Question about ignition temp/activation energy/flash point (1 Viewer)

[LuthienAdrianna]

My text book says something like:

'Hydrocarbons do not undergo spont. combustion at room temp because of the high activ. energy of these reactions. The presence of a flame/spark can provide the activ. energy required for some molecules to react. The release of heat enerygy accompanying the reaction of these molecules is sufficient to maintain a high temp. so the reaction continues. This is why the flash point for fuels is much lower than the ignition temperature.

Can someone explain that last part a bit more clearly? :\

 

[Dreamerish*~]

The flash point of a fuel is the lowest temperature at which it can form an ignitable mix with air. At this temperature the vapor may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined at which the vapor continues to burn after being ignited. Neither of these parameters is related to the temperatures of the ignition source or of the burning fuel, which are much higher.

All flammable liquids have vapor pressure, which is a function of temperature. As the temperature increases, the vapor pressure increases. As the vapor pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence it is the temperature which determines the concentration of evaporated flammable liquid in the air under equilibrium conditions. Different flammable liquids require different concentrations of the fuel to be present in the air to sustain combustion. The flash point is that minimum temperature at which there is enough evaporated fuel in the air to start combustion

 

[LuthienAdrianna]

Yes... I understand everything that comes before that sentence, but I don't get how that last part relates to anything else in the passage. It's like saying some dogs are big and some are small, that's why some cats are black. ><

 

[Sparcod]

Flash point relates to volitility. The flash point is the min. temp. a substance has to reach to form a combustible vapourised mixture with air. A slight spark can set this off.

Ignition temp. is the min. temp it has to reach for the hydrocarbon to start reacting with the oxygen. At ignition temp, the spark is enough to get it up to the ignition temp so anything above the flash point is DANGEROUS!