2017 H2 Chemistry JC1 & 2 students post your questions here

last SG · 2016-12-14T03:15:46+00:00

http://www.bedokfunlandjc.com ------------------------------------------------------------------------------This thread is open to all 2017 Singapore JC stud...

UltimaOnline SG

Seeing how you've been asking me A level Chem qns since 5 years back, you're a private tutor or school teacher, not a student, correct?

Why is it that for a Daniel Cell, why do we need a porous pot to keep the solution separated? What is the purpose for the separation of solutions? Like wise for a typical galvanic cell where one beaker contains Zn metal and ZnSO4 while the other contains CuSo4 and Cu metal, connected by a salt bridge. Would it not have been better for the zinc and copper metal to be dipped into the same conducting electrolyte (say H2SO4), instead of two separate solution, to generate electricity?

If the solutions mix, there is no concern about the unbalanced charge built-up?

A Daniell cell is of course, a specific type of Galvanic-Voltaic cell with the following setup :

https://en.wikipedia.org/wiki/Daniell_cell

If, as you suggest, "the zinc and copper metal to be dipped into the same conducting electrolyte (say H2SO4), instead of two separate solution", then the electrons would only transfer in the redox reaction that would occur only between the electrolyte H+ reduced and the Zn oxidized, and no electrons would flow in the external wire, in which case the cell would have failed in it's objective to do work (eg. operate a light bulb).

H4x0ru5 SG

Seeing how you've been asking me A level Chem qns since 5 years back, you're a private tutor or school teacher, not a student, correct?

A Daniel cell is of course, a specific type of Galvanic-Voltaic cell with the following setup :

https://en.wikipedia.org/wiki/Daniell_cell

Ok if that's the case, what about changing the electrolyte to aq NaCl? So now I have two different metals dipped into the same beaker containing aq NaCl. Do I need a porous pot/any concerns of charge built up? In fact, by taking away the salt bridge/porous pot, and changing the electrolyte from aqueous salt of the metal to aq NaCl, is it still a Daniel Cell?

UltimaOnline SG

If NaCl(aq) is used, no redox reaction occurs, not even between the electrolyte and Zn electrode, as would have occurred if an acid was used. Either way, no electrons would flow in the external wire, so the setup would still be a failure.

You would then not need a porous pot, nor would you have any concerns of a charge build up, because no redox reaction would occur, as the setup would fail.

It would no longer be a Daniell cell, or any type of Galvanic-Voltaic cell. It would be a failure.

H4x0ru5 SG

You would then not need a porous pot, nor would you have any concerns of a charge build up, because no redox reaction would occur, as the setup would fail.

It would no longer be a Daniell cell, or any type of Galvanic-Voltaic cell. It would be a failure.

Why would the set up fail then? This set up is always seen in typical O/A level guidebooks.

UltimaOnline SG

Why would the set up fail then? This set up is always seen in typical O/A level guidebooks.

Understandable, you tutor both O and A levels.

At O levels, the diagram of the setup is often simplified, because O level students are not expected to understand why or how a salt bridge is necessary.

At A levels, the student must draw a proper setup separating the cathode and anode (to force electrons to flow in the external wire), and a salt bridge is necessary to allow the electrons to flow.

That's why your previously suggested setup (which doesn't separate cathode and anode) fails.

glitter58 SG

for reaction between conc. sulfuric acid and hcl, will the salt formed be na2so4 or nahso4? which reaction is more widely used?

glitter58 SG

also, if the qn part iii states "using the answers in part i and ii" do i use the 3sf answer or the 5sf intermediate working for calculation in qn part iii?

UltimaOnline SG

for reaction between conc. sulfuric acid and hcl, will the salt formed be na2so4 or nahso4? which reaction is more widely used?

Depends on which reactant is in excess. For Group 17 purposes, use NaHSO4. In other contexts, see which equation is more relevant to the question and/or final products.

UltimaOnline SG

also, if the qn part iii states "using the answers in part i and ii" do i use the 3sf answer or the 5sf intermediate working for calculation in qn part iii?

For A level Chemistry purposes, Cambridge will accept either. No worries.

UltimaOnline SG

Why would the set up fail then? This set up is always seen in typical O/A level guidebooks.

Also, it's likely that some setups you're referring to, do not need salt bridges or separate compartments, because they're not Galvanic-Voltaic cells, but are electrolytic cells.

O levels test mainly on the latter, while A levels test equally on the former and the latter both.

glitter58 SG

why doesnt fluorine follow the trend of decreasing bond energy from cl2 to br2 to i2?

glitter58 SG

when to use the term "delocalised electrons" and "mobile electrons"? what is the difference between these two terms?

UltimaOnline SG

why doesnt fluorine follow the trend of decreasing bond energy from cl2 to br2 to i2?

This anomaly is one of those interesting tricks (I have an entire BedokFunland JC collection of such fascinating Chemistry anomalies) that I usually only share with and reveal the answer to my own BedokFunland JC students, and I won't spoonfeed these answers here. But you should be able to find the answer for yourself quite easily with Google.

Cambridge is unlikely to ask A level students this question, but if they do, they'll phrase it as, "Suggest a reason for this anomalous trend" to function as a distinction-type question to separate the A graders from the rest. While one can of course easily google out the answer, the point is students don't have access to the internet during the exam paper, which is where exploring beyond the basic syllabus throughout the year will help prepare determined students to get A grade.

UltimaOnline SG

when to use the term "delocalised electrons" and "mobile electrons"? what is the difference between these two terms?

As with many other such terminology pairs (eg. heat vs reflux, energy vs enthalpy, shielding vs screening, oxidation state vs oxidation number, resonance vs mesomeric, etc), in most cases they can be used interchangeably and Cambridge will generally accept either term, except in specific contexts where one term is clearly more appropriate than the other.

To explain why molten or aqueous ionic compounds and metals conduct electricity, it's better to use "mobile". To describe the resonance effect, eg. why amides are much less basic than amines, it's compulsory to use "delocalized".

UltimaOnline SG

http://mothership.sg/2017/04/we-found-that-colourful-talad-drink-in-toa-payoh-so-you-dont-have-to-squeeze-at-artbox/

Tuk Tuk Cha now selling colour-changing gradient drinks too

http://mothership.sg/2017/05/tuk-tuk-cha-now-selling-colour-changing-butterfly-pea-gradient-drink-too/

UltimaOnline SG

It’s all Chemistry, yo. Different colours reflect different pH values in the different regions of the drink in the cup, different degrees of protonation vs deprotonation, different molarities of conjugate acids vs conjugate bases, different extents of molecular conjugation or overlap of p-orbitals with delocalized electrons, different magnitudes of energy differences between lower and higher molecular orbitals, different wavelengths of visible light absorbed when the electrons transit. It’s all Chemistry, yo. The most important subject at A levels.

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