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Lighting Cans of Alcohols

1. a. You are presented with three identical metal containers of about 5 litres capacity, each about half full of fuel. The containers have a short neck and are normally closed by a screw cap of about 30mm diameter. The cans contain:

(i) petrol (gasoline, flashpoint -43oC),
(ii) methyl alcohol (methanol, flashpoint 11oC), and
(iii) paraffin (kerosine, flashpoint 50oC).

Explain concisely what would happen if the top was taken off and a lit match was immediately dropped into the headspace of each of the cans. Assume that the ambient temperature, and thus that of the liquids, is 15oC. (11 marks)

b. State briefly the relationship between the flash point and auto-ignition temperature of a series of related chemicals, for example methane, propane, butane and pentane. (Completing the table below will help you.) (3 marks)

Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
(n)Pentane C5H12


Solution Preview

According to Wikipedia, "The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark."

Whereas, "The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is removed."

Therefore, when we look at the containers given in the question, we can draw the following conclusions regarding what would happen if a lit match was dropped into the space of each can. Based on the definitions above, we are assessing the flash point. Another way to understand flashpoint ...

Solution Summary

The solution examines what happens when petrol, methyl alcohol and paraffin is lit with a match in a can. The relationship between flash point and auto ignition temperature is discussed.