Chemistry Questions (3 Viewers)

[Siddy123]

Re: ethanol as a reneweable resource?

Ethanol is renewable because we can grow cane sugar which we can make ethanol from

beast.

 

[golgo13]

Re: Urgent, heat of combustion question!

So changing the apparatus, so maybe use an aluminum can instead of foam cup, better sealing of the calorimeter, use an electronic thermometer instead of an alcohol/lead one. For reliability the only one i can think of is repeating it but i guess u can also use different measurements since molar heat of combustion should be constant the weight shouldn't matter too much and it gives you a range to work with, I hope that helps

 

[someth1ng]

Re: Urgent, heat of combustion question!

So changing the apparatus, so maybe use an aluminum can instead of foam cup, better sealing of the calorimeter, use an electronic thermometer instead of an alcohol/lead one. For reliability the only one i can think of is repeating it but i guess u can also use different measurements since molar heat of combustion should be constant the weight shouldn't matter too much and it gives you a range to work with, I hope that helps

I believe you mean mercury.

For the reliability, you should make more tests to determine why there is an anomaly (was it an outlier result due to error or is the results actually unreliable?)

 

[BlugyBlug]

Re: Urgent, heat of combustion question!

Also electronic balance with more decimal points for whatever you're weighing (accuracy)

 

[someth1ng]

Re: Urgent, heat of combustion question!

Also electronic balance with more decimal points for whatever you're weighing (accuracy)

I would be careful here, more decimal points does NOT necessarily mean that it is more accurate - it may appear more accurate since it seems to have smaller error but that's assuming that it is perfectly calibrated.

Let's say, a balance is accurate to 10 decimal places but there is a problem such that as any pressure on the balance will cause an extra 0.1g to be added - is it still accurate? Of course not!

 

[golgo13]

Re: Urgent, heat of combustion question!

I guess what u can do is zero it to the container which you are taking it from and take measure how much you take out but i don't think that makes much difference

 

[girlworld_club]

production of materials propagation question?

what occurs during the process of propagation and also

what steps occur before and after propagation to produce polyethylene.

 

[theind1996]

Re: production of materials propagation question?

what occurs during the process of propagation and also

what steps occur before and after propagation to produce polyethylene.

Initiation --> propagation --> termination.

You need to explain each step and draw a structural formula for each step.

First step is initiation. An initiator molecule is added to start the polymerisation, usually peroxide such as benzoyl peroxide. Peroxides are generally used since the O-O bond is easier to break than bonds in most other molecules. The initiator is decomposed by laser light or heat to form an initiator radical, which reacts with a monomer unit to form a monomer radical.

Condensed Structural formula:

R−O∙ + CH2=CH2 ----------> R−O−CH2−CH2∙

Second step is proagation. The ethylene monomer radical reacts with another ethene monomer, forming a dimer (two monomers that have been bonded together) radical. More and more alkene monomers are added and the polymer chain grows. Back biting may occur leading to chain branching.

Condensed Structural formula:

R−O−CH2−CH2∙ + n[CH2=CH2] -----------> R−O−[CH2−CH2]n−CH2−CH2∙

Final step is termination. Chain growth terminates when two polymer radicals of variable chain length react with each other, forming a longer chain. The initiator molecule is part of the chain and is not present by the end of the reaction.

Condensed Structural formula:

R−O−[CH2−CH2]n−CH2−CH2∙ + R−O−[CH2−CH2]n−CH2−CH2∙ -----------> R−O−[CH2−CH2]n−O−R

 

[girlworld_club]

Does anyone have jacarda chemsitry

i had the link for it but stupidly deleted it. anyone?

thanks soo much. i desperately need it.

 

[girlworld_club]

HDPE and LDPE

i do not quite understand how the two are made. Can any suggest a good video which explains it.

 

[Madridismo]

Re: HDPE and LDPE

They are polymers, which means they are made up of tens of thousands of ethlyene monomers including a catalyst - this sort of info should be in your chemistry textbook. (Polymerization)

HD = High Density, this means this polyethlyene is strong and rigid because on the molecular level there are less branches and the ethleyne are packed tightly in a nice arranged order.
LD = Low Density, this means this polyethylene is weak, malleable (or flexible) because on the molecular level there are more branches that produce dispersion forces and so aren't packed as tightly as the HDPE.

That's the most simplest way I guess.

These videos are good. Just search through the channel for the chem ones
http://www.youtube.com/user/letslearnscience

 

[Dan895]

Re: HDPE and LDPE

Also HDPE is produced under low pressure and temperature using the 'Ziegler-Natta' process and LDPE is produced under high temperatures and 1000-3000 times atmospheric

 

[Madridismo]

Re: HDPE and LDPE

Also HDPE is produced under low pressure and temperature using the 'Ziegler-Natta' process and LDPE is produced under high temperatures and 1000-3000 times atmospheric

You don't have to know the actual numbers but just the basic conditions + catalyst used.

 

[HeroicPandas]

Re: HDPE and LDPE

They are polymers, which means they are made up of tens of thousands of ethlyene monomers including a catalyst - this sort of info should be in your chemistry textbook. (Polymerization)

HD = High Density, this means this polyethlyene is strong and rigid because on the molecular level there are less branches and the ethleyne are packed tightly in a nice arranged order.
LD = Low Density, this means this polyethylene is weak, malleable (or flexible) because on the molecular level there are more branches that produce dispersion forces and so aren't packed as tightly as the HDPE.

That's the most simplest way I guess.

this doesn't show how HDPE and LDPE are made

 

[Madridismo]

Re: HDPE and LDPE

this doesn't show how HDPE and LDPE are made

Damnit didn't read that properly >.> Thanks for pointing it out

 

[wannabesurgeon]

Re: HDPE and LDPE

View attachment 2013-01-28 160121.pdf

I know its not a video but it explains it to me fairly well. Hope it helps!

 

[nightweaver066]

Re: HDPE and LDPE

http://www.youtube.com/watch?v=TkJ8h9WiagU

 

[hayabusaboston]

Re: HDPE and LDPE

lol dude, just read it in conquering chem, people on this thread are pretty much quoting directly from that book anyway.

 

[Madridismo]

Re: HDPE and LDPE

This is Ahmed Shah Idil's notes, not sure if state ranking but they are top quality and very thorough!
It might confuse you but it's still good to look at!

View attachment 9.2 - Production of Materials.doc

 

[someth1ng]

Re: HDPE and LDPE

Both are made by the same initiation (radical attacks an ethene molecule), propagation (monomer radical then reacts with more ethene molecules - repeating reaction) and termination (two growing chains react together making a large, stable polymer) steps. The difference is what is used as the initiator/"catalyst" and the conditions.

In LDPE, an organic peroxide initiator is used such as benzoyl peroxide with conditions as high pressure (typically 1000-3000 atm) and high temperatures (300C).
In HDPE, a catalyst is used which is also the initiator (Ziegler-Natta catalyst) containing triethylaluminium and titanium(III) chloride (TiCl3) compound.