By Robert Hazen, Ph.D., George Mason University
A chemical reaction occurs when two or more chemical substances combine to form a different substance, or when one chemical breaks down to form other materials. Given the relatively small number of common chemical elements, it might appear chemistry is a rather limited field. After all, with only chemical elements, how many combinations could there be?
Art of Chemistry
Think about a 1:1 ratio of two elements, the binary compounds. Well, we have 80 elements, each of which can combine in a 1:1 ratio with the remaining 79. So that’s 80 times 79, and you have to divide by two because a combination of A with B is the same as a combination of B with A. That gives you a total of 3,160 1:1 element combinations from the common chemical elements.
Well, you need to try each of those 1:1 combinations, those 3,000-plus combinations, at many different temperatures, at many different pressures, and, of course, at many different ratios, because you might have, try 100:1 or 1,000:1 or 10:1 or 2:1 or whatever you want to do, all those ratios. So that leads ultimately to millions upon millions of combinations, even with the binary compounds.
And then you have to consider three elements together or four elements. There are more than a billion different combinations of five elements in a 1:1:1:1:1 ratio. And then you have to vary the ratios. And then you have to vary the temperature. And you have to vary the pressure. For all intents and purposes, chemistry is a vast, infinite subject of study.
Indeed, you can’t just go into the laboratory and randomly mix chemicals and hope to have anything useful come out. You have to have a plan. So the art of chemistry is the ability to predict which chemical combinations are going to provide something useful, which will give you a new synthesis condition that can be used to help humanity.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Possible Chemical Reactions
All chemical reactions involve the rearrangement of electrons through the breaking and re-forming of chemical bonds. You have to take atoms apart, stick them back together in a new way. Now, while there are countless specific reactions, most fall under a few general types of reactions. Let’s take oxidation-reduction reactions.
Now, oxidation-reduction reactions include many of the common reactions with oxygen, which is a highly reactive gas. It’s the gas that forms 20 percent of our atmosphere that we breathe. Oxidation reactions occur when oxygen— or any other element, for that matter—accepts electrons. Oxygen is element 8; so it wants two electrons. And when oxygen gains those two electrons, that’s an oxidation reaction.
Learn more about nuclear fission and fusion reactions.
Oxidation
Typically, oxygen reacts with a metal. So, for example, if you have iron—a metal—iron gives up electrons, oxygen takes them, and you can form the chemical reaction called rusting. So you form iron oxide from iron metal and oxygen. There are other, much more rapid forms of oxidation.
For example, burning. If you burn a hydrocarbon, you produce hydrogen oxide, H2O, or water, and carbon dioxide, the oxide of carbon. And you get lots of heat, and you get lots of energy. So all of these oxidation reactions produce heat as a byproduct, and that’s one of the reasons why they’re very useful.
Now, humans and other animals also use oxidation reactions. In fact, when we breathe in oxygen, we’re able to gain energy by oxidizing various hydrocarbons that we eat as our food. There are other elements that undergo this kind of oxidation reaction too.
Fluorine, the most violently reactive of all elements, wants to gain one electron, and so it oxidizes almost anything. That’s why fluorine gas is so dangerous. It killed or injured so many early chemists who tried to study it.
Reduction
The opposite of oxidation is called reduction. It’s just the opposite. It’s when oxygen gives up electrons. Now, since we’re such an oxygen-rich atmosphere, and since every oxygen atom wants electrons, reduction reactions don’t often occur just in the normal atmosphere. But in a very, very hot fire, you can produce a reducing environment. And that’s how iron ores are smelted.
Iron ore is an iron oxide. So you take iron oxide, you put it in a hot furnace along with charcoal—that’s carbon—and typically, calcium oxide—that’s called lime. The typical reaction is the iron oxide combines with carbon to form carbon oxide, CO2, or carbon dioxide, and iron metal. And that’s how an iron smelter works.
All of the oxidation-reduction reactions involve the movement of electrons. That’s the key here. So when iron metal is oxidized, the iron atoms lose electrons, the oxygen gains electrons. When iron ore is smelted, on the other hand, the iron atoms gain electrons.
Learn more about carbon’s unparalleled ability to form covalent bonds.
Battery Action
Batteries rely on oxidation-reduction reactions. The two terminals on most batteries are places where oxidation and reduction reactions occur. The negative terminal of many batteries is in contact with a chemical such as zinc metal. And that zinc metal readily gives up electrons and enters a solution as Zn2+ ions.
Now, the positive terminal of the battery is in contact with a solution containing metal ions, typically copper ions, Cu2+ ions. And as the battery is used up, copper is precipitated on that positive terminal. So you have a transfer of electrons with oxidation-reduction reactions going on. Now, when a circuit of the battery is closed, electrons flow from one terminal to the other.
When the battery goes dead, it means that either all of your oxidants or all of your reductants have been used up. And quite a few batteries can be recharged just by forcing electrons to flow the other way, and you re-set up the oxidants and reductants, and you can use your battery over again.
Common Questions about Oxidation and Reduction Reactions
Different combinations with various ratios result in staggering amounts. It’s close to infinity when we factor into account that all these materials can have chemical reactions or physical ones at different pressures, temperatures, etc.
Both are chemical reactions but are the opposite of each other. When oxygen gains electrons, it’s called oxidation, and when it loses them, it’s called reduction.
It means that the chemical reactions that led to the battery working are no longer possible, which means that either all the oxidants or all the reductants have been used up.
