Chemistry 3/4 2013 Thread

[psyxwar]

How exactly is a "soluble" compound classified, considering all salts dissolve to some extent in water? Do we only consider salts that dissociate completely (or near to completion) soluble, or is there a certain Ksp threshold at say 298K?

[lzxnl]

I wouldn't try putting a threshold on "soluble". As you've hinted, solubility is a continuum that, as you've correctly pointed out, varies with temperature. There are some no-brainers, like most phosphate salts whose log(pKsp) values may be greater than 1. Then there are salts like calcium hydroxide which are sparingly soluble. It won't really matter for VCE. In VCE, things are either soluble or they're not. We don't need to worry about the solubility of sodium hydroxide decreasing with temperature, for instance. Keep things simple.

[Bad Student]

The activation energy for the reaction A + B ---> C is greater than the activation energy for the reverse (opposite) reaction C -----> A + B. Is the reaction A + B ----> C exothermic or endothermic explain?

[psyxwar]

The activation energy for the reaction A + B ---> C is greater than the activation energy for the reverse (opposite) reaction C -----> A + B. Is the reaction A + B ----> C exothermic or endothermic explain?

Well, this means that it takes more energy for A+B to reach the transition state than C, meaning that A+B-->C must be endothermic (as the enthalpy of C is greater than the enthalpy of A and B combined).

I wouldn't try putting a threshold on "soluble". As you've hinted, solubility is a continuum that, as you've correctly pointed out, varies with temperature. There are some no-brainers, like most phosphate salts whose log(pKsp) values may be greater than 1. Then there are salts like calcium hydroxide which are sparingly soluble. It won't really matter for VCE. In VCE, things are either soluble or they're not. We don't need to worry about the solubility of sodium hydroxide decreasing with temperature, for instance. Keep things simple.

Thanks.

[lzxnl]

Draw out a reaction energy profile diagram and compare the activation energies of an exothermic and endothermic reaction.

[Alwin]

Draw out a reaction energy profile diagram and compare the activation energies of an exothermic and endothermic reaction.

my paint skills haha:

[lolipopper]

i have a question. how do you select a suitable electrolyte for a galvanic cell?

eg.
Q21. An experimental fuel cell that uses methanol as the fuel has the half equations:
                      CH3OH(g) + H2O(l) + 6OH–(aq)  --->  CO2(g) + 6H2O(l) + 6e–
                                   O2(g) + 2H2O(1) + 4e–  --->  4OH–(aq)
a   Write the equation for the overall cell reaction.
b   Which reaction occurs at the positive electrode of the cell?
c   Suggest a suitable electrolyte for the cell.

[teletubbies_95]

positive electrode= anode= oxidisation occurs
c) electrolyte has to be an ion conductor,right? So the electrolyte is the hydroxide ion, and as the oxidisation reaction is a backward reaction , there isnt an electrode , so it would require an inert electrode like graphite. Hope that helps

[lolipopper]