bromine water-br2(aq)? HOBr? Question on prac (1 Viewer)

[kooltrainer]

for the experiment with bromine water and cyclohexene.. my tutor wrote :
C6H10 + Br2 (aq) --> C6 H10 Br2

but one thing i dun get is that my other teacher wrote
C6H10 + HOBr --> ...
and they're both called bromine water? whats the difference?
From the experiment that i did, the resultant has 2 layers; water and 1,2-dibromocyclohexane . how come the first equation doesnt have water??

also, if there are two layers formed, which layer is water? (top or bottom?)

 

[Mattamz]

HOBr is Hypobromous acid. I agree with your tutors equation.

Also the first equation does have water:

C6H10 + Br2 <b>(aq)</b> --> C6 H10 Br2

Ill take a stab in saying that the top layer is 1,2-dibromocyclohexane since it would have a density lower than water.

 

[twilight1412]

the way i remember which one is less dense is by looking at how the molecule looks =)

cyclohexane has a hell of a lot more atoms so for the same mass there would be less cylcohexane meaning its less dense =)

so basically i think a general rule SHOULD be

bigger the molecule the less dense it is =)

there are exceptions of course but i think it applies to most things

 

[kooltrainer]

yehh i knew water was on bottom but teacher said waters on top.. she saw the bottom of test tube decolourises and she knows that the bromine moves towards cyclohexene rather than the water. From this observation, she says hexane is on bottom and water is on top..
Also, why does high molar mass = less density??

 

[kooltrainer]

hows difference between HOBr and Br2 (aq) :S which to use in particular case??
im stuck.. =/

 

[jah_lu]

it doesnt matter which one you use, both will be accepted in the hsc
bromine water is actually an equilibrium between Br2 and HOBr
Br2+H20-->HBr+HOBr (dunno how to do the equilibrium sign)
Br2 is brown, Hobr is colourless
it is actually the HOBr that reacts with the alkene, therefore reducing the concentration of HOBr.
By Le Chatelier's principle, the equilibrium will shift to the right to counteract the change and produce more HOBr
this will in turn decrease the concentration of Br2-thus alkenes decolourise bromine water from brown to colourless (as concentration of brown br2 is decreasing)

however, i'd strongly recommend using Br2, even if it is incorrect as it is accepted and it also simplifies explanations, formulas, equations, diagrams etc
its a real hassle to explain what happens if it reacts with HOBr as you can see from above
if you just say it reacts with Br2, you can say Br2 was brown, reacted with alkene, less Br2 left, so becomes colourless
much much much easier