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ACIDS and BASES
Acid – Base theories
Naming acids and bases
Oxides
Reactions and properties of acids and bases
Strengths of acids and bases
Acid and Base Theories
1) Arrhenius Theory
• An acid is a substance that gives H+ ion,
when dissolved in water.
For example, hydrochloric acid reacts with water to
form hydrogen ions which are transferred to a water
molecule to form a hydronium ion (H3O+).
But simply the reaction is: HCl
H+ + Cl-
Acids which have one ionizable hydrogen atom per molecule
are called monoprotic acids.
Example:
HNO3
H+ + NO3Acids which have two ionizable hydrogen atom per molecule
are called diprotic acids.
Example:
H2SO4
HSO4−
⇌
H+ + HSO4−
H+ + SO42−
Acids which have three ionizable hydrogen atom per molecule
are called triprotic acids.
Example:
H3PO4 ⇌ H+ + H2PO4–
H2PO4– ⇌ H+ + HPO42–
HPO42– ⇌ H+ + PO43–
• A base is a substance that gives OH- ion,
when dissolved in water.
NaOH → Na+ + OH−
Ca(OH)2 → Ca2+ + 2OHReaction of NH3 produce OH-:
NH3 + H2O → NH4+ + OHso it is a base.
Limitations of the Arrhenius theory
Hydrochloric acid is neutralized by both sodium
hydroxide solution and ammonia solution. In both cases,
you get a colourless solution which you can crystallize to
get a white salt - either sodium chloride or ammonium
chloride.
These are clearly very similar reactions. The full
equations are:
NaOH(aq) + HCl(aq)
NaCl(aq) + H2O(l)
NH3(aq) + HCl(aq)
NH4Cl(s)
In the sodium hydroxide case, hydrogen ions from
the acid are reacting with hydroxide ions from the
sodium hydroxide - in line with the Arrhenius theory.
However, in the ammonia case, there don't appear
to be any hydroxide ions!
You can get around this by saying that the ammonia
reacts with the water, it is dissolved in to produce
ammonium ions and hydroxide ions:
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
This is a reversible reaction, and in a typical dilute
ammonia solution, about 99% of the ammonia remains
as ammonia molecules. Nevertheless, there are
hydroxide ions there, and we can squeeze this into the
Arrhenius theory.
However, this same reaction also happens between
ammonia gas and hydrogen chloride gas.
NH3(g) + HCl(g)
NH4Cl(s)
In this case, there aren't any hydrogen ions or
hydroxide ions in solution - because there isn't any
solution. The Arrhenius theory wouldn't count this as an
acid-base reaction, despite the fact that it is producing
the same product as when the two substances were in
solution.
Naming Acids and Bases
A. Naming Acids:
The name of the
acid is determined
based on the name of
the anion, specifically,
based on the ending
of the anion
name. The three
possibilities are listed
here:
Anion
Name
Acid
Name
-ide
Hydro-ic
acid
-ite
-ous acid
-ate
-ic acid
Common Anions
Fluoride
F-
Chloride
Cl-
Bromide
Br-
Iodide
I-
Sulfide
S2-
Nitride
N3-
Sulfite
SO32-
Nitrite
NO2-
Chlorite
ClO2-
Hypochlorite
OCl-
Phosphate
PO43-
Hydrogen phosphate
HPO42-
Dihydrogen phosphate
H2PO4-
Nitrate
NO3-
Sulfate
SO42-
Hydrogen sulfate
HSO4-
Perchlorate
ClO4-
Chlorate
ClO3-
Carbonate
CO32-
B. Naming Bases
Simply use the normal rules for naming
compounds; ionic or covalent depending
on the elements in the compound.
