When a burner is lit, flames come out. Those flames are composed of ultra-hot gases. Where did they get their heat from? They got their heat from a redox reaction. Oxygen reacted with methane (or propane) to form carbon dioxide and water vapor. Redox reactions are very important in life. In a redox reaction, there is a transfer of electrons.
There are several concepts associated with redox reactions. The first concept is the electron. When a substance is oxidized, it loses electrons; when it is reduced, it gains electrons. Electrons are negatively charged particles. When they are attached to an atom, it becomes more negative (or less positive). When they are removed from an atom, it becomes more positive (or less negative) For example:
Cl + e(-) à Cl(-) (reduction)
Zn(2+) + 2 e(-) à Zn (reduction)
Zn à Zn(2+) + 2 e(-) (oxidation)
Cl(-)à Cl + e(-) (oxidation)
You notice the terms oxidation and reduction. What do they mean? Reduction occurs when electrons are attached to an atom. When a neutral chlorine atom has an electron chemically attached to it, it becomes a chloride ion. When a zinc ion has two electrons attached to it, it becomes a zinc atom. When reduction occurs, the element gains electrons to become more negative (or less positive).
There is another term called oxidation. Oxidation is the opposite of reduction. It occurs when electrons are removed from an atom. When oxidation occurs, the element loses electrons to become more positive (or less negative). When a zinc atom has two electrons removed from it, it becomes a zinc ion. When a chloride ion has an electron removed from it, it becomes a chlorine atom. Many times, an ion is much different from a neutral atom. For example, chlorine is a noxious, greenish-yellow gas, while the chloride ion is colorless and nonpoisonous. Zinc is a bluish-white metal, while the zinc ion is colorless and needed in your body in small quantities.
Oxidation and reduction happen at the same time. Neither one can happen without the other. For example, when hydrogen and chlorine react explosively, this is the net reaction:
H2 + Cl2 à 2 HCl
But this is a redox reaction: oxidation and reduction are happening in this reaction. The hydrogen is being oxidized to hydrogen ion; the reaction is:
H2 à 2 H(+) + 2 e(-)
Since oxidation is going on, something has to be oxidizing it. Normally a substance is oxidized by an oxidizing agent. Oxidizing agents come in various strengths. Some substances are harder to oxidize, while others are easier. Strong oxidizing agents can oxidize practically anything that will oxidize, while weak oxidizing agents can only oxidize substances that will easily oxidize. In this reaction, the oxidizing agent is the chlorine. But there is also reduction going in. The chlorine is being reduced to the chloride ion; the reaction is:
Cl2 + 2 e(-) à 2 Cl(-)
Since reduction is going on, something has to be reducing it. Normally it is reduced by a reducing agent. Reducing agents come in various strengths. Some substances are harder to reduce, while others are easier. Strong reducing agents can reduce practically anything that will reduce, while weak reducing agents can only reduce substances that will easily reduce. In this reaction, the reducing agent is the hydrogen. The reducing agent does the reducing, but gets oxidized in the process. The oxidizing agent does the oxidizing, but gets reduced in the process. You may notice that the two electrons released by the hydrogen gas are absorbed by the chlorine. Remember that when a substance releases electrons, oxidation is going on. When a substance gains electrons, reduction is going on. Here is a table that shows the relationships.
Happens to the reducing agent
Loss of electrons
Done by the oxidizing agent
Happens to the oxidizing agent
Gain of electrons
Done by the reducing agent
I will give some more complicated examples now. Here is the analysis of the reaction with ammonia and chlorine:
2 NH3 (g) + 3 Cl2 (g) à N2 (g) + 6 HCl (g)
Here are the half-reactions which occurred.
2 N(3-) à N2 (g) + 6 e(-)
3 Cl2 (g) + 6 e(-) à 6 Cl(-)
The nitrogen gave electrons away, so it was oxidized. The chlorine accepted an electron, so it was reduced. Since the chlorine was reduced, it was the oxidizing agent, and since the nitrogen was oxidizing, it was the reducing agent. The electrons gave away and taken are the same number.
Another example of a redox reaction that occurs in common everyday life is the corrosion of iron. Iron reacts with oxygen and water to form a hydrated iron oxide. Since the reaction involves the flow of electrons, adding salt water increases the rate of corrosion.
Here are some redox experiments that can be done in the home. One is the electrolysis of water. Take a pencil lead and a screw; put them into a container of water which contains a small amount of baking soda in it. Connect the pencil lead to the positive electrode and the screw to the negative electrode of a 9 volt battery or a power supply of similar voltage. You will see some bubbles being produced. The reaction is:
2 H2O à 2 H2 + O2
This is the net reaction. When solutions undergo electrolysis (the passing of electric current through the solution), the oxidation occurs at the positive electrode and the reduction occurs at the negative electrode. That is why to get this reaction you should not use a metal anode (the positive electrode). The metal will be oxidized and eaten away. The redox reactions are:
4 H(+)+ 4 e(-) à 2 H2
2 O(2-)à O2 + 4 e(-)
The hydrogen ions are being reduced at the cathode to hydrogen gas. It may ignite when a wooden splint is applied to it. The oxide ions are being oxidized to oxygen gas. It will make a glowing ember flare up.
Another experiment is electrolysis of water (containing sodium bicarbonate) with copper electrodes. You will notice a blue precipitate (insoluble substance) form, which will gradually turn black if it stays wet. The reaction is:
Cu (s) + 2 H2O (l) à Cu(OH)2 (s) + H2 (g)
The copper hydroxide is produced at the anode, and the hydrogen gas is produced at the cathode. Here are the individual redox reactions:
Cu (s) à Cu(2+) + 2 e(-)
2 H(+) + 2 e(-) à H2 (g)
Oxidation is occurring with the copper, oxidizing it to copper ion. Hydrogen ions are reduced to hydrogen gas at the cathode. The bubbles will be visible. The copper wire at the anode will become thinner and thinner, and you will see a greenish-blue substance being produced. Copper is reddish-brown, while the copper ion is greenish-blue or sometimes black. If you heat the copper hydroxide, it will lose two of its hydrogen and one of its oxygen to form water; the remaining oxide ion will combine with the copper ion to form black copper oxide.
Redox reactions are very important and essential to life. The way the human body is powered is the redox reaction between glucose and oxygen. They can be harnessed also in many useful ways.