## 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

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).

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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