alkanes physical properties

Higher and foundation tier

Some of the physical properties of hydrocarbons depend on the size of the molecules, these properties include: boiling and melting points, viscosity and flammability. Viscosity is how runny or how easily a substance will flow and flammability is how readily a substance catches fire. These properties are particularly important for hydrocarbons, since their main use is as fuels. The table below contains the melting and boiling points of the first 10 alkanes, can you spot any trends or patterns in the data?

alkane molecular formula boiling point/0C melting point point/0C state at room temperature
methane CH4 -161 -182 gas
ethane C2H6 -88 -183 gas
ethane C2H6 -88 -183 gas
propane C3H8 -42 -188 gas
butane C4H10 -0.5 -138 gas
pentane C5H12 36 -130 liquid
hexane C6H14 69 -95 liquid
heptane C7H16 98 -90 liquid
octane C8H16 126 -57 liquid
nonane C9H20 151 -51 liquid
decane C10H22 174 -30 liquid

The trend in the boiling point is fairly obvious, the larger the molecule the higher the boiling point. This trend is much as expected, the larger the molecule the larger its molecular mass and the more intermolecular bonding it makes so its boiling point increases. The general trends is similar with the melting points. The first 4 alkanes are all gases, and pentane (C5H12) to hexadecane (C16H34) are all liquids at room temperature. The viscosity of the alkane liquids changes with chain length, the shorter the chain length the more runny and the more easily it flows, that is it is less viscous. The longer the carbon chain the more viscous or thick the liquid is. A volatile substance is one that evaporates easily. The smaller the alkane molecule the more volatile it is, this trend is mirrored in the boiling points of the alkanes.

The flammability of alkanes

flash point of alkanes Alkanes are used mainly as fuels. Obviously if an alkane is going to be used as a fuel it must be flammable. But just how flammable are alkanes? Consider the following example:
A liquid is placed in an evaporating basin as shown in the image opposite. If the liquid has a low boiling point it will be volatile, that is it will evaporate easily. If it evaporates easily then above the liquid will be lots of vapour, if the liquid has a high boiling point it will not be volatile and there will be very few vapour (gas) particles above the liquid. It is the amount of particles in the gas phase above the liquid surface that will determine how flammable it is. When a lit splint is placed above a flammable liquid, it is not the liquid but the vapour above the liquid which burns.

You may hear on CSI or fire fighters talk about the flash point of a substance. The flash point is the point at which there is just enough vapour present that when a flame is applied it will flash up BUT there is not enough vapour to keep the flame going. The substance will not burn. However the fire point is the point at which there is enough vapour present to sustain combustion and the fuel will continue to burn. Roughly speaking the fire point is about 100C higher than the flash point. The lower the flash point the more flammable the fuel will be. The table below gives the flash points for several alkane molecules.

alkane methane propane pentane heptane decane
flash point/oC -188 -104 -49 -7 46

Trends in flammability

testing how dirty a flame is The trend in the flash points and fire points are very clear, the larger the molecule the higher is its flash point and hence the less flammable it will be. It is also worth noting that the smaller the hydrocarbon molecule the more cleanly it burns. Large chain hydrocarbons are much more difficult to ignite than smaller chain hydrocarbons and they burn with very dirty sooty flames. You may also hear people refer to incomplete combustion as dirty combustion because of the soot it produces. You can easily see the soot in a flame by placing a cup or in the image opposite a clean white evaporating basin, after a few seconds in the flame the nice clean evaporating basin is black with soot.

Key Points

Practice questions

Check your understanding - Questions on the alkanes physical properties.