Examples – Assigning Oxidation Numbers and Identifying Redox Reactions

Example 1: Combustion of Methane

When methane (CH4) is combusted (fig. 1), it reacts with oxygen (O2) to form carbon dioxide (CO2) and water (H2O):

CH+ 2 O→ CO+ 2 H2O

photo a methane flame of a gas stove burner

Figure 1: Methane flame of a gas stove burner. [S. Depolo, licensed under the Creative Commons Attribution 2.0 Generic license, https://www.flickr.com/photos/stevendepolo/4226949238]

In order to figure out, if this is a redox reaction and, if it is, which element is oxidized and which element is reduced we start by assigning oxidation numbers to the individual compounds:

Compound

Rules applied

Balancing

Oxidation numbers

Glucose CH4

The oxidation number of hydrogen is usually +1.

4 hydrogen gives

+4

 

The sum of the oxidation numbers in a compound is zero.

The oxidation number of carbon must be

-4

Oxygen O2

Pure elements have the oxidation number 0.

 

0

Carbon dioxide CO2

The oxidation number of oxygen is usually -2.

2 oxygen gives

-4

 

The sum of the oxidation numbers in a compound is zero.

The oxidation number of carbon must be

+4

Water H2O

The oxidation number of hydrogen is usually +1.

2 hydrogen gives

+2

 

The oxidation number of oxygen is usually -2.

1 oxygen gives

-2

The sum of the oxidation numbers in a compound is zero.

Works out well. The balance is zero.

 

Looking at the reaction equation with the assigned oxidation numbers

we can now see, that carbon’s oxidation number increases from -4 to +4, while the oxidation number of oxygen decreases from 0 to -2. Carbon loses electrons and is oxidized. Oxygen gains electrons and is reduced. The oxidation number of hydrogen does not change. It is neither reduced nor oxidized.

Example 2: Nitrification

As mentioned before in this course, nitrogen (N) is an essential plant nutrient. Many fertilizers contain nitrogen in the form of ammonium (NH4+). In the soil, bacteria convert ammonium into nitrate (NO3-). This process is called nitrification. Even though plants use NO3- as a nitrogen source, it also readily dissolves in water and therefore rapidly leaches from the soil, eventually contaminating ground and surface waters. In surface water increased NO3- concentrations cause an increased growth of algae (fig. 2).

photo of a river exhiting algal bloom

Figure 2: The addition of excess nutrients to surface waters causes an increased growth of algae. [F. Andrews, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license, https://en.wikipedia.org/wiki/File:River_algae_Sichuan.jpg]

The formation of NO3- from NH4+ can be described by the following equation:

NH4+ + 2O2 → NO3- + 2H+ + H2O

After assigning oxidation numbers in this equation

we see that nitrogen is oxidized (the oxidation number increases from -3 to +5) and oxygen is reduced (the oxidation number decreases from 0 to -2).

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Sources:

Last modified: Thursday, 6 October 2016, 5:13 PM