Example:
NaOH: Sodium hydroxide
Ca(OH)2: Calcium hydroxide
NH3: Ammonia
Example:
a) Name the following acids and bases:
NaOH: Sodium hydroxide
H2SO3: Sulfurous acid
H2S :
Hydrosulfuric acid
H3PO4: Phosphoric acid
Ammonia
NH3:
Hydrocyanic acid
HCN:
Ca(OH)2: Calcium hydroxide
Fe(OH)3: Iron (III) hydroxide
H3P:
Hydrophosphoric acid
b) Write the formulas of the following acids and bases:
Hydrofluoric acid:
Hydroselenic acid:
Carbonic acid:
Lithium hydroxide:
Nitrous acid:
Cobalt (II) hydroxide:
Sulfuric acid:
Beryllium hydroxide:
Hydrobromic acid:
HF
H2Se
H2CO3
LiOH
HNO2
Co(OH)2
H2SO4
Be(OH)2
HBr
Oxides
Nonmetal
Oxides
CO2, SO2,
SO3 etc.
show acidic
properties
(acid
anhydride)
CO, NO,
N2O
are neutral
(have 1
oxygen
atom in the
formula)
Metal
Oxides
Na2O, BaO
etc. show
basic
properties
(basic
anhydrides)
Amphoteric
metals show
both basic
and acidic
properties
such as Al
and Zn
Acidic Property of Nonmetal Oxides
•
The oxides of nonmetals are usually acidic except
NO, N2O and CO (They are neutral)
CO2
+ H2O
H2CO3
SO2
+ H2O
H2SO3
SO3
+ H2O
H2SO4
N2O5 + H2O
2HNO3
Cl2O + H2O
2HOCl
P4O10 + H2O
4H3PO4
•
Monoxides of halogens are acidic such as Cl2O,
Br2O.
•
Oxides of some metals at high oxidation states show
acidic properties such as Mn2O7, CrO3.
Acidic nonmetal oxides react with bases to form salts.
SO3 + 2KOH
K2SO4 + H2O
ACID ANHYDRIDES
 Any oxygen-containing substance which will
produce an acid, when dissolved in water, is
called an ACID ANHYDRIDE.
 When sulfur dioxide (SO2) is dissolved in
water, sulfurous acid is formed.
SO2 + H2O
H2SO3
 When sulfur trioxide (SO3) is dissolved in
water, some sulfuric acid is formed.
SO3 + H2O
H2SO4
ACID ANHYDRIDES
• Carbon dioxide dissolved in water is in
equilibrium with carbonic acid:
CO2 + H2O ⇌ H2CO3
• The hydration equilibrium constant at 25°C
is Kh= 1.70×10−3: hence, the majority of the
carbon dioxide is not converted into
carbonic acid and stays as CO2 molecules.
•
Basic Properties of Metal Oxides
Oxides of metals are usually basic.
Na2O + H2O
2NaOH
BaO + H2O
Ba(OH)2
•
Some metal oxides can not dissolve in
water but they can dissolve in acidic solutions.
MnO + 2HCl
MnCl2 + H2O
CrO + 2HCl
CrCl2 + H2O
• Basic oxides react with acids to form salts.
CaO + H2SO4
CaSO4 + H2O
BASIC ANHYDRIDES
 Any oxygen-containing substance which will
produce a base, when dissolved in water, is
called a BASIC ANHYDRIDE.
 If sodium oxide (Na2O) is added to water,
sodium hydroxide, a base, is formed.
Na2O + H2O
2 NaOH
ANHYDRIDES
 Anhydride means "without water", so anhydrides may
be classified as acids or bases with all the water
removed.
 It might be more accurate and understandable if we
say anhydrides are acids or bases with all the
hydrogen removed in the form of water.
 Given a particular acid or base, we can determine its
anhydride by removing two hydrogen atoms and one
oxygen atom from its molecule (or molecules). For
example:
2 HClO
Cl2O + H2O
Ca(OH)2 CaO + H2O
 The acid anhydride for hypochlorous acid, HClO, is
Cl2O. The basic anhydride for calcium hydroxide is
CaO.
ANHYDRIDES
 In predicting anhydrides,
 ENOUGH H2O UNITS MUST BE REMOVED TO
LEAVE THE ANHYDRIDE WITHOUT ANY
HYDROGEN.
 To form the anhydride for an acid like H3PO4,
remove three water molecules from two
phosphoric acid molecules to produce the
anhydride, P2O5.
2 H3PO4 P2O5+ 3 H2O
Amphoteric Oxides
Oxides amphoteric metals are also
amphoteric.
Al2O3 + HCl
AlCl3 + H2O
Al2O3 + 2NaOH + 3H2O
2NaAl(OH)4
(sodium tetrahydroxoaluminate)
Properties and Reactions of
Acids and Bases
A. Properties of Acids:
• Are corrosive
• They taste sour
• They form solutions w/ pH less than 7 at 25°C.
