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Oxidation and Reduction
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Oxidation and Reduction
Oxidation and reduction is what electrochemistry revolves around. It is the basis of the whole unit of electrochemistry.
Oxidation:
In simple terms, oxidation is the process of losing electrons during a chemical process. In a chemical reaction, the substance which loses electrons is oxidized.
Reduction:
Reduction is the process of gaining electrons during a chemical process. The substance in a chemical reaction that gains electrons is reduced.
Redox reactions (reduction+oxidation) are reactions in which one substance gets oxidized, and the other gets reduced. Double displacement and neutralization reactions are NOT redox reactions.
To determine which is reduced and which is oxidized, we must look at the charges.
Example Equation:
Zn + Cu → Zn + Cu
The writing above shows the charges of these atoms. If there is no charge above the substance, the charge is 0 (If you are unfamiliar with charges, first learn about ions)
Zinc goes from having a charge of 0 to a charge of 2+, so it must have lost electrons. This tells us that zinc has been oxidized.
Copper begins with a charge of 2+ which then changes to 0, meaning it has gained electrons. Copper has been reduced.
Redox reactions are often split into two separate equations, oxidation and reduction. To do this, write the oxidation portion and reduction portion separatly.
Oxidation: Reduction:
Zn → Zn Cu → Cu
However these half reactions need to be balanced in terms of charges. To do this, we add electrons. The charge on the product side of the oxidation half reaction is +2, but we need it to be 0 on both sides. So we add 2 electrons (2e ) with a charge of -2 to the product side.
Oxidation:
2+ -
The reactant side of the reduction reaction has a charge of +2, so we add 2 electrons to make the charge 0 on both sides.
Reduction:
2+ -
Cu +2e → Cu
Oxidation numbers:
An oxidation number is the possible charge of an atom which shows how many electrons are gained/lost/shared. However, we are not always given the oxidation number when dealing with compounds.
Example Equation:
CuO + H → Cu + H O
Oxidation number of elements:
1st group: +1
2nd group: +2
13th group: +3
14th group: +4
15th group: -3
16th group: -2
17th group: -1
18th group: 0 (noble gases - full outer shell)
These elements often have these charges, but they may vary. Elements in between do not have a "normal" charge (charges vary). To know all the possible charges for an element, refer to an oxidation chart as shown on the bottom of this page.
Finding Oxidation Numbers:
When finding oxidation states in compounds, remember that their oxidation number must add up to its charge, in this case 0.
(Remember that an element by itself will always have an oxidation number of 0)
First compound:
Copper does not have a "set" oxidation number, so we will take the standard oxidation number of oxygen which is -2. Because the sum must be equal to 0, the oxidation number of copper must be +2.
Next:
H
Next:
H O
Now that the oxidation states are known, we can determine what is oxidized and what is reduced.
An increase in oxidation number means loss of electrons, which is oxidation. For example, the number going from -2 to 0.
An decrease in oxidation number means the gain of electrons, which is reduction. For example, the number going from +2 to -1
Write the oxidation numbers above the equation to find out what is oxidized and what is reduced.
+2 -2 0 0 +2 -2
|
___________
Now split into balanced half reactions:
CuO + H → Cu + H O
2+
2+ 2+
Zn → Zn + 2e
2 2
2 2
Oxidation:
Reduction:
CuO
2
|
2+
Molecules of elements by themselves will always have an oxidation number of 0.
As we know from before, the oxidation number of oxygen is -2. The oxidation number of hydrogen is +1, however there are two hydrogen atoms making its oxidation number +2.
2
___________
↑
↑
and
reduction
H → H O
2 2
CuO → Cu
Periodic Table of Oxidation States by Compound Interest, licensed under CC BY 4.0. No changes were made. View the license: https://creativecommons.org/licenses/by/4.0/
Cu
oxidation
Periodic Table of Oxidation States
Periodic table by 2012rc, CC BY 3.0, via Wikimedia Commons
Summary:
- Oxidation is loss of electrons
- Reduction is gain of electrons
- Find oxidation number to split into half reactions
- Half reactions are balanced using electrons