Download oxidation–reduction reaction

Table of Contents
Chapter 20: Oxidation-Reduction
20.1: Oxidation and Reduction
20.2: Balancing Redox Equations
20.3: Half-Reactions
Table of Contents
Chapter 20: Oxidation-Reduction
20.1: Oxidation and Reduction
Oxidation-Reduction: Basic Concepts
Electron Transfer and Redox Reactions
Oxidation-Reduction: Basic Concepts
Electron Transfer and Redox Reactions
• A reaction in which electrons are transferred
from one atom to another is called an
oxidation–reduction reaction.
• Also called redox
reactions
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Oxidation-Reduction: Basic Concepts
Electron Transfer and Redox Reactions
• Single-replacement and combustion reactions
always involve the transfer of electrons from
one atom to another.
• So do many, but not all, synthesis and
decomposition reactions.
Oxidation-Reduction: Basic Concepts
Electron Transfer and Redox Reactions
Oxidation-Reduction: Basic Concepts
Changes in oxidation number
• The oxidation number of an atom in an ionic
compound is the number of electrons lost or
gained by the atom when it forms ions.
• Oxidation numbers are tools that scientists
use in written chemical equations to help
them keep track of the movement of electrons
in a redox reaction.
Oxidation-Reduction: Basic Concepts
Rules for determining
oxidation numbers
1. All atoms in elemental form have an
oxidation number of 0.
• This is true for elements that exist as
polyatomic molecules such as O2, Cl2,
H2, N2, S8.
Oxidation-Reduction: Basic Concepts
Rules for determining
oxidation numbers
2. Each element in a compound gets the number
it would have if it were an ion.
• Split ionic compounds into known ions.
E.g. NH4NO3 becomes NH4+ & NO3− ,
–
2+
Ca = +2, Br = –1.
NH3
N = -3.
H = +1.
Si = +4.
SiCl4
Cl = -1.
Oxidation-Reduction: Basic Concepts
Rules for determining
oxidation numbers
3. Nonmetals usually have negative oxidation
numbers.
• Hydrogen is +1 when bonded to
nonmetals and -1 when bonded to metals.
• Double check the oxidation number of
any element on the 2nd period and lower
Oxidation-Reduction: Basic Concepts
Rules for determining
oxidation numbers
4. The sum of the oxidations numbers in a
neutral compound is zero.
(+1) + (-1) = 0
(+2) + 2(-1) = 0
NaCl
CaBr2
2(+1) + (+6) + 4(-2) = 0
Na2SO4
Oxidation-Reduction: Basic Concepts
Rules for determining
oxidation numbers
5. The sum of the oxidation numbers of the
atoms in a polyatomic ion is the charge
on the ion.
(-3) + 4(+1) = +1
NH4+
(+4) + 3(-2) = -2
SO32-
Basic Assessment Questions
Try it out!
Determine the oxidation number of the
boldface element in each of these compounds.
Li2SiO3
O = -2
Li = +1
Si = + 4
Al4C3
Al = +3
C=–4
Table of Contents
Chapter 20: Oxidation-Reduction
20.2: Balancing Redox Equations
Review
• Define a “redox reaction”
• Why do we use oxidation numbers?
• What are the oxidation numbers for reach of the
following elements in the following compound:
PO43Cu3N2
P=+5
O=-2
N=-3
Cu=+2
Oxidation-Reduction: Basic Concepts
How do oxidation and reduction differ?
• Oxidation is the loss of electrons from
atoms of a substance.
• If a substance loses electrons another substance
must take those electrons.
Oxidation-Reduction: Basic Concepts
How do oxidation and reduction differ?
• Reduction is the gain of electrons by atoms
of a substance.
• Half-reaction: an equation showing either
the oxidation or the reduction half of a redox
reaction (always includes the electrons)
Electrochemistry: Basic Concepts
Identify which ½ rxns are oxidation
and which are reduction
1. Fe → Fe2+ + 2e2. Te2+ + 2e- → Te
oxidation
reduction
3. IO4- + 2e- → IO6-
reduction
4. Al → Al3+ + 3e-
oxidation
5. I2 +2e- → 2I-
reduction
Oxidation-Reduction: Basic Concepts
How do oxidation and reduction differ?
• Can oxidation occur without reduction?
• Oxidation and reduction are complementary
processes; oxidation cannot occur unless
reduction also occurs.
Oxidation-Reduction: Basic Concepts
How do oxidation and reduction differ?