• They turn litmus dye from blue to red
• They conduct electricity (electrolyte)
• They react with active metals to form salt and
H2 gas.
Mg + 2HCl
MgCl2 + H2
•
The acids which do not contain oxygen in their
structures can not react with semi noble metals Cu,
Hg, Ag.The oxy acids react with these metals
producing gases other than H2.
Cu + 2H2SO4
3Ag + 4HNO3
CuSO4 + SO2 + 2H2O
3AgNO3 + NO + 2H2O
• They react with metal carbonates and hydrogen
carbonates to give a salt, water and carbon
dioxide, which appears as effervescence
(bubbles).
Na2CO3 + 2HCl
NaCl + H2O + CO2
CH3COOH (aq)+NaHCO3 (aq)NaCH3COO(aq) +H2O (l) + CO2
ethanoic acid metalh hydrogen
salt
water carbon
carbonate
dioxide
• They react with bases to form salts
and water.
HCl + NaOH

(neutralization)
NaCl + H2O
H+ (aq) + OH- (aq)  H2O(l) (net ionic
equation)
•
•
•
•
•
•
B. Properties of Bases
They have bitter taste
Aqueous solutions of bases, known as alkalis (for
example aqueous sodium hydroxide), have a slippery
feel.
They turn the litmus dye from red to blue
They react with fats in the skin to form soaps
They conduct electricity (electrolyte)
The most common bases are the oxides,
hydroxides and carbonates of metals, but a
number of other compounds, such as ammonia
and amines also act as bases.
• They only react with amphoteric metals: Zn, Al
Zn + 2NaOH 
Na2ZnO2 + H2
2Al + 6 NaOH 
2Na3AlO3 + 3H2
• If they are soluble in water they give a
solution with pH>7
• They react with acids to form a salt. For
example, calcium oxide will react with
hydrochloric acid to form calcium chloride
and water:
CaO (s) + 2 HCl (aq)  CaCl2 (aq) + H2O (l)
base
acid
salt
water
• Amphoteric metals have both acidic and basic
properties such as Al, Zn, Sn, Pb, Cr
Al + 6HCl
AlCl3 + 3H2
2Al + 6NaOH
2Na3AlO3 + 3H2
• Oxides and hydroxides of amphoteric metals are
also amphoteric.
Al2O3 + HCl
AlCl3 + H2O
Al2O3 + 2NaOH + 3H2O
2NaAl(OH)4
ZnO + 2 HCl
ZnCl2 + H2O
ZnO + 2NaOH + H2O
Na2Zn(OH)4
Neutralization
Examples of Acids & Bases
Acids
Bases
HCl
H2SO4
HNO3
Juices, Soda
NaOH
Ca(OH)2
KOH
Soap,
Ammonia,
Baking Soda
2.Bronsted-Lowry Theory
A Bronsted-Lowry (BL) acid is defined as any substance that
can donate a hydrogen ion (proton) and a Bronsted-Lowry
base is any substance that can accept a hydrogen ion (proton).
Thus, according to the BL definition, acids and bases must come
in conjugate pairs. For example, consider acetic acid dissolved
in water:
CH3COOH + H2O
CH3COO- + H3O+
Act as an acid
Act as a base
Conjugate acid-base pairs:
1.
CH3COOH and CH3COO2.
H2O and H3O+
Act as a base
Act as an acid
Label Bronsted-Lowry acids and bases in the
following reactions and show the direction of proton
transfer.
H+
1.
H2O + H2O
OH- + H3O+
Acid
Base
Base
Acid
H+
2. NH3 + H2O
Base
Acid
NH4+ + OHAcid
Base
When a Bronsted-Lowry acid has given up its proton,
it is capable of getting back that proton and acting as a base.
Conjugate base is what is left after an acid gives up a proton.
The stronger the acid, the weaker the conjugate base. The
stronger the base, the weaker the conjugate acid.
The relationship between the Bronsted-Lowry
theory and the Arrhenius theory
The Bronsted-Lowry theory doesn't go
against the Arrhenius theory in any way - it just
adds to it.
Hydroxide ions are still bases because they
accept hydrogen ions from acids and form water.
