prelim chem energy assignment questions (1 Viewer)

[jtran]

Hello people of BOS,:wave:

Im trying to finish my chem assignment but im stuck with a few questions that i cant google, if anyone can help with any question(s), please reply before sunday night (it's due on monday), thanks alot! Here are the questions:
1. Milk kept at room temperature can sour in as little as a day, but can keep in a refrigerator. Use energy profile and molecular kinetic energy distribution diagrams to explain these observations.

2. a) Explain why the flash point for pentane (- 49°C) is much lower than its ignition temperature (260°C).

b) Account for the explosive risks in small motor boats, wheat storage silos and underground coal mines.

c) Explain how white phosphorus, which has much lower ignition temperature than butane, can have a lower risk of explosive combustion.
Thanx again,
jtran XD

 

[eddy11]

1. Would have something to do with the rate of chemical reactions increasing with heat (souring of milk is a chemical reaction), as heat increases average kinetic energy also increases, meaning particles collide more often and thus react more often. dunno about those diagrams though, haven't covered them

2. a.dunno yet

b. An explosive reaction occurs when fuel molecules (molecules that can burn inc. wheat and coal particles, and obviously petrol vapour) are well mixed with air and there is no limit on the flow of fuel to the combustion area be it a small boat, mine or silo. When there is a good mixture with the right concentrations, and a spark or ignition occurs, this spark starts a reaction that accelarates so quick that the heat release and gas pressure creates an 'explosion.' In small and or confined spaces e.g. silo, there is this potential as the wheat particles cannot move out and so remain mixing with the air if there is no ventilation.

c. as explained above, you need a mixture of fuel and air for an explosion, in addition to the spark or ignition, without this mixture even if it had a very low ignition temp, it would not explode or burn. Assuming that white P is a solid, it would mix not with the air as efficiently as butane (a gas), hence not form an explosive mixture and not explode, regardless of ignition temp.

 

[jtran]

Thanx, eddy11! now im trying to put those answers into a more formal & scientific way...

 

[nandayo]

Herro.

In response to 2)

The ignition temperature for all hydrocarbons will be significantly higher than the flash point. This is because as you probably know, flash point is the temperature/pressure at which the pure fuel can form a ignitable mix with air..imagine the petrol vapours at a petrol station at a normals day temperature (quite low).

Ignition temperature however is the temperature at which the hydrocarbon (in this case pentane) can spontaneously combust and continue to burn without any addition of heat. Obviously this is going to be much higher.

Once again you can probably jazz that up to sound a bit more scientific.