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Alkanes 12,645 views

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Alkanes are hydrocarbons that only consist of single bonds. The ratio of carbon atoms to hydrogen atoms in alkanes is always in the form of n: 2n+2.

Alright. Let's talk about alkanes. And alkanes are hydrocarbons, compounds consisting of hydrogen and carbon, that only consist of single bonds, okay? Just single bonds in this carbon chain, this carbon back bone. So the formula for this is going to be CNH2N+2. Okay. So let's talk about several different types of alkanes you might see and come across.

So the first one, the simplest one is methane. Methane is CH4 consisting of one carbon and four hydrogen. And this is [IB] when you use methane, you [IB] this is like a natural gas, gas that might come from our body, something like that is methane. Okay? And notice the prefix meth, is dealing with one carbon, okay?

Next one is ethane. Ethane is dealing with two carbons. The prefix eth, is talking about two carbons and this is typically using c talking about different types of reactions [IB] and some things like that. This is ethane is typially used as a catalyst. Meaning that it helps the reaction move along. In fact three carbons we're going to take the three carbon chain it's going to have a prefix of prop. Meaning three carbons. And this is used, and you're talking about propane gas, propane tanks and things like that. You've probably heard of that when you go camping. You use that with fuel, those are you simplest fuel used for cooking and heating. We're actually transferring over from wood to propane more than, more times than not because it's actually a hotter flame. And so you're that's when you're dealing with propane and butane is also used as a fuel. Bute when talking about four carbons, four carbon chain and, but that doesn't combust as readily, so we use it for smaller things. These small lighters, things that need a smaller flame you're going to use butane, because this combusts a little bit with more energy than butane does.

Next is pentene. And now we're back to the prefix as we know pent, hex, hept, meaning it has five carbons that you should know when naming covalent compounds. Same, the same prefix that they use there. And this is in production of plastics. Plastics are also made up of hydrocarbons and so this is made up of a chain of hydrocarbons with five of them. So this prefix meth will always mean one carbon. Eth will always mean a chain of two carbons. Prop will always mean a chain of three and bute will always mean a chain of four. The reason they actually don't have the prefixes that we're used to is because these were actually discovered before they even knew what they looked like. So this was discovered before they were actually be able to determine the chemical composition of compounds. So these are just made up based off of the Chemist that were discovering them back in the 1800's. So an easy way for me to remember this is the [IB] mother eats peanut butter. It's just something I use and I tell my students it will help them remember that it's meth, eth, prop and but. I'm dealing with the naming of these alkanes.

Alright. Let's talk about the properties of alkanes. Alkanes can be either in straight chains, like this, you see and all the ends of these little lines are going to be hydrogen or it can be branched. So we can have four carbons arranged in this way where it's branched up. So there's many many many different ways that these carbon chains can actually show up. So when dealing with the properties of alkanes, these will come into play. But let's do the basics. Let's talk about water versus methane. So here's what water looks like structurally and this is what methane looks like structurally. Okay we know they're pretty, we know they're pretty similar, they're not very big particles. One has 18, mass 18 grams per mole and this methane is 16 grams per mole. So it's pretty much the same but they're actually drastically different. This state of matter at room temperature is going to be water, liquid state. But methane, we already talked about this in natural gas is actually going to be gaseous at room temperature. And why do we think that is? Let's talk about like what's actually chemically going on within this structure that's different.

We know this is a polar molecule. Not only is it a polar molecule due to the fact that it is bent and it has an, it's asymmetrical, but also that the fact that the bond between oxygen and hydrogen is polar. Electronegativity difference is very extreme. So we have a extremely polar molecule within water. Methane is symmetrical as you can see and not only is it symmetrical making the compound non-polar, carbon and hydrogen have very similar electronegativities. [IB] Everything within this is non-polar. So it has no partial charges anywhere whereas water has partial charges making the oxygen partially negative and hydrogen partially positive. That's one major difference. So, that's one. Look at the boiling point. The boiling point of water is 100 degree celsius as we know. This is -162 degrees celsius, drastically different. Why do we think that is?

Well, let's look at what's actually keeping when we're talk about boiling point, that's what actually keeping the particles together. So high boiling point means they're held together strongly in that liquid state and it's going to take a lot of energy to actually break apart that liquid state. This actually, -162 meaning it doesn't like to be in a liquid state. they're not attracted to each other. Methane particles are not attracted to one another at all. While water particles are attracted very strongly to each other. So, the forces, because this is non-polar, this is going to have London forces, very very weak intermolecular forces. Water has very strong intermolecular forces, there's hydrogen bonding, the strongest intermolecular force you can have. So methane is drastically different from water and these are the properties that we go along with and they're reasons why the characteristics, the fact that the hydrocarbons are alkanes especially are gaseous at room temperature and they behave the way that they do. So these are the general understanding of alkanes, our simplest hydrocarbons.