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Tips on Ranking Melting Points of Different Hydrocarbons - Concept
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.
Tips on ranking melting points or boiling points of different hydrocarbons. First we need to talk about what influences boiling points and melting points of any substance, not just hydrocarbons. So I’m going to write this up. My thought it should be going off in my head, is intermolecular forces, you probably know them a lot as IMF, are most influential things regarding melting and boiling points.
So let’s think about IMF. So now that we know that IMF have a huge factor on melting and point, before we even get into reminding you how this operates. We know that some hydrocarbons actually have different IMF, so let’s go over the IMF real quick. We have going from the weakest IMF is London Dispersion. This also might be called Van Der Waals. Then you have Dipole Dipole, and then you have H-bonding. And these are the strongest.
So the weakest ones actually have the lowest melting point and boiling point and H-bonding has the strongest. Ias the highest melting point and boiling point.
So you know if we’re dealing with like an alcohol verses an alkanes, alcohol has H-bonding and then alkanes have London Dispersion. So you’ll definitely going to pick alcohol before the alkane. The alkane is going to boil first and then the alcohol is going to boil last, in this combination. But what if they’re all the same?
Both have the same IMF then what? Write this example below and we have Isomers; pentane 2-methylbutane, and 2-dimethylpropane. They are all actually isomers of each other, they all have the back bone of C5H12. And so therefore they’re pretty even and they help us explain how this works, because they are very similars to each other but structurally different.
They all have the IMF of London Dispersion, all London Dispersion Forces (LDF). What if I told you the boiling point of this is 36 degrees, would you expect these to be higher or lower than 36 degrees? Well let’s think. What keeps things together and in its like liquid state are the attractions that they have towards each other. So the electrons in London Dispersion, this is induced dipoles. Let’s say that electrons flow over here at some particular moment and time, and make this partially negative, and this partially positive. I’ll let you know this partially negative end, is very negative, because all these electrons were able to come over. And this becomes very polar due to the string of electron sources.
So electrons can float over here they all can be very far over here and then next they will all be far away from here. So this can create a very polar molecule. So from that you could look over here and they’re going to do the same thing. However this is not going to be as polar because they are so close together. When they’re more ball like, they don’t actually have a dipolar end. So this is actually as most a nonpolar because it is more ball like. They’re not able to get that serious pull on one side because all of them will come together anyway. So you’re not going to get that positive end and negative end because they’re all already kind of clumped together. There is no end in sight.
Here is the same thing because you have this guy it’s going to also be less polar than the actual pentane string or pentane backbone itself.
So actually, the boiling point for this, is 28 degrees Celsius. And the boiling point for this, is going to be even less, 9 degrees Celsius. So this going to boil first and it’s because of the ball structure. Anything that’s in a ball structure it’s going to boil first, boil away. It has a very low IMF, very weak IMF.
This is going to boil second because it’s not as ball like is 2,2-dimethylpropane but it isn't a straight chain as pentane. And this is going to stay together the longest, due to its very polar ends, well induced polar. So hopefully even this trick can even work when dealing with H-bonding all compounds. If H-bonding if they’re ball together like this, they’re not going to be as polar. You want to get something that's straight chained that’s going to have the highest boiling point. It’s going to stay together the longest.
So hopeful that helped and when you’re asked question regarding breaking of boiling and melting points of hydrocarbons.
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