Question Video: Predicting the Trend in Flammability for the Alkane Homologous Series Chemistry • 7th Grade

In a part of the homologous series of alkanes, from methane to hexane, what happens to the flammability as the number of carbon atoms increases?

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Video Transcript

In a part of the homologous series of alkanes, from methane to hexane, what happens to the flammability as the number of carbon atoms increases?

A homologous series is a family of compounds that have the same functional group, and thus similar chemical properties, and the same general formula, with each compound in the series differing from the next by a simple structural unit. The question mentions the alkane homologous series, specifically from methane to hexane. Methane, with one carbon atom, is a small molecule with this displayed formula. Ethane, with two carbon atoms, is slightly longer with this displayed formula. Propane is longer by one carbon atom, butane even longer, pentane even longer with five carbon atoms, and hexane the longest of this portion of the alkane homologous series, with six carbon atoms.

We know the alkanes are hydrocarbon molecules. That is, they only consist of hydrogen and carbon. All bonds are saturated. That is, all bonds are single bonds. And all have the same general formula C𝑛H2𝑛+2, where 𝑛 is a nonfractional number, such as one, two, three, et cetera. And each successive member differs from the previous or the next by a simple structural unit, in this case CH2, one carbon atom and two hydrogen atoms. Since the compounds are identical except for a progressive addition of a CH2 unit, it makes sense that any property will change progressively with an increasing number of carbon atoms. We could say properties change progressively, especially physical properties.

We are asked what happens to the property of flammability as the number of carbon atoms increases, in other words with increasing chain length or molecule size. A flammable substance catches fire or ignites immediately when exposed to a flame. As we go from methane with one carbon atom through to hexane with six carbon atoms, so the molecules become larger. Although all these alkanes are combustible and can burn in oxygen, the more carbon atoms there are or the larger the molecule, the stronger the van der Waals forces of attraction are. As a result, more energy is required to vaporize these larger or longer molecules. And so the bigger hydrocarbon molecules are more likely to undergo incomplete combustion. And therefore they burn with smokier flames.

The smaller molecules in this series have weaker van der Waals forces of attraction between their particles as a result of the small size of these molecules. The very small alkanes are gases at room temperature and are already in the vapor phase. Even if the temperature was colder, less energy would be required to convert these molecules to the vapor phase because of these weak van der Waals forces of attraction. And these compounds tend to undergo complete combustion if there’s sufficient oxygen present and burn with a blue nonsmoky flame.

So what happens to the flammability as the number of carbon atoms increases? The flammability decreases.

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