Reef Chemistry Question of the Day #128 Elements in Polyethylene

Randy Holmes-Farley

Reef Chemist
View Badges
Joined
Sep 5, 2014
Messages
66,555
Reaction score
62,861
Location
Arlington, Massachusetts, United States
Rating - 0%
0   0   0
Reef Chemistry Question of the Day [HASHTAG]#128[/HASHTAG]

Polyethylene is widely used by reef aquarists for many things. It is a good choice as it is nontoxic, resistant to most chemicals we use, and can withstand the temperatures we are interested in.

For example, I use it in the form of Brute trash cans for sumps, limewater storage, RO/DI storage, new salt water storage, and refugia.

What elements are needed to comprise more than 99% of the weight of polyethylene used in objects such as trash cans?

A. C, H and O
B. C, H and N
C. C, H, N, and O
D. C and H
E. C and N
F. C and O
G. C only

Bonus:

What elements are needed to comprise more than 85% of the weight of polyethylene used in objects such as trash cans?

A. C, H and O
B. C, H and N
C. C, H, N, and O
D. C and H
E. C and N
F. C and O
G. C only

Good luck!























.
 

JimWelsh

Valuable Member
View Badges
Joined
Nov 5, 2011
Messages
1,547
Reaction score
1,679
Location
Angwin, CA
Rating - 0%
0   0   0
Ethylene = C2H4. Polyethylene = (C2H4)n, where n is some positive integer. Answer is D: C and H. The atomic mass of carbon is 12.0107, and that of hydrogen is 1.00794, so carbon is 12.0107/(12.0107+2*1.00794) = 85.6% of the weight of polyethylene. Bonus answer is G: C Only.
 

Isoprene

Valuable Member
View Badges
Joined
Feb 28, 2012
Messages
1,032
Reaction score
121
Location
Tallahassee, FL
Rating - 0%
0   0   0
I think? Just tryin to help. Love your chem articles randy, best out there, you are one of the reasons I dove so deep into this hobby. Brought my love for chemistry into what I consider the husbandry of the most beautiful animals the planet has to offer. Sincere thanks for that.
 

Isoprene

Valuable Member
View Badges
Joined
Feb 28, 2012
Messages
1,032
Reaction score
121
Location
Tallahassee, FL
Rating - 0%
0   0   0
True, its answer D and the bonus is answer G. The base unit of the polymer or monomer is C2H4 carbon being 24.02 g/mol and hydrogen 4.04 g/mol for a total molecular weight of 28.06 g/mol taking 24.02g/mol and dividing by the total molecular weight provides approx 85% of the monomer molecular weight making up the crystalline structure of whatever material being produced.
 
OP
OP
Randy Holmes-Farley

Randy Holmes-Farley

Reef Chemist
View Badges
Joined
Sep 5, 2014
Messages
66,555
Reaction score
62,861
Location
Arlington, Massachusetts, United States
Rating - 0%
0   0   0
And the answer is...D and G, as folks have correctly noted.

Ethylene is CH2=CH2, which is a gas. To make polyethylene you attach very large numbers of them together in a long chain. In the polymerization the double bond between the carbon atoms in ethylene breaks down to a single bond and new bonds are formed to a second ethylene and the chain grows.

Eventually the polymer becomes ...-CH2-CH2-CH2-CH2-...

So the main components are carbon and hydrogen.

The only other elements that could end up actually in a polyethylene molecule would be those that might be attached to the start of the polymer, or at the end. What those are will depend on how you make the polyethylene, but in the normal types of polyethylene used to make something like a Brut trash can, those end groups are a very tiny fraction of the total molecule, which has thousands or even hundreds of thousands of CH2 groups.
 

Mastering the art of locking and unlocking water pathways: What type of valves do you have on your aquarium plumbing?

  • Ball valves.

    Votes: 73 51.8%
  • Gate valves.

    Votes: 72 51.1%
  • Check valves.

    Votes: 36 25.5%
  • None.

    Votes: 31 22.0%
  • Other.

    Votes: 9 6.4%

New Posts

Back
Top