hello and welcome to an introduction to reaction types. There are many many types
of reactions in chemistry, and any introductory text will usually present
these five types of reactions. In this video we will look at what characterizes
each reaction type so that you can identify each type of reaction.
Placing reactions in to categories can be very helpful when you are doing a reaction
that you are unfamiliar with, so that you can predict what might happen, or it can
help you design a reaction to get a particular outcome. Identifying a
reaction type helps you to predict products, and the second part of this
topic, you will find in a series of links to videos on how to predict products.
Our first reaction type is called combination/synthesis. Synthesis is a
much broader term than combination, so we will stick with just combination.
Basically we are combining atoms, or molecules, to create a new molecule.
For example zinc and sulfur combined to produce zinc sulfide. For each example we
will make a model for the reaction type, and importantly find a feature of the
chemical equation that tells us the reaction type. For combination, the model
is simply two substances, represented by A and B, combining to produce a single
product made up of A and B, which is represented by C. So very importantly, we
can identify combination reactions as having a single product. Another example
is hydrogen and oxygen combining to make water. Notice it is not balanced and for
the sake of brevity we will not balance any equations in this video. We can see
that these reactions have only one product. And that identifies them as
combination reactions, so two substances react to form one product.
Second reaction type is decomposition. In the example hydrogen peroxide decomposes
into two substances, oxygen and water. The model is a single compound,
breaks apart or decomposes into two substances, and the identifying feature
is seeing that there is one reactant, which breaks apart into component substances.
Another example is the decomposition of barium carbonate.
We see in the equation there is one reactant which decomposes into component elements
and/or compounds. Click on this link for predicting products in decomposition.
The third reaction type is combustion including hydrocarbon combustion.
Let's look at the model and identifying feature first, and leave out hydrocarbon
combustion for now. We see in the model that oxygen is a reactant and that is
what identifies a combustion reaction.
In particular this is a rapid reaction with
oxygen that would constitute combustion. There are many many reactions constantly
occurring all around us and in our bodies that are substances reacting with
oxygen in the air or in ourselves, but these are not combustion. We would notice
combustion immediately because it gives off lots of heat and has a flame!
In other words fire! Fire is another word for combustion.
One example is iron. At high enough temperatures metals will burn.
Another example is the combustion of hydrogen to produce water. Notice that both of these
also fit into the category of combination. So oxygen is a required reactant
and what is reacting with it is called fuel, a common word we use with any fire.
However any fire that we may see in our everyday lives, is almost
exclusively a hydrocarbon combustion.
So let's take a look at hydrocarbon combustion.
We still have the requirement of oxygen being a reactant, but now if
the other reactant, or the fuel, is a hydrocarbon, then the products are always
carbon dioxide and water. Hydrocarbons come in two slightly different forms.
The first one we see here as CxHy, the other is CxHyOz, with the x, y, & z representing a
huge range of subscripts. For example CH4 is methane, or we could have ethane,
propane, octane, and waxes have in the vicinity of 24 carbons, and there are
many, many more. If any of them react with oxygen then
the products are carbon dioxide and water. That second type of hydrocarbon
you need to be able to identify is just CxHy that has been oxidized to some
degree, in other words has oxygen in the formula, so that its general formula is
CxHyOz, such as formaldehyde, CH2O, or ethyl alcohol, glycerin, glucose, complex
carbohydrates, and thousands of others. They are all hydrocarbons that will
react with oxygen to produce carbon dioxide and water. So you need to be able
to identify a hydrocarbon as having in general the formula CxHy or CxHyOz.
Reaction four is single replacement. And some textbooks call it single
displacement but it's the same thing. Our example is aluminum reacting with zinc
chloride, the identifying feature is an element, usually a metal, reacting with an
ionic compound. You may notice from the products that
the two metals switch places. This is what our model represents: one element,
one compound, the arrow represents the reaction, which is where the metals switch
places to become the products. The arrow represents the reaction which is where
the metals switch places to become the products. Notice the neutral aluminum
becomes an ion, whereas the zinc ion becomes neutral. This is somewhat
simplified, please see the product prediction video link for single
replacement to get a more in-depth view of what is occurring here. For this video
know the identifying feature of one element reacting with one compound and
that is a single replacement. Reaction 5 is double replacement or double
displacement in some textbooks, and this is more appropriately referred to as a
precipitate reaction and more advanced textbooks. Our example is aqueous
magnesium sulfate reacting with aqueous zinc fluoride. Aqueous tells us the
compounds are dissolved in water, and this is the identifying feature to look for.
Two aqueous ionic compounds on the reactant side.
You may have noticed from the products that the two metal ions switch places to become
the products. And this is what our model represents, the arrow represents the
reaction, the ions switch places, becoming products. The arrow represents
the ions switching places and becoming products. The magnesium is now written
with the fluoride on the product side, and the zinc is with the sulfate.
So we identified double replacement as having two aqueous ionic compound reactants.
This is your basic double replacement, please see the product prediction link
to get a much more in-depth view of what is happening, so that you can
successfully predict products in a double replacement.
This has been a Mad Scientist production.
SEEYA!