Faculty of Biotechnology
General Chemistry
Lecture 6
Dr. M. Abd-Elhakeem
 Valence:
The valence of an element can be defined as a
measure of its combining power or the number
of electrons an atom must gain, lose, or share
to have a full or stable outer electron shell.
elements sometimes gain, lose, or share
electrons in order to have eight electrons in
their outer shell. This observation led to the
development of the octet rule, which states
that outer electron shells prefer to have eight
electrons even though the shell may not be
full. (Octet means a group of eight.)
HOW you can calculate the oxidation number
To determine the oxidation state of an element
in a molecule, follow the following steps.
 Determine the oxidation state of all elements
forming the molecule (from the guiding table)
except the element of interest.
 The sum of all oxidation state (including the
unknown oxidation state) must be equal to
zero or to the net charge on the molecule.
have two parts
 Reactants - the substances you start with
 Products- the substances you end up with
Reactants Products
The reactants turn into the products.
 the
arrow separates the reactants from the
products
 The + sign = “and”
 (s) after the formula -solid
 (g) after the formula -gas
 (l) after the formula -liquid
 (aq)
after the formula - dissolved in water,
an aqueous solution.
 used after a product indicates a gas (same
as (g))
 used after a product indicates a solid
(same as (s))
indicates a reversible reaction


 , 
 shows
heat
that heat is supplied to the reaction
Pt
 is
used to indicate a catalyst used supplied,
in this case, platinum.
A
substance that speeds up a reaction
without being changed by the reaction.
 Enzymes are biological or protein catalysts.
 When
atoms combine, they do so in definite
ratios of intact atoms to produce compounds
with definite composition
 Combine
- put together
 2 elements, or compounds combine to make
one compound.
Ca +O2 CaO
SO3 + H2O  H2SO4
 decompose
= fall apart
 one reactant falls apart into two or more
elements or compounds.
electricity
 NaCl

 Na + Cl2

 CaCO
3   CaO + CO2
 One
element replaces another
 Reactants must be an element and a
compound.
 Products will be a different element and a
different compound.
K + NaCl  Na + KCl
F2 + LiCl  LiF + Cl2
 Metals
replace metals (and hydrogen)
K + AlN 3N
Zn + HCl  ZnCl2
 Think of water as HOH
 Metals replace one of the H, combine with
hydroxide.
Na + HOH  NaOH
 Two
things replace each other.
 Reactants must be two ionic compounds or acids.
 Usually in aqueous solution
 Will only happen if one of the products



doesn’t dissolve in water and forms a solid
or is a gas that bubbles out.
or is a water molecule formed.
AgNO3 + NaCl → AgCl + NaNO3
Precipitate
NaOH + HCl → NaCl + H2O
Water molecule
Na2CO3 + HCl → NaCl + CO2 + H2O
gas evolved
 Usually
in a combustion reaction oxygen
combines with another compound to form
carbon dioxide and water. An example of a
combustion reaction is the burning of
naphthalene:
C10H8 + 12 O2 ---> 10 CO2 + 4 H2O
-
Acids: Arrhenius defined an acid as a
compound that donates protons (H+) in
solution.
HOH
HCl -----> H+ + Cl –
HOH
H2SO4 -----> H+ + HSO4 –
 Bases.
Arrhenius defined a base as any
compound that donates hydroxyl (OH-) ions in
solution.
HOH
NaOH -----> Na+ + OH –
HOH
KOH -----> K+ + OH -
 Acids
change blue litmus paper to red.
 Acids
have a sour taste. Like lemon taste.
 Acids
react with metals to release hydrogen
gas. For example:
Zn + 2H+ ------> Zn ++ + H2
 Acids
react with carbonates and bicarbonates
to form carbon dioxide. For example:
CaCO3 + 2HCl ------> CaCl2 + H2O + CO2 ↑
 Acids
react with bases to form salts and
water (neutralization reaction). For example:
HCl + NaOH -------> NaCl + H2O
 Bases
change red litmus paper to blue.
 Bases
possess a bitter taste and feel soapy
when in contact with the skin.

Bases react with acids to form salts and
water (neutralization reaction).
 acid
is any compound (charged or uncharged)
capable of donating a proton.
 A base is defined as a charged or uncharged
substance capable of accepting a proton.
NH3 + H2O
 By
NH3 + H+ + OH
-
NH4 + + OH –
accepting a proton from water, ammonia
has effectively increased the concentration
of hydroxyl ions in the solution.
Strong acids: are acids that are completely
or nearly 100% ionized in their solutions
Strong bases: are bases completely ionized
in solution
KOH = K+ + OH-
HA = H+ + AWeak acids and bases are only partially
ionized in their solutions
Type
Formula
Hydrogen
halides
HCl
HBr
HI
Oxyacids
of halogens,
HClO3
HClO4
HBrO3
HBrO4
HIO3
HIO4
Sulfuric acid
H2SO4
Nitric acid
HNO3
Name
Formula
Sodium
hydroxide
NaOH
Potassium
hydroxide
KOH
Cesium
hydroxide
CsOH
Calcium
hydroxide
Ca(OH)2
 For
dissociation reaction of acid and base it
is the equilibrium between dissociated
molecules and undissociated molecules
CH3COOH
CH3COO- + H+
Ka = [CH3COO-(aq)][H+(aq)] / [CH3COOH(aq)]
So it is very high in strong acid and very low in
weak acid
pKa = - log Ka
CH3COOH = CH3COO- + H+
0.100
0.0042
0.0042
Kb is the dissociation constant of base
 Acids
donate protons (hydrogen ions, H+) in
solution. Thus, the acidity of a solution must
be related to this property.
 (1)
Litmus paper: Litmus paper is a paper
coated with a dye, which is red in an acid pH
or blue in a basic pH.
(2) pH paper: pH paper works on the same
principle as litmus paper but uses several
different dyes.
(3) pH meter: The most accurate tool for pH
measurement is the pH meter. This makes
use of an electrode dipped into solution and
is accurate to about 0.01-pH unit.
 It
is an ionic compound that contains a
positive ion other than hydrogen and a
negative ion other than hydroxyl.
 (1)
Strong acid and strong base. Neutral salt
HCl+ NaOH ---> NaCl + H2O
 (2) Weak acid and weak base. Neutral salt
2H2CO3 + Fe(OH)2 ---> Fe(HCO3)2 + 2H2O
 (3) Strong acid and weak base. Acidic salt
2HCl + Fe(OH)2 -----> FeCl2 + 2H2O
 (4) Weak acid and strong base. Basic salt
H2CO3 + NaOH -----> NaHCO3 + H2O
Na Cl
Base part acid part
Add OH Add H
NaOH St HCl St (Neutral)
Fe Cl2