2011 33c) HSC (1 Viewer)

[IR]

Hey guys,
Problem is, I was unsure about 33c) in:

Paper: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2011exams/pdf_doc/2011-hsc-exam-chemistry.pdf

Marking Guidelines: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2011exams/pdf_doc/chemistry-marking-guide-11.pdf

So I was wondering why the marking guidelines doesn't consider Cl at anode. The litmus paper has changed colour to red from blue so why isn't it Chlorine cos Oxygen cant decolourise. I do realise that its low conc but the litmus paper has changed colour so how does that work?

 

[fatima96]

Cl isn't considered at the anode since the solution is 0.05M concentration, making it very dilute. Thus, at the anode, instead of CL being oxidised, water is oxidised because it is more abundant.

ANODE: 2H2O ----> o2 + 4H+ +4e-

Since H+ is produced, it makes the solution acidic thus causing the litmus paper to turn red. The bubbles were because of the Oxygen gas being produced, as can be seen in the above oxidation eqn.

At the cathode, reduction of water again takes place, since we learnt that it is a dilute NaCl soln (the electrolysis of dilute NaCl is simply the electrolysis of water)

2H2O + 2e- ----> H2 + 2OH-


Since OH- ions are produced, the environment at the cathode is basic, causing litmus to turn blue, bubbles observed was H2 gas being formed, as can be seen in eqn.


Hope this helped, rep me if it did