• LEO the lion says GER (LEO GER)
• Loss of Electrons is Oxidation
• Gain of Electrons is Reduction.
• OIL RIG
• Oxidation Is Loss
• Reduction Is Gain
Oxidation-Reduction: Basic Concepts
Oxidation Number in Redox Reactions
spectator ion
Oxidation-Reduction: Basic Concepts
Oxidizing and Reducing Agents
Oxidized
• Loses electrons
• Oxidation number increases
• Reducing agent: it is the substance that
reduces another substance by giving its
electrons.
Oxidation-Reduction: Basic Concepts
Oxidizing and Reducing Agents
Reduced
• Gains electrons
• Oxidation number decreases
• Oxidixing agent: it is the substance that
oxidizes another substance by accepting its
electrons.
Oxidation-Reduction: Basic Concepts
TRY IT OUT! Oxidizing and Reducing Agents
0
0
+1
-1
• Reducing agent = potassium (K)
• Oxidizing agent = bromine (Br2)
(always reactants)
Oxidation-Reduction: Basic Concepts
Identifying Oxidation–
Reduction Reactions
• The following equation represents the redox
reaction of aluminum and iron.
• Identify what is oxidized and what is reduced
in this reaction.
• Identify the oxidizing agent and the reducing
agent.
Oxidation-Reduction: Basic Concepts
Identifying Oxidation–
Reduction Reactions
• Oxidized: Al
• Reducing agent: Al
• Reduced: Fe3+
• Oxidizing agent: Fe3+
The iron ion accepts the three electrons lost
from aluminum in the reduction process.
Basic Assessment Questions
Try it out!
For each of the following reactions, identify what is
oxidized, what is reduced, the oxidizing agent & the
reducing agent.
Zn + NiSO4 → Ni + ZnSO4
Ox: Zn
Red: Ni2+
Ox ag: Ni2+
Red ag: Zn
Ox: I–
Red: Br2
Ox ag: Br2
Red ag: I–
Table of Contents
Chapter 20: Oxidation-Reduction
20.3: Half-Reactions
• Writing half-reactions
1. Net ionic equation
2. Split into groups, add electrons
2Al → 2Al3+ + 6e-
2Fe3+ + 6e- → 2Fe
3. Reduce
• Oxidized: Al
• Reduced: Fe3+
• Red. Agent: Al
• Ox. agent: Fe3+
Oxidation-Reduction: Additional Concepts
• Writing half-reactions
0
0
+3
-1
3Br2 + 2 Al → 2 AlBr3
Net ionic eq: 3Br2 + 2Al → 2Al3+ + 6Br3Br2 + 6e- → 6BrBr2 + 2e- → 2BrReduced
2Al → 2Al3+ + 6eAl → Al3+ + 3eOxidized
Ox agent
Red agent
Oxidation-Reduction: Basic Concepts
Oxidizing and Reducing Agents
• A common application of redox chemistry
is to remove tarnish from metal objects,
such as a silver cup.
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Oxidation-Reduction: Basic Concepts
Oxidizing and Reducing Agents
• Several oxidizing and reducing agents play
significant roles in your daily life.
• For example, when you add chlorine bleach
to your laundry to whiten clothes, you are
using an aqueous solution of sodium
hypochlorite (NaClO), an oxidizing agent.
Oxidation-Reduction: Basic Concepts
Oxidizing and Reducing Agents
• Hydrogen peroxide
(H2O2) can be used
as an antiseptic
because it oxidizes
some of the vital
biomolecules of
germs, or as an
agent to lighten hair because it
oxidizes the dark pigment of the hair.
It’s your time!
• Start bringing scientific calculators to class
End of Topic Summary File
Oxidation-Reduction: Basic Concepts
Redox and Electronegativity
• The chemistry of oxidation–reduction
reactions is not limited to atoms of an
element changing to ions or the reverse.
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movie clip.
Oxidation-Reduction: Additional Concepts
Half-Reactions
• The oxidation half-reaction shows the loss
of electrons by aluminum.
• The reduction half-reaction shows the gain
of electrons by bromine.
Oxidation-Reduction: Additional Concepts
Half-Reactions
• Balance half-reactions by multiplying their
equations so that the electrons cancel
2 Al → 2 Al3+ + 36 e6 3 Br2 + 62e- → 2Br
3Br2 + 2 Al → 2 Al3+ + 6 Br3Br2 + 2 Al → 2 AlBr3