Structure and Physical Properties

Transcript

0:00Introduction

Hello everyone! Okay, let us start with a small puzzle. Pentane and two, two dimethyl propane have the same formula, so the same mass. Yet pentane boils at three hundred and nine kelvin, and the other one at only two hundred and eighty three kelvin. Now the strange part: the one that boils lower melts higher! It melts at two hundred and fifty six kelvin, while pentane melts at just one hundred and forty three kelvin. How can that be? Hold on to that question. In this video, we look at the structure and the physical properties of alkanes, and questions like this come up often in JEE, so let's begin!

0:41Alkanes: saturated, open-chain hydrocarbons

So, what exactly is an alkane? Alkanes are saturated, open-chain hydrocarbons: only single bonds, and no ring. Their general formula is C n H two n plus two. The small n written below the C counts the carbon atoms, and the small two n plus two below the H counts the hydrogen atoms. Okay? Now a quick one for you. Put n equal to one, so just one carbon atom. How many hydrogen atoms will it carry? Two times one, plus two, gives four, yes, exactly! So the formula is C H four, with a small four below the H, and that is methane. Right? Now the structure. Every carbon in an alkane is s p three hybridised. The raised three counts the three p orbitals that mix with one s orbital. Clear? And the bonds? The carbon carbon bond, the little dash between the two carbon symbols, is one hundred and fifty four picometres long. The carbon hydrogen bond is one hundred and twelve picometres; p m on screen is short for picometre. Both bonds are strong, and almost non-polar. Remember these two words, because they explain what comes next. With me?

1:57Why alkanes are called paraffins

Now think about this. Strong bonds, and almost non-polar. So tell me, will an acid or a base attack an alkane easily? The answer is no, because alkanes are unreactive towards acids, bases, oxidising agents and reducing agents. But note the words in brackets on screen: under ordinary conditions. Okay? That is why alkanes are also called paraffins, which means little affinity. They simply do not want to react much. Got it? So which reactions do they show at all? Their only important reactions are free-radical reactions, and reactions at high temperature. Keep that in mind. Right?

2:39Physical state and boiling point

Now, the physical properties. First, the state at two hundred and ninety eight kelvin, which is room temperature; the capital K is kelvin. C one to C four, meaning one to four carbon atoms, are gases. C five to C seventeen are liquids, and C eighteen onwards are waxy solids. Okay? Next, the boiling point. Each time we add one more C H two group to the chain, with a small two below the H, the boiling point rises by about twenty to thirty kelvin. Clear? But why? What holds these non-polar molecules together, and why does it get stronger as the chain grows? It is the van der Waals forces between the molecules. These forces grow with the surface area, and a longer chain has more surface. Stronger forces to overcome, so a higher boiling point. With me? Now, here is a trap. Isomers have the same formula, so the same mass. So they must have the same boiling point, right? No, no, careful, because the boiling point follows surface contact, not mass. Among isomers, the more branched the chain, the smaller its surface of contact. So more branching means a lower boiling point. Got it?

3:57Melting point: it is about packing

Now the melting point, and here the rule changes. The melting point depends on how well the molecules pack together in the solid crystal. Okay? So a symmetrical molecule, which packs better, melts higher. Right? And one more pattern to remember. Along the series, the melting point does not rise smoothly; it zigzags. Going from an odd to the next even number of carbon atoms, it jumps up a lot, but from an even to the next odd number, it rises only a little, because chains with an even number of carbons pack better. Clear? Now a topper's trick. Boiling follows surface contact, and melting follows packing. Two different questions, so never mix them up. Okay?

4:41Solubility and density

Next, solubility. Alkanes are non-polar molecules. So will an alkane dissolve in water? The answer is no; alkanes are insoluble in water. They cannot form hydrogen bonds with water, so mixing would only break the hydrogen bonds between the water molecules, with nothing in return. Got it? But they do dissolve in non-polar solvents. And alkanes are less dense than water. With me?

5:07Example 1: alkane formula from molar mass 100 g/mol

Time for the first example. An alkane has a molar mass of one hundred grams per mole; the g mol to the power minus one on screen means grams per mole. Find its molecular formula. Pause the video and try it first. Done? Okay, let's check. Use the general formula, C n H two n plus two. Each carbon counts twelve, and each hydrogen counts one. So the molar mass, capital M, is twelve times n for the carbons, plus, in brackets, two n plus two for the hydrogens. That is fourteen times n, plus two. Okay? Now set fourteen times n, plus two, equal to one hundred. So fourteen times n is ninety eight, and n is ninety eight over fourteen, which gives seven. The double arrow on screen means gives. Clear? Always check your answer. Seven carbons give twelve times seven, which is eighty four, and two times seven plus two gives sixteen hydrogens. Eighty four plus sixteen is one hundred, perfect! So the alkane is C seven H sixteen. One line of algebra, and the formula is done. Got it?

6:20Example 2: 2,2-dimethylpropane boils lower, melts higher

Now the harder one, and it is our puzzle from the start. Pentane boils at three hundred and nine kelvin, and two, two dimethyl propane at two hundred and eighty three kelvin. Yet two, two dimethyl propane melts at two hundred and fifty six kelvin, and pentane at only one hundred and forty three kelvin. Explain this. Pause the video and try it first. Done? Okay, let's check. Both are C five H twelve, so they are isomers with the same mass. Pentane is a chain of five carbons. Two, two dimethyl propane is a central carbon carrying four C H three groups; the brackets with a small four after them mean four such groups. So it is compact, nearly spherical, like a ball. With me? First, boiling. Boiling follows surface contact, and the compact, nearly spherical molecule has the least surface contact. So it boils lower: two hundred and eighty three is less than three hundred and nine, and the pointed sign on screen means less than. Okay? Now melting. Its forces are weaker, so it should melt lower too, right? No, no, wait, because melting is a different question! Melting follows packing, and two, two dimethyl propane is the most symmetrical isomer, so it packs best in the crystal. So it melts higher: two hundred and fifty six is greater than one hundred and forty three, with the sign now opening the other way. Clear? So, the full answer: branching lowers the boiling point, and symmetry raises the melting point. And that solves our puzzle from the start!

8:02Quick revision: formula, boiling, melting, solubility

So, quick revision. Alkanes are C n H two n plus two, every carbon is s p three, and their strong, almost non-polar bonds make them unreactive. The boiling point goes up by about twenty to thirty kelvin for each C H two, and branching lowers it. The melting point follows packing, so symmetry raises it. And alkanes are insoluble in water, and less dense than water. Now go and try the practice problems on this page. All the best!