LEARNING OUTCOMES
C12-1-10: Identify reactions as redox or non-redox. Include:
oxidizing agent, reducing agent, oxidized substance, and
reduced substance.
DEFINING OXIDIZING AND REDUCING AGENTS
Based on the below reaction:
We can see that:
•
The copper atom is oxidized because its oxidation number increases
(becomes more positive), indicating a loss of electrons
•
Each silver atom is reduced---the gain of electrons is indicated by a
decrease in the oxidation number
OXIDIZING AND REDUCING AGENTS
•
•
•
•
•
The two processes of reduction and oxidation cannot occur without one
another
Since silver is reduced, it causes copper to be oxidized
• Any substance that causes the oxidation of another substance is called
an oxidizing agent
• In this example, AgNO3 is the oxidizing agent
Since copper is oxidized, it causes silver to be reduced.
• Any substance that causes the reduction of another substance is called
a reducing agent
The oxidizing agent becomes reduced and the reducing agent becomes
oxidized
Analogy: travel agent
• A travel agent books tickets and holidays for you to travel. They don't do
the traveling, but they are responsible for your travels. They are the
agents of travel.
IDENTIFYING OXIDIZING AND REDUCING
AGENTS
Ex 1) Nitric acid reacts with hydrogen sulfide according to the
balanced equation below. Identify the substance oxidized, the
substance reduced, the oxidizing agent and the reducing agent
for the burning of propane.
2 HNO3(aq) + 3 H2S(g) → 2NO(g) + 3 S(s) + 4 H2O(l)
Step 1: Assign oxidation numbers
IDENTIFYING OXIDIZING AND REDUCING
AGENTS
Step 2: Determine substances oxidized and reduced
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•
•
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S is oxidized, loses electrons.
N is reduced, gains electrons.
H2S is the reducing agent.
HNO3 is the oxidizing agent.
NOTE: oxidized and reduced substances are always atoms. The
oxidizing and reducing agents are the compounds that undergo the
change.
ELECTRONS TRANSFERRED
A balanced chemical equation allows us to determine how many
electrons are transferred in a redox reaction
To determine the number of electrons transferred in a redox
reaction:
1. Assign oxidation numbers
2. Determine atoms gaining or losing electrons
3. Multiply the number of electrons lost or gained by the number of
atoms gaining or losing the electrons
EXAMPLE: ELECTRONS TRANSFERRED
Example: for the redox reaction Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag How many
electrons are transferred?
Step 1 & 2:
Step 3:
• Each copper atom loses 2 electrons. There is one copper atom = total of 2
electrons lost
• The silver atoms gain 1 electron each. There are 2 silver atoms = 2
electrons gained
• 2 electrons lost + 2 electrons gained = total of 2 electrons transferred
EXAMPLE: ELECTRONS TRANSFERRED
Example 2) How many electrons are transferred in the reaction
below:
2 HNO3(aq) + 3 H2S(g) → 2NO(g) + 3 S(s) + 4 H2O(l)
Step 1 & 2:
EXAMPLE: ELECTRONS TRANSFERRED
Step 3:
COMMON OXIDIZING AGENTS
• Strong oxidizers are those that
readily accept electrons, that is,
they will take or steal electrons
from many substances. Strong
oxidizing agents are usually
atoms with high positive
oxidation numbers, like +5, +6
and +7.
• Usually very reactive because
they will take electrons from
almost any substance, in order
to lower their oxidation number.
Common
Oxidizing Agents
Reaction
Products
O2
O2-, H20, CO2
F2, Cl2, Br2, I2
F-, Cl-, Br-, I-
MnO4-
Mn2+
Cr2O72-
Cr3+
HNO3
NO, NO2
H2O2
O2, H2O
COMMON OXIDIZING AGENTS
Example : Chlorine
• Often used in pools, drinking water and sewage to kill bacteria.
• Chlorine oxidizes the cell membrane of bacteria causing the
membranes to be disrupted and the bacteria die.
• Hydrogen peroxide, H2O2, works in a similar manner when used
as a disinfectant on cuts and scrapes.
COMMON REDUCING AGENTS
• Good reducing agents are good sources of electrons. That is,
reducing agents give up electrons easily.
• Examples of good reducing agents include metals that are easily
oxidized, such as Na, K, Mg, Al, Zn and Fe.
• Other reducing agents are those substances that burn easily, like
hydrogen gas and hydrocarbons.