The Alkanes (GCSE Chemistry)


Uploaded by freeeschool on 04.09.2012

Transcript:
GCSE Chemistry – The Alkanes
Hello and welcome! In today’s video, we are looking at the Alkanes and this is Part
2 for the Crude Oil video that was done previously. The first thing that I’d like to do is just
reminders about Crude oil; how the fractions are separated out from Crude oil. You may
remember this diagram that we had before. We saw that all the different parts of Crude
oil where separated out by the process of fractional distillation.
We also looked at a pattern in the type of molecule that we had. We talked about molecule
size being very small. We talked about viscosity which is the thickness of the liquid that
came out. We talked about the flammability and how molecules size increases as we went
down boiling point. We didn’t mention, but boiling point increases as you go down. Viscosity
gets thicker so viscosity increases as you go down and flammability decreases as you
go down these different fractions.
Now, what I want to do today is look a little more detail about the chemical nature of the
compounds that we get. The first thing to know and remember is that all these compounds
of what we call hydrocarbons and that means that they contain two elements only. Those
elements are hydrogen and quite simply, carbon. They contain hydrogen and carbon only; nothing
else. No other element is present. All these chemicals are called hydrocarbons and they
contain the element hydrogen and carbon only.
Next thing we can do is look at the structure of the hydrocarbon. So here, we have an example
of what hydrocarbons might look like. The carbon atoms are the black ones down the center
and the hydrogen ones are the ones all around the side. We can look at this and we can actually
work out a formula for this particular hydrocarbon. This one, we can count the number of carbons.
We have nine carbons and we have 20 hydrogens. The formula for this particular hydrocarbon
would be C9H20; as simple as that.
There is another way of representing this hydrocarbon and that’s by actually using
the letters so we can put the letters down in the middle of the carbon and the hydrogens
are on the side. So these are the two ways to be able to recognize the structure of hydrocarbons,
either by a key and a diagram or the actual letters themselves.
The next thing I want to look at is that actual features of these hydrocarbons. Here’s an
example of a small hydrocarbon. The key point that I would like you to remember is that
firstly, you see no double bonds in this hydrocarbon molecule. There are no double bonds and that
means that all these bonds between the atoms are single lines. If it was double bond, you
would see these two lines. Because there are no double bonds, we say it’s an Alkane.
We also described it as saturated. Saturated is another way of saying that it contains
no double bonds, so this is a saturated hydrocarbon.
We have the general formula which is CnH2n+2. I’ll explain what that means a little bit
more in a moment. These are the key features of the Alkanes. We say that they are saturated
hydrocarbons; they contain no double bonds in their structure and this is the general
formula.
What I mean by the general formula is you can use this to work out the formula for any
Alkanes. I’ve done one here for you. This letter here, the “n” represents the number
of carbon atoms. If we know the number of carbon atoms, we can work out the number of
hydrogen atoms. This example here, we got C3 and so n is 3 in this case. So we say C3
and we can say H8. The reason why H is 8 because H2(3)+2. So what we have done is instead of
putting n, we put a 3 in there. So 2(3) is a 6 + 2 = 8. We can use this general formula
to work out the formula for any Alkane.
So here, I have four examples of Alkanes. These are ones that you probably will need
to remember: Methane, Ethane, Propane and Butane. These are the number of carbon atoms
present in each one. I’ll write a formula up here to remind us. CnH2(n)+2.
So for this one here, we have C1 and if we substitute the 1 for the n, we have 2(1) +2
is 4. We have C1H4 and because we only 1 there, we don’t generally write the 1 there. It
just becomes CH4. So what you could probably do here is pause the video and have a look
to see if you can work out all the rest of them. I’ll go through the answers in a moment.
Okay, this one here is C2H6; C3H8. This one is C4H10. Okay. We can see a pattern going
up in this. You should also be out to look at one that doesn’t have a pattern like
this. For example, if you were given an Alkane that had 17 Carbon atoms, what would that
be? Very simply it is C17H36.
So that’s basically it. We have hydrocarbons which are made of hydrogen and carbon only.
The Alkanes are the ones that have no double bonds; they are saturated and have the general
formula CnH2(n)+2 and you should be able to work out the formula for any of those Alkanes
given that general formula.
[end of audio – 6:34] GCSE Chemistry – The Alkanes Page…1