An acid produces hydrogen ions in solution
because it reacts with the water molecules by
giving a proton to them.
The hydrogen chloride / ammonia problem
This is no longer a problem using the
Bronsted-Lowry theory. Whether you are talking
about the reaction in solution or in the gas state,
ammonia is a base because it accepts a proton
(a hydrogen ion).
If it is in solution, the ammonia accepts a
proton from a hydronium ion:
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
If the reaction is happening in the gas state, the
ammonia accepts a proton directly from the
hydrogen chloride:
NH3(g) + HCl(g)
NH4Cl(s)
3. Lewis Theory
A Lewis acid is a chemical compound,
A, that can accept a pair of electrons from
a Lewis base, B, that acts as an electronpair donor, forming an adduct, AB.
A + :B → A—B
A Lewis base is also a Brønsted-Lowry
base.
The Bronsted-Lowry theory says that
they are acting as bases because they are
combining with hydrogen ions. The reason
they are combining with hydrogen ions is
that they have lone pairs of electrons which is what the Lewis theory says. The
two are entirely consistent.
But what about other similar reactions
of ammonia or water, for example?
• Ammonia reacts with BF3 by using its lone
pair to form a co-ordinate bond with the
empty orbital on the boron.
Co-ordinate (dative covalent) bonding
A covalent bond is formed by two atoms
sharing a pair of electrons. The atoms are held
together because the electron pair is attracted
by both of the nuclei.
In the formation of a simple covalent bond,
each atom supplies one electron to the bond.
A co-ordinate bond (also called a dative
covalent bond) is a covalent bond (a shared pair
of electrons) in which both electrons come from
the same atom.
The reaction between ammonia and
hydrogen chloride
Representing co-ordinate bonds
In simple diagrams, a co-ordinate bond
is shown by an arrow. The arrow points
from the atom donating the lone pair to the
atom accepting it.
Dissolving hydrogen chloride in water
to make hydrochloric acid
Lewis acids
Lewis acids are electron pair acceptors. In
the above example, the BF3 is acting as the
Lewis acid by accepting the nitrogen's lone pair.
On the Bronsted-Lowry theory, the BF3 has
nothing about it.
Then what makes HCl a Lewis acid?
Chlorine is more electronegative than
hydrogen, and that means that the hydrogen
chloride will be a polar molecule. The electrons
in the hydrogen-chlorine bond will be attracted
towards the chlorine end, leaving the hydrogen
slightly positive and the chlorine slightly
negative.
The lone pair on the nitrogen of an ammonia
molecule is attracted to the slightly positive
hydrogen atom in the HCl. As it approaches it,
the electrons in the hydrogen-chlorine bond are
repelled still further towards the chlorine.
Eventually, a co-ordinate bond is formed
between the nitrogen and the hydrogen, and the
chlorine breaks away as a chloride ion.
Relative Strengths of acids and
Bases
The strength of an acid depends on how easily the
proton H+ is lost or removed from an acid
Two factors determine the acid strength:
1. The polarity of H atom: The more polarized the
bond is, the more easily the proton is removed
and greater the acid strength.
2. The size of the atom X (in HX): The greater the
atom X, the weaker is the bond and greater the
acid strength.
• Periodic Trends for Binary Acids:
Down a group: Sizes of the atoms increase.
HF
HCl
Acidity increases
HBr
HI
Across a period: Polarity of the bond increases.
CH4 NH3
H2 O
HF
Acidity inreases.
• Oxyacids:
HOF
HOCl
Acidity decreases. H-O bond
HOBr
ionizes more easily when the
HOI
oxygen atom is bonded to a
more electronegative atom.
• For a series of oxyacids:
HClO
HClO2
HClO3
HClO4
Acidity increases
As the number of oxygen atoms increases,
The oxidation number of central atom (Cl)
increases. This increases the ionization of
O-H bond. Therefore, acid strength
increases.
• Polyprotic Acids and Their Anions:
H3PO4
H2PO4-
H2CO3
HCO3-
H2SO4
HSO4-
HPO42Acidity decreases
Organic Acids
Organic acids have carboxyl group
(COOH). They are weak acids.
Example:
HCOOH: Formic acid
CH3COOH: Acetic acid

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