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!
