Chem Ch 7 Notes (Part 3)
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 oxidationreduction, or redox, reactions. (See page 189-190 in text).
2. The oxidation number (or oxidation state)
a. A number that represents the number of electrons ________________________________________.
______________________________________________________
b. The number has a _______________________value if an atom has lost one or more electrons, a
______________________ value if an atom has gained one or more electrons, and is ___________ 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 for electrons 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 said to be ______________ when its oxidation number increases
(becomes more positive) in a chemical reaction.
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 said to be _____________ when its oxidation number decreases (becomes more negative) in a
chemical reaction..
Examples: O0  O2- (oxygen gained 2 electrons)
Cr6+  Cr3+ (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. 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 an nonmetal (or anion), and an
oxidation number of -1 when in it’s combined with a metal (cation).
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 -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.