Many very important chemical reactions involve oxidation-reduction. The burning of fuels, the corrosion of metals, photography, and batteries all involve oxidation and reduction.

When an element loses an electron, it is said to be oxidized. When an element gains and electron, it is said to be reduced. Oxidation and reduction always occur together. When something is oxidized, something else is reduced. Oxidation-reduction reactions are often called redox reactions.

The process by which a metal combines with water, air, or other materials in the atmosphere is called corrosion. The rusting of iron is an example of corrosion. In the rusting of iron, the iron loses electrons to the oxygen in water. So in rusting the iron is oxidized and the oxygen in water is reduced. The result of this corrosion is a product called iron oxide. Iron oxide is rust. Aluminum is also oxidized by the oxygen in water, or in air, to form aluminum oxide. However, aluminum oxide does not crumble like rust. A thin layer of aluminum oxide will protect the layers of aluminum below it so that corrosion does not continue.

Zinc also loses an electron to oxygen to form zinc oxide. Zinc oxide, like aluminum oxide, is not crumbly. The thin layer of zinc oxide protects the metal below so that it cannot be oxidized. Therefore, zinc is often used to coat and protect iron from rusting. The process of coating iron with zinc is called galvanization. Nails are iron galvanized with zinc to protect the iron from rusting. Paint will also protect iron from the oxygen which causes rusting.

Some metals corrode easily, others resist corrosion. Metals which give up electrons easily (corrode easily) are said to be active. The activity of a metal refers to its tendency to give up electrons. When metallic elements are arranged according to their activity, this is known as an activity series. Magnesium, aluminum and chromium are the most active metals. Gold, platinum and silver are the least active metals. Refer to the activity series, table 12-2 on page 544. On a sheet of paper, copy this activity series. Turn this in at the beginning of class tomorrow as a 25 point assignment. This will be used to help you answer questions on the next test.

In the activity series, a metal will give up electrons to any metal which is below it on the activity series. When elements gain or lose electrons, they are no longer called elements but ions. Elements which gain electrons are called negative irons. Elements which lose electrons are called positive ions.

Any more active metal will give up electrons to a less active metal. This will serve to protect the less active metal from corrosion. For example, steel ships often have bars of zinc attached to the sides of the ship. As the steel is corroded by the oxygen of water and air, the zinc will give up electrons to the steel and protect it from corrosion.

The substance which gains electrons causes the oxidation. Therefore, it is called the oxidizing agent. The substance which loses the electrons causes the reduction. Therefore, it is called the reducing agent.

Household bleach (sodium hypochlorite) is a very strong oxidizing agent. It removes electrons from stains and causes the stains to wash away. Hydrogen peroxide is also an oxidizing agent. It can remove electrons form the color pigment of hair causing it to lose its color. Ozone is also a very strong oxidizing agent. Ozone can damage vegetation, and can even damage plastic and rubber products, such as tires.

Substances called antioxidants are added to food to prevent oxidation and the resultant spoilage. These substances act as preservatives by reacting with the oxygen in the air. This prevents the oxygen from oxidizing the food.

Any active metal will give up electrons to a less active metal. This principle is used to make electrochemical cells (batteries). As the more active metal gives up electrons, they flow along a conductor to do work, and are accepted by the less active metal. To do this, the metals must be placed in a liquid or paste substance which is a good conductor of electricity. Such a substance is called an electrolyte. Acids and bases are good electrolytes.

To make a simple battery, you can insert a copper penny and a silver dime into a lemon. Using tape, attach wire to the penny and to the dime. Electrons will flow from the copper to the silver. This flow of electrons can be used to power appliances which require low electrical voltage, such as a digital watch.

Oxidation-Reduction Study Sheet