Reduction of Metallic Oxides: Examples & Equations

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Metallic oxides are metals bonded with oxygen. In certain reactions, the metals in the metal oxides are reduced. They initially have a positive charge and end up with no charge. In this lesson, we will go through what happens in the reduction process using some examples.

From Charged to Neutral

Libraries loan out books. When the patron has read the book, they return it to the library. This is similar to what happens with metals and their electrons. Metals are like the library, they give away electrons (books), and get the electrons (books) back in a reduction reaction. Let's pretend we are checking out a book on metallic oxide reduction to learn the details of this process.

Superscripts and Electrical Charge

Before we get into oxidation and reduction, we have to discuss superscripts, which are numbers written above an element's symbol. For example, the +2 superscript in Zn+2 is the charge on the ion. There is no superscript on an element's symbol when it is not in a bond with another element because it is neutral. A compound will have multiple elements that have bonded. Each atom in the compound has a charge even though no superscripts are shown. Next, we will discuss the reactivity series.

Activity Series

Many people think if you put any chemical compounds together, a reaction occurs. This is not true. Imagine putting a piece of gold into some crushed graphite from a pencil. Nothing happens. A chart called the reactivity series shows the relative reactivities of elements. This chart helps inform us which reactions will occur and which won't occur.

The elements written in green are the metals in the metal oxides in our upcoming examples. The elements in red will be used in the reactions.

If you want to reduce a metallic oxide, you have to use an element to react with it that is higher on the reactivity series. For example, we will use carbon to reduce zinc oxide. Carbon is more reactive (higher on the reactivity series) compared to zinc. You can't reduce magnesium oxide with carbon because magnesium is more reactive than carbon. Now let's get into oxidation and reduction.

Oxidation and Reduction

Metals initially give up electrons, which is called oxidation. This gives them a positive charge equal to the number of electrons they gave up. For example, if a zinc atom loses two electrons it becomes Zn+2. If the two electrons lost from zinc are picked up by oxygen, it becomes O-2. When something gains electrons it is called reduction. A trick to remember which is which is to use the commonly used phrase ''OIL RIG'', which stands for ''oxidation is losing (electrons), and reduction is gaining (electrons).'' Since these ions are oppositely charged, they stick together like opposite ends of two magnets forming zinc oxide, ZnO. This an example of an ionic compound because it was formed from two oppositely charged ions.

The zinc ion has a +2 charge, and the oxygen has a -2 charge. These charges cancel. These superscripts were included for illustrative purposes.

Reduction of Metallic Oxides

Zinc Oxide Reduced with Carbon

Just like when the library gets its books back, zinc can get back two electrons (be reduced) in a special chemical reaction with carbon. The zinc ion separates from the oxide ion and gains two electrons to become a neutral zinc atom. Let's look at the reaction whereby zinc is reduced.

ZnO (s) + C (s) + heat → Zn (g) + CO (g)

  • (s) stands for solid
  • (g) stands for gas

Notice the zinc produced is a gas! Zinc metal boils at over 900° C, which indicates the extreme temperatures required for this reaction to happen. The zinc gas is cooled, which turns it into a liquid. Upon further cooling, it solidifies.

The zinc metal produced has no charge because it initially had a +2 charge, and gained two electrons (-2 charge). The carbon atom was initially neutral (no superscript) and lost the electrons that zinc took. This resulted in oxygen having a -2 charge. The positively charged carbon bonded with the negatively charged oxygen.

Carbon starts the reaction neutral, but loses two electrons during the reaction giving it a +2 charge

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