alkanes???

Question

alkanes have a high melting point (i worked out that much) but how does this influence their use as fuels?

Answer 1

Alkanes with a small number of carbon atoms have very low melting and boiling points (that's why methane is a gas at room temperature).
Very heavy alkanes (ones with lots of carbon atoms) have higher melting points.

If you want to burn something as a fuel, you need it to vapourise before it will ignite.
The lower the boilling point, the quicker it will vapourise and the quicker it will catch fire.

Heavier alkanes are therefore more difficult to burn.

Answer 2

The melting point of alkanes depends on the length of the carbon chain. In fact, the chain has to be at least 5 carbons long for the alkane to be a liquid at room temperature. Methane, ethane, butane and propane are all alkane gasses. The alkane used as a motor fuel is octane and gasoline engines are designed around the combustion characteristics of this compound. As the carbon chain increases in length, oils are the result. Past a chain length of 20, solids predominate. The extreme upper limit is the plastic polyethylene which has a chain length in excess of 200 carbons. Polyethylene's melting point is in fact above its ignition point.

Alkanes make good fuels because they consist of nothing except hydrogen and carbon. The combustion of carbon releases more heat energy than most other elements which will burn, such as Sulfur. Also, carbon dioxide and water are inert compared to the combustion products of sulfur or phosphorus. Liquid and gaseous alkanes are also very portable as fuels. Sulfur and phosphorus are solid materials. The combustion of Magnesium releases more heat energy than carbon and the oxides are not as toxic as those of Sulfur, but Magnesium is not as abundant as Carbon. Also, Magnesium is a solid material.

Answer 3

Alkanes do NOT all have high melting points.
The simplest alkane is methane, which is a gas at room temperature. Consequently, does not have a high melting nor boiling point.
It should be appreciated that as the alkane chain length increases then the boiling and melting points rise as well!!.
The chain length
C1 - C4 are gases at room temperature.
C5 - C10 are volatile liquids at room temp.
C11 - C16 are liquids
C17 - and above are waxes - none crystalline solids.

Generally, the longer the chain length, then more energy is released on burning as there are more bonds to break, but they often have an high activation energy to ignite them.
The volatile liquids and gases are the best fuels because they readily mix with atmospheric oxygen, and require only a low activation energy.

Answer 4

The shorter the chain of an Alkane (as gases), the lower is the Ignition temperature when mixed with the correct amount of air.
As the chains become longer, and in liquid form, they have increasingly higher Vaporisation and Ignition temperatures.
Above a chain length of around 18 Carbons, they are generally in a solid or semi-solid state (Waxes) and, therefore require more heating to melting point, further heating to vaporisation and have an even higher Ignition temperature.

Answer 5

You are partially right in your statement that alkanes have a high mp.

For alkanes to have high mp, they need to have a long carbon chain, normally 5 C atoms and above.

Fuels are meant to be burnt, so as to generate heat and light. Hence, you dont want a fuel that will either take a long time to light up or difficult to burn in the first place. For this, you need a fuel with relatively low mp, so as to generate heat and light quickly.

Hope it helps!

Answer 6

they have single bonds, and they use method distillation