dilute and concentrated products of electrolysis (1 Viewer)

[mr EaZy]

im gonna check this out soon, but to be sure,
can anyone describe the differences in using diluted / concentrated brine in electrolysis.

thnx

 

[mitochondria]

I strongly suggest you to "check it out" frist Research is an important part in learning and you actually learn more if you do the work before you have attempt to solve it.

Hint: Electrolysis occurs when an external potential difference is applied (usually the reaction is being forced to occur in the reverse direction), V=IR, suppose R is relatively constant, then I is proportional to V. I = q/t, so the larger the V the larger the I and more electrons are produced at the cathode. Now think about the use of brine in an electrolytic cell and then how will the amount of mobile charged particles in the solution will affect the process Good luck!

 

[mr EaZy]

i asked the question so that i could come back here when someone has replied.
what i have is this:

by using electrolytic cells, the non spontaneous reaction is forced to proceed. i dont see what u were getting at with the equation V=IR, maybe its too complicated.

the nacl in low concentrations acts like an electrolyte. in this case, only water would be
reduced at the cathode: 2H20 +2e- ------> h2 (g) + 2(OH)- IS this right?

hold on this is only for nacl in high concentr4ations. this is how we get NaOH

im pretty sure nacl in low concentrations will produce Na0Cl-.
ill have to check up on my equations

 

[mr EaZy]

in the mean time, can u explain if this is true:

all electrolytic reactions are re4versible coz both the spontaneous and non spontaneous reactions are equally able to proceed.

this is what im getting because some textbooks and sources on bos uses a reversible sign in all theier equations

 

[nesstar]

i think the difference is that concentrated brine will result in production of Na whereas dilute brine results in production of NaOH. i think. it relates to what u were getting at b4 where at low conc the water is reduced in preference to Na...

 

[mervvyn]

i think the difference is that concentrated brine will result in production of Na whereas dilute brine results in production of NaOH. i think. it relates to what u were getting at b4 where at low conc the water is reduced in preference to Na...

I disagree - electrolysis of molten NaCl produces Cl2 gas and Na metal. Electrolysis of concentrated NaCl solution produces Cl2 gas and OH- ions. I think the difference is that in a dilute solution of NaCl, the Na and Cl ions act more as an electrolyte rather than a reactant, and so H2 gas is produced, and O2 gas instead of Cl2 gas by oxidising OH- ions to water and O2.

 

[nesstar]

my bad - i was getting confused with molten...
we don't need to know the difference between conc and dil brine, do we?

 

[mervvyn]

Strictly speaking, no - it's not mentioned in the syllabus
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/pdf_doc/chemistry_stg6_syl_03.pdf
but that said, it would be worth knowing that if the brine is too dilute, O2 will be formed, it might be a problem you need to identify with an experiment or something like that.

 

[nesstar]

ok...well, i did some further research too...so now i can see where you're coming from...
these are the equations if anyone still wanted them:

Dilute NaCl solution
Cathode: 2H2O + 2e- → H2 + 2OH-
Anode: 2H2O → O2 + 4e- + 4H+
Net ionic: 4H2O + 2e- → H2 + O2 + 2OH- + 4e- + 4H+

Concentrated NaCl solution
Cathode: 2H2O + 2e- → H2 + 2OH-
Anode: 2Cl- → Cl2 + 2e-
Net: 2H2O + 2Cl- + 2e-→ H2 + 2OH- + Cl2 + 2e-

 

[Riz]

the syllabus actually says
"to predict and explain the different products of the electrolysis of aqueous and molten sodium chloride"
so we need to know this, but basicially its just wat mervvyn said...with dilute brine, O2 is produced but with concentrated brine, Cl2 is produced. but why??
oh can someone explain to me the difference between aqueous and molten NaCl?
(sorry, i m hopeless in chem)

 

[nesstar]

Molten NaCl will result in the reduction of Na+ ions to Na. This Na would need to be mixed with water in order to produce NaOH. Aqueous NaCl results in the reduction of water instead of Na ions, resulting in formation of OH- ions. Theses ions then combine with Na ions to form NaOH.
aqueous --> NaOH formation; molten --> Na formation

 

[mervvyn]

I think this is what you were asking:
Aqueous NaCl is a solution of NaCl dissolved in water - dilute doesn't have much NaCl, concentrated has a fair bit, and saturated can't dissolve any more.
Molten NaCl is solid NaCl that has been heated until it is a liquid, but there is no water involved.