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Lesson #1 |
Net
Ionic and Half Reactions |
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Topics
Introduction to Redox
Net ionic equations and Half- reactions
Spontaneity of Redox Reactions
Half reactions and net ionic equations.
Writing oxidation Half reactions
Let's examine the reaction between magnesium and oxygen to form magnesium
oxide. Look at the video of the reaction below
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Reaction between Magnesium
and oxygen
Set the quicktime movie to the start and watch the redox reaction
presented on the video between Magnesium metal and oxygen in the air.
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The balance of the overall reaction seen above can be written as follows.
2 Mg (s) + O2 (g) -------> 2 MgO
1) What is happening to magnesium in this reaction?
Answer: Magnesium is losing electrons to become
a positive ion . This is the process of oxidation.
The reaction we can write for this process is called the oxidation half
- reaction.
Mg0 (s) --------> Mg 2+ + 2 e -
( electrons )
Since two magnesium atoms are reacting a total of 4 electrons
are lost by the magnesium
2) What is happening to the Oxygen in this reaction?
Answer: Oxygen is gaining electrons to become
a negative ion. This is the process of reduction.
The reaction we write for this process is called the reduction half-
reaction.
O0 (g) ----------> O 2- + 2e -
Since oxygen is a diatomic molecule two oxygen atoms are undergoing
reduction and a total of 4 electrons are gained
Note: The total number of electrons
lost in oxidation = the total number gained in reduction.
This is true of all redox reactions
Key idea |
In
all redox reactions the total number of electrons lost in oxidation
= total number gained in reduction.
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Demonstration:
Aluminum and Iodine
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Watch the demonstration of powdered aluminum
reacting with iodine. A little water is added to speed up the reaction.
This is also a redox reaction. Try to identify
what is oxidized (lost electrons) and what is reduced ( gained
electrons ).
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Oxidizing and Reducing Agents
Using the reactions above:
Since the oxygen caused the magnesium to lose electrons it is termed the
oxidizing agent. (The substance that is reduced is always the
oxidizing agent)
Since the magnesium caused the oxygen to gain electrons it is termed the
reducing agent. (The substance that is oxidized is always the
reducing agent)
The equation below summarizes the above discussion.
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2 Mg(s)
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+
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O2 (g)
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2 MgO (s)
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Undergoes oxidation
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Undergoes
reduction
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Reducing agent
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Oxidizing agent
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Aluminum and Iodine (see demonstration)
Determine the oxidizing and reducing agent in the aluminum and iodine reaction.
Create a similar table for this reaction.
Determining if a reaction involves oxidation and reduction.
How can we tell if a reaction is redox or methasis?
In order to determine if the reaction involves an exchange of electrons,
it is often necessary to write a net ionic equation. Net ionic equations remove
all chemical species that do not change during the reaction. In order to determine
the net ionic equation, chemists write a total ionic equation in which all
ions are written separately. The following link provides a review of how to
write net ionic equations.
Writing Net ionic equation:
These general guidelines can be used to write the total and net ionic equations.
- The non ionic equation is balanced
- All ionic substances in aqueous form in the nonionic equation are written
as separate ions.
- All acids in aqueous form in the nonionic equation are written as separate
ions
- All solids , liquids and gases are written unchanged.
- To write net ionic equation the common aqueous ions are canceled or removed.
What is left is the net ionic equation.
Examine the reaction of zinc with hydrochloric acid , described by the nonionic
equation below
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Zn (s)
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+
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2 HCL (aq)
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ZnCl2 (aq)
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+
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H2 (g)
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a) Writing total ionic equation.
- Zinc is a solid so is left alone: Zn (s).
- HCl is an acid in aqueous form so is written as separate ion ; 2 H+
(aq) and 2 Cl - (aq) .
- ZnCl2 (aq) is an ionic substance so is written as separate
ions ; Zn 2+ (aq) and 2 Cl- (aq).
- Hydrogen is a gas and is therefore left alone : H2 (g).
Total Ionic equation then is:
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Zn (s)
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+
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2H+ (aq)
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+
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2Cl - (aq)
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Zn 2+ (aq)
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+
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2Cl- (aq)
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+
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H2 (g)
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b) Writing the net ionic equation
- We cancel common aqueous ions ( 2 Cl - (aq) ).
Net ionic equation is:
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Zn (s)
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+
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2H+ (aq)
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Zn 2+ (aq)
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+
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H2 (g)
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Using Net ionic equation to determine if reaction is Redox.
Let's examine the net ionic equation in the example above:
a) What has happened to Zinc in this equation? It went from Zinc atoms (
Zn (s) ) to Zinc ions ( Zn 2+ ). This is what process? Answer
b) What has happened to the hydrogen in this equation? It went from hydrogen
ions ( H + (aq) ) to Hydrogen atoms. This is what process? Answer
c) What are the oxidizing and reducing agents? Answer
d) What is the oxidation half reaction and the reduction half reactions for
the redox reaction above. Answer
Examples and Assignments
Example
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Write the total and net ionic equation for
the following reaction and determine which substance is oxidized
and reduced and write the oxidation and reduction half reactions.
Cu (s) + AgNO3 (aq) ---> Cu(NO3)2
(aq) + 2 Ag (s)
click on the eye to view answer
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Assignment : Half
reactions and net ionic equations
Lab : Single replacement reactions
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Lab : Single replacement
reaction
click on the test tube and complete the lab
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