Transcript Oxidation-Reduction (Redox) Reactions

Chapter 20
20.1

Salt on icy roads can make
driving safer, but the salt
that clings to the metallic
parts of cars can cause them
to corrode or rust relatively
quickly. This corrosion is
one example of a chemical
reaction called oxidationreduction. You will learn
about oxidation-reduction
reactions.
20.1

Oxidation originally meant combining with
oxygen.
20.1
 A Bunsen burner oxidizes
the methane in natural gas
to carbon dioxide and
water.
20.1
 Carbon is oxidized when charcoal burns.
20.1
 Iron is oxidized when it rusts.

Today the meaning is different:
◦ Oxidation means the loss (or partial loss) of
electrons.
◦ Reduction means the gain (or partial gain) of
electrons.
 They are interdependent: one can’t occur without the
other.
LEO the lion says GER…
20.1
◦ Redox reactions that form ions involve a total loss
and gain of electrons.
20.1
20.1
 The substance that loses electrons is called the
reducing agent. The substance that accepts electrons
is called the oxidizing agent.
20.1
◦ Redox reactions with covalent compounds involve
the partial loss and gain of electrons.
20.1
20.1

When a metal corrodes it combines with
oxygen.
◦ Iron, a common construction metal often used in
the form of the alloy steel, corrodes by being
oxidized to ions of iron by oxygen.
20.1
◦ Aluminum resists corrosion because it forms a
protective coating of aluminum oxide.
20.1
◦ Painting a surface protects it from corrosion.
Chromium metal also serves as a protective
coating.
20.1
◦ Zinc blocks attached to the steel hull of this ship
oxidize instead of the iron, preventing corrosion.
20.2

When aluminum or magnesium
powder in fireworks are heated
to high temperatures in the
explosion, they burn with an
intense white light. As
elements burn, their oxidation
numbers change. You will learn
how oxidation and reduction
are defined in terms of a
change in oxidation number.
20.2
◦ An oxidation number is a positive or negative
number assigned to an atom to indicate its charge
or partial charge when bonded to another atom.
 In a compound, the more electronegative atom will get
a negative number, and the less electronegative atom
will get a positive number.
20.2
 The oxidation number of any element in the free or
uncombined state is zero.
20.2
20.2
 Oxidation numbers are often written above the
chemical symbols in a formula.
20.2
K2CrO4
Cr oxidation
number = +6
Cr2O3
Cr oxidation
number = +3
“Multiply down; divide up” to find an unknown oxidation state.
20.2
◦ An increase in the oxidation number of an atom or
ion indicates oxidation.
◦ A decrease in the oxidation number of an atom or
ion indicates reduction.
20.2
20.2
 When an iron nail is placed in a copper(II) sulfate
solution, the iron reduces Cu2+ ions in solution and is
simultaneously oxidized to Fe2+ The iron becomes
coated with metallic copper.
20.3

When concentrated
hydrochloric acid (HCl) and
nitric acid (HNO3) are mixed
with gold, oxidation and
reduction reactions produce
gaseous nitrogen monoxide
(NO) and soluble, stable
AuCl4– ions. You will learn
how to write and balance
chemical equations for redox
reactions such as this one.
20.3
◦ If the oxidation number of an element in a reacting
species changes, then that element has undergone
either oxidation or reduction. Therefore, the
reaction as a whole must be a redox reaction.
 Single replacement reactions are always redox, and
double replacement reactions are never redox.
20.3
K + H2O KOH + H2
2 Zn + HCl ZnCl2 + H2
20.3
 A redox reaction takes place between nitrogen and
oxygen when lightning bolts heat the air.
20.3
 A color change can signal a redox reaction.

Shows either the oxidation or reduction
portion of a redox reaction and it shows the
exchange of electrons.

An equation that shows an atom or ion losing
one or more electrons while its oxidation
number increases is an oxidation halfreaction.
◦ The law of conservation of mass must be followed.
◦ Also, the law of conservation of charge applies.
◦ Ex:

An equation that shows an atom or ion
gaining one or more electrons while its
oxidation number decreases is a reduction
half-reaction.
◦ Must follow the law of conservation of mass and
charge.
◦ Ex:
1.
2.
3.
4.
5.
Assign oxidation numbers.
Leave out spectator ions (no change in ox #).
Write half-reactions to show conservation of
mass.
Add electrons to show conservation of
charge.
Balance using coefficients if necessary.
*Make sure the # electrons lost = # electrons
gained.
1.
Mg + Fe(NO3)2  Fe + Mg(NO3)2
2.
Zn + 2 HCl  H2 + ZnCl2
3.
2 Al + 3 ZnCl2  3 Zn + 2 AlCl3


Show only participating atoms and ions.
Spectator ions and electrons are left out.
◦ Still have to be balanced!
◦ Mass and charge must be conserved!
1.
2.
3.
4.
Balance mass. Use coefficients if necessary.
Write half-reactions.
Balance charge. Multiply half-reaction(s) by
a coefficient to balance electrons.
Incorporate coefficients from the halfreactions into the equation.
1.
H+ + Al  H2 + Al3+
2.
Fe3+ + Ni  Fe2+ + Ni2+
3.
Cr3+ + Mg  Cr + Mg2+