Unit 11:
Classification of
Chemical Reactions
Cypress Creek HS: Chemistry 1K
Chapter 10
Part 3: Redox
Redox Reactions
 Electron transfer reactions are called
oxidation-reduction or redox reactions.
 Occurs when oxidation numbers change.
 Results in the generation of an electric
current (electricity)
 Therefore, this field of chemistry is often
called electrochemistry.
Terminology for Redox
 OXIDATION
 loss of electron(s); increase in oxidation number
 REDUCTION
 gain of electron(s); decrease in oxidation number
 OXIDIZING AGENT
 electron acceptor; species oxidizes another and is itself reduced.
 REDUCING AGENT
 electron donor; species reduces another and is itself oxidized.
Ways to Remember Redox
 Reduction can’t happen without an oxidation to
provide the electrons – they are companions!
OIL RIG
Oxidation is loss
Reduction is gain
LEO the lion says GER
Lose electrons – oxidation
Gain electrons – reduction
Oxidation Numbers
 The oxidation number of an
atom is the number of
electrons lost or gained
when it forms ions.
 Oxidation numbers are written
with the sign before the
number, whereas ionic charge
is written after the number.
Oxidation number: +3
Ionic charge: 3+
Common Oxidation Numbers
Rules for Oxidation Numbers
1. The oxidation number of an
uncombined atom is zero.
Ex: Mg, Ca, O2, Cl2, S
2. The oxidation number of an
uncombined ion is equal to the
charge on the ion.
Ex: the oxidation number of a
Ca2+ is +2, and Br– is –1
Rules for Oxidation Numbers
3. The oxidation number of oxygen in
compounds is –2.
4. The oxidation number of hydrogen
in compounds is +1.
5. The oxidation number of group 1A
elements in compounds is +1.
6. The oxidation number of group 2A
elements in compound is +2.
Rules for Oxidation Numbers
7. The oxidation number of the
more electronegative atom in a
compound is the charge of its
ion.
Ex: In SiCl4, chlorine is more
electronegative, so chlorine has
an oxidation number of –1.
Rules for Oxidation Numbers
8. The sum of the oxidation
numbers in a neutral compound
is zero.
9. The sum of the oxidation
numbers of the atoms in a
polyatomic ion is equal to the
charge on the ion.
Determining Oxidation Numbers
 What is the oxidation number of chlorine in KClO3
(potassium chlorate)?
 Oxidation numbers add up to zero.
 The oxidation number of oxygen in compounds is –2.
 Group 1A elements have a +1 oxidation number.
+1 ? -2
(+1) + (?) + (-2●3) = 0, so Cl = +5
KClO3
 What is the oxidation number of sulfur in SO32–
(sulfite ion)?
 Oxidation numbers add up to –2.
 The oxidation number of oxygen in compounds is –2.
? -2
SO3
(?) + (-2●3) = -2, so S = +4
Oxidation and Reduction
 Zinc is oxidized from zinc metal to the Zn2+ ion.
H+ is the oxidizing agent.
 Each H+ is reduced and combine to form H2.
Zn is the reducing agent.
Oxidation and Reduction
 To see how oxidation numbers change, start by
assigning numbers to all elements in the balanced
equation.
 There is no change in the oxidation number of
potassium – it is called the spectator ion.
Oxidation and Reduction Practice
 Identify what is oxidized and what is reduced:
 Aluminum is oxidized, iron is reduced
 Identify the oxidizing agent and the reducing
agent:
 Aluminum is the reducing agent, iron is the oxidizing
agent.
 Identify the spectator ion:
 Oxygen
Half-Reactions
 The oxidation process and the reduction
process of a redox reaction can each be
expressed as a half-reaction.
 For example, consider the unbalanced equation for
the formation of aluminum bromide.
Half-Reactions
 The oxidation half-reaction shows the loss of
electrons by aluminum.
 The reduction half-reaction shows the gain of
electrons by bromine.