Ch 7 Notes (Part 3)
Ch 7.5, 17.1, 17.2, 17.3 Oxidation-Reduction (Redox) Reactions
1. Reactions involving the transfer of electrons between substances in a chemical process are
called oxidation-reduction, or redox, reactions.
2. The oxidation number (or oxidation state)
a. A number that represents the number of electrons gained, lost, or unequally shared by an
atom.
b. The number has a positive (+) value if an atom has lost one or more electrons, a negative
(-) value if an atom has gained one or more electrons, and is zero if an atom has the number of
electrons it has in its free (uncombined) state.
c. Changes in the oxidation numbers of atoms in a chemical reaction indicate which atom
lost electrons and which atom gained electrons.
3. Oxidation numbers and ionic compounds (metal plus a nonmetal).
a. Electrons from a metal (cation) are transferred to a nonmetal (anion).
b. The oxidation number of an atom that has gained or lost electrons is the same as the
positive or negative ionic charge for the atom as determined by its group on the periodic table.
4. Oxidation numbers and covalent compounds (two nonmetals).
a. Electrons shared between nonmetals in a covalent bond are not always shared equally.
Electrons are assigned to the atom with the greater electronegativity (attraction for electrons).
Elements in the upper right portion of the periodic table have higher electronegativity values.
F > O > N > Cl > . . .
F is more electronegative than O, which is more electronegative than N, and so on . . .
b. The element with the greater attraction for electrons gains a negative oxidation number,
and the element with the lesser attraction gains a positive oxidation number.
5. Oxidation-reduction (redox) reactions.
a. These are synthesis, decomposition, single displacement, and combustion reactions.
b. In redox reactions the oxidation numbers of atoms change between the products and
reactants. This indicates which atoms lost electrons and which atoms gained electrons.
c. Oxidation occurs when the oxidation number of an atom increases (goes towards a
positive value) as a result of losing electrons. An atom is oxidized when its oxidation number
increases (becomes more positive).
Examples:
Fe0  Fe3+ (iron lost 3 electrons)
C2+  C4+ (carbon lost 2 electrons)
d. Reduction occurs when the oxidation number of an element decreases as a result of
gaining electrons. An atom is reduced when its oxidation number decreases (becomes more
negative).
Examples: O0  O2Cr6+  Cr3+
(oxygen gained 2 electrons)
(chromium gained 3 electrons)
e. It is the flow of electrons from oxidized atoms to reduced atoms that causes redox
reactions to form products. Every oxidation (loss of electrons) requires a reduction (gain of
electrons). In a balanced reaction the numbers of electrons lost equals the numbers of electrons
gained.
f. Oxidizing agent:
g. Reducing agent:
f. In other types of reactions, such as precipitation and acid-base reactions, there is no
transfer of electrons and, hence, no atoms are oxidized or reduced.
Remember these words:
OIL = oxidation is loss of electrons
RIG = reduction is gain of electrons
Rules for assigning oxidation numbers (oxidation states):
1. The oxidation number of an element in its free (uncombined) state is 0.
Ex: Na = 0, Mg = 0, Fe = 0, H2 = 0, Cl2 = 0, O2 = 0 (Remember HONClBrIF for diatomic
elements.)
2. The oxidation number of a monoatomic (single atom) ion is the same as its ionic charge.
Ex:
Na+ = +1, Mg2+ = +2, Fe2+ = +2, Fe3+ = +3, Pb4+ = +4, Cl- = -1, S2- = -2, N3- = -3,
and so on
3. Hydrogen is assigned an oxidation number of +1 when it’s combined with a nonmetal, and
an oxidation number of -1 when in it’s combined with a metal.
Ex:
In HCl the H has an oxidation number of +1; in MgH2 each H has an oxidation
number of -1
4. Metals always have positive oxidation numbers.
5. Nonmetals can have negative or positive oxidation numbers in compounds.
a. Fluorine is always -1 (it is the most electronegative element and strongly attracts shared
electrons).
b. Oxygen is always -2, except in peroxides where it is -1, and when combined with
fluorine, where it is +2.
c. Other nonmetals (e.g., carbon, chlorine, bromine, iodine, sulfur, nitrogen, phosphorus)
can have positive oxidation numbers when they share electrons with a more electronegative
nonmetal, such as oxygen.
6. The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation
numbers in a polyatomic ion is the charge on the ion.
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To determine which atoms are oxidized and reduced in a chemical reaction start by assigning
oxidation numbers to all the atoms in the reactants and products. Then determine which atom
was oxidized (went from a lower to a higher oxidation number when it lost electrons) and which
atom was reduced (went from a higher to lower oxidation numbered when it gained electrons).
Don’t forget OIL RIG.