper
stays red).

Acids React with Active Metals

Acids react with active metals to
form salts and hydrogen gas:

HCl(aq) + Mg(s) → MgCl2(aq) + H2(g)

This is a single-replacement reaction

Acids React with Carbonates
and Bicarbonates
HCl + NaHCO3
Hydrochloric acid + sodium bicarbonate

NaCl + H2O + CO2
salt + water + carbon dioxide

An old-time home remedy
for relieving an upset
stomach

Effects of Acid Rain on Marble
(marble is calcium carbonate)
George Washington: George Washington:
BEFORE acid rain AFTER acid rain

Sulfuric Acid = H2SO4
4 Highestvolume
production of any
chemical in the U.S.
(approximately 60 billion pounds/year)

4 Used in the production
of paper
4 Used in production of
fertilizers
4 Used in petroleum
refining; auto batteries

Nitric Acid = HNO3
of fertilizers
of explosives
4 Nitric acid is a volatile
acid – its reactive
components evaporate
easily
4 Stains proteins yellow
(including skin!)

Hydrochloric Acid = HCl
4 Used in the “pickling” of
steel
4 Used to purify
magnesium from sea
water
4 Part of gastric juice, it
aids in the digestion of
proteins
4 Sold commercially as
Muriatic acid

Phosphoric Acid = H3PO4
4 A flavoring agent in
sodas (adds “tart”)
4 Used in the
manufacture of
detergents
4 Used in the
manufacture of
fertilizers
4 Not a common
laboratory reagent

Acetic Acid = HC2H3O2
(also called Ethanoic Acid, CH3COOH)

4 Used in the manufacture
of plastics
4 Used in making
pharmaceuticals
4 Acetic acid is the acid
that is present in
household vinegar

Properties of Bases (metallic hydroxides)
n React with acids to form water and
a salt.
n Taste bitter.
n Feel slippery (don’t try this either).
n Can be strong or weak electrolytes
in aqueous solution
n Change the color of indicators (red
litmus turns blue).

Examples of Bases
(metallic hydroxides)
 Sodium hydroxide, NaOH
(lye for drain cleaner; soap)
 Potassium hydroxide,
KOH (alkaline batteries)
 Magnesium hydroxide, Mg
(OH)2 (Milk of Magnesia)
 Calcium hydroxide, Ca
(OH)2 (lime; masonry)

Bases Affect Indicators

Red litmus paper
turns blue in contact
with a base (and blue Phenolphthalein
paper stays blue). turns purple in base
(pH above 9.0)

Bases
have a
pH
greater
than 7

Bases Neutralize Acids
Milk of Magnesia contains
magnesium hydroxide,
Mg(OH)2, which neutralizes
stomach acid, HCl.

2 HCl + Mg(OH)2

Magnesium salts can cause
diarrhea (thus they are used
MgCl2 + 2 H2O as a laxative) and may also
cause kidney stones.

Acids and Bases

litmus paper

pH less than 7 pH greater than 7
sour
taste ______ bitter
taste ______
bases
react with ______ acids
react with ______
proton (H1+) donor proton (H1+) acceptor
turn litmus red turn litmus blue
lots of H1+/H3O1+ lots of OH1–
react w/metals don’t react w/metals
Both are electrolytes. (they conduct electricity in soln)

Strength
n Acids and Bases are classified according
to the degree to which they ionize or
dissociate (break apart into ions) in water:
–Strong are completely ionized in
aqueous solution; this means they ionize
100 %
–Weak ionize only slightly in aqueous
solution
n Strength
is very different from
Concentration

Strength
n Strong- completely ionizes when
dissolved (100 % ionization)

n KOH (Potassium Hydroxide) is a
strong base- it completely ionizes
(100%) in water.

n HCl(Hydrochloric Acid) is a strong
acid - it completely ionizes (100%)
in water

Concentration
n Concentration:How many acid/base
molecules are present in a solution

n Concentrated = A LOT of acid/base
molecules. High concentration is above
1.0 M (M = Molar = moles/L)

n Dilute
= A little of acid/base molecules.
Low concentration is below 1.0 M

Strength vs. Concentration
n The words concentrated and dilute tell
how much of an acid or base is dissolved
in solution - refers to the number of
moles of acid or base in a given volume

n Thewords strong and weak refer to the
extent of ionization of an acid or base.
Strong = 80% - 100% ionization
n Weak = 10% - 30 % ionization

Common Acids
Strong Acids (dissociate ~100%)

hydrochloric acid: HCl H1+ + Cl1–
-- stomach acid;
pickling: cleaning metals w/conc. HCl

sulfuric acid: H2SO4 2 H1+ + SO42–
-- #1 chemical; (auto) battery acid

nitric acid: HNO3 H1+ + NO31–
-- explosives; fertilizer

Common Acids (cont.)
Weak Acids (dissociate very little)
acetic acid: CH3COOH H1+ + CH3COO1–
-- vinegar; naturally made by apples
hydrofluoric acid: HF H1+ + F1–
-- used to etch glass
citric acid, H3C6H5O7
-- lemons or limes; sour candy
ascorbic acid, H2C6H6O6
-- vitamin C
lactic acid, CH3CHOHCOOH
-- waste product of muscular exertion

carbonic acid, H2CO3

-- carbonated beverages
-- CO2 + H2O H2CO3

rainwater H2CO3: beverage carbonation
in air dissolves
limestone
(CaCO3)

H2CO3: cave formation H2CO3: natural acidity of lakes

How to measure pH with wide-range paper

1. Moisten the pH
indicator paper strip
with a few drops of 2.Compare the color to
solution, by using a the chart on the vial –
stirring rod. then read the pH value.

Some of the
many pH
Indicators
and their
pH range

Acid-Base Indicators
n Although useful, there are limitations
to indicators:
–usually given for a certain
temperature (25 oC), thus may
change at different temperatures
–what if the solution already has a
color, like paint?
– the ability of the human eye to
distinguish colors is limited

Acid-Base Indicators
n A pHmeter may give more accurate
pH values
–some are large, others portable
–works by measuring the voltage
between two electrodes; typically
accurate to within 0.01 pH unit of the
true pH
–Instruments need to be calibrated

Indicators chemicals that change color,
depending on the pH
Two examples, out of many:

litmus…………………
red in acid,
blue in base
acid base

phenolphthalein……..
clear in acid,
pink in base acid base
l
e
a
b
pink
r

Measuring pH
litmus paper
Basically, pH < 7 or pH > 7.
phenolphthalein

pH paper -- contains a mixture of various indicators
-- each type of paper
measures a range of pH
-- pH anywhere from 0 to 14
universal indicator -- is a mixture of several indicators
-- pH 4 to 10
4 5 6 7 8 9 10
R O Y G B I V

Measuring pH (cont.)
pH meter -- measures small voltages in solutions
-- calibrated to convert voltages into pH
-- precise measurement of pH

Acids and Bases

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