Now that you understand chemical reactions, it’s time to start classifying them into smaller groups. You may wonder why this is something that’s important, and frankly, that’s not a bad question.
You see, the six types of reaction are only one of many different ways to categorize chemical reactions. For example, your teacher may teach you things like “redox reaction” and “nuclear reaction” as types of reaction, or may just rename some of the ones that I’ve listed below. This isn’t something you should concern yourself with because it doesn’t really matter.
The real reason we learn about the types of reaction is that it allows us to figure out what will happen in a chemical reaction. Whether you refer to a synthesis reaction as being “synthesis” or “redox”, the main thing is that you’re able to use this label to do important stuff. As you’ve probably already figured out, it’s doesn’t really matter what you name something, as long as that naming scheme gives you useful information.
In this tutorial, you’ll learn about what are generally considered the six types of chemical reaction. Though not particularly interesting, this will come in very handy when you start learning about how to predict reaction products.
Reaction type 1: Combustion reaction
A combustion reaction occurs when stuff burns. OK… I assume you already knew that, so I’ll just give you the chemical definition:
Combustion reactions take place when a compound containing carbon and hydrogen reacts with oxygen to make water vapor, carbon dioxide, and heat.
This sounds annoying, I know, but it’s really not. Consider the general form of a combustion reaction:
CₘHₙ + O₂ → CO₂ + H₂O
Basically, if anything containing C and H reacts with oxygen gas, you end up with carbon dioxide and water vapor. And lots of heat.
One example of combustion takes place when cyclohexane (which is present in crude oil) is burned:
C₆H₁₂ + 9 O₂ → 6 CO₂ + 6 H₂O
Reaction type 2: Synthesis reaction
A synthesis reaction is a reaction in which simple compounds are combined to make a more complex one. An analogue you might see in everyday life if that of making a turkey sandwich – you start with two slices of bread and a slice of turkey and end up with a sandwich. The general form for a synthesis reaction reflects this:
A + B →C
One specific example of this sort of reaction occurs when sodium and chlorine gas combine to make sodium chloride:
2 Na + Cl₂ →2 NaCl
For the reactions you’ll run into, it’s likely that the reagents will be fairly simple and the products will be easy to predict. Keep in mind, however, that the real world doesn’t work that way.
Reaction type 3: Decomposition reaction
A decomposition reaction is one where a molecule breaks apart into simpler ones. For example, I would guess that Johannes Sebastian Bach is in much simpler pieces now than when he was buried in 1750.
In the chemical world, the general form for a decomposition reaction is this:
C → A + B
And an example of it occurs when hydrogen peroxide breaks apart to form oxygen gas and water:
2 H₂O₂ → 2 H₂O + O₂
Reaction Type 4: Single displacement reaction
Also known as a “single replacement reaction”, this type of reaction occurs when a pure element switches places with an element in a chemical compound. Essentially, two atoms switch places, where one of the atoms isn’t stuck to anything else. The general form of this reaction is:
A + BC → B + AC
In this case, the elements A and B switched places. This type of a reaction is also a very common type of redox reaction, which you’ll learn about when you’ve done some more chemistry.
Many metals will boil when you place them into a strong acid. For example, if you put magnesium into hydrochloric acid, you’ll get the following single-displacement reaction:
Mg + 2 HCl → H₂ + MgCl₂
Because hydrogen is a gas, bubbles can be seen during this reaction.
Reaction type 5: Double displacement reaction
Also called a double “replacement” reaction, this type of reaction occurs when the cations of two chemical compounds switch places. The general form for this reaction is:
AB + CD → CB + AD
In which you can see that A and C switched places. If you were, for some reason, interested in adding hydrochloric acid to silver nitrate, you’d see the following double displacement reaction:
HCl + AgNO₂ → AgCl + HNO₂
Silver chloride will eventually degrade in light to form silver metal and chloride gas, so if you’re willing to wait around, you’ll eventually be able to have a very small amount of impure silver and a very small amount of unbelievably toxic gas to play with. Which, now that I mention it, seems like a very bad idea.
Reaction type 6: Acid-base reaction
If you combine an acid with a base, you’ll get water and something else. Well, this isn’t entirely true, depending on your definition of an acid and base, but for most practical purposes, we can assume that this works out pretty well. In any case, acid-base reactions are pretty much the same thing as double displacement reactions, except that water is one of the things that’s made:
HA + BOH →BA + H₂O
As you can see, the H and B switched places, which is where the water came from. Aside from that, it’s the same thing as a double displacement reaction.
One example of an acid-base reaction occurs when nitric acid is neutralized with sodium hydroxide:
HNO₃ + NaOH → NaNO₃ + H₂O
Incidentally, I would recommend that if you do this particular reaction you be very careful, as both nitric acid and sodium hydroxide are very caustic.
How to tell which type of reaction you’ve got:
If you’re not sure what type of reaction you’re dealing with, ask yourselves these questions in order. When you say “yes” to any of the questions, you’re done and that’s the type of reaction you’ve got. (If you answer the questions out of order, you’ll probably get it wrong, which is why I put in order in italics).
- Does the equation contain O₂, CO₂, and H₂O? Yes = combustion rxn.
- Do simple things make something more complex? Yes = synthesis rxn.
- Does something complex break apart? Yes = decomposition rxn.
- Are there any pure, unbonded elements? Yes = single displacement.
- Is water a product of this reaction? Yes = acid-base. No = double displacement.
An example of how to use this: Determine the type of reaction here: H₂O → 2 H₂ + O₂
Answering the questions:
- The reaction contains O₂ and H₂O, but not CO₂. Keep moving.
- Simple things don’t make something more complex. Keep moving.
- Something complex breaks apart to make simpler things. This is a decomposition reaction.
It’s as simple as that!
- The lovely Yvette Gonzalez-Nacer: By ARTEST4ECHO (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons. For the record, Ms. Gonzalez-Nacer was 29 at the time of this writing, so don’t email me about being a creepy dude.
- Bach: Elias Gottlob Haussmann [Public domain], via Wikimedia Commons.
- BP fun: By John Kepsimelis, U.S. Coast Guard [Public domain], via Wikimedia Commons.
- Oily pelicans: By International Bird Rescue Research Center [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)%5D, via Wikimedia Commons.
- Hot tub: Image courtesy of David Castillo Dominici at FreeDigitalPhotos.net.
- Nitric acid: “Nitric acid fuming” by W. Oelen – http://woelen.homescience.net/science/index.html. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Nitric_acid_fuming.jpg#/media/File:Nitric_acid_fuming.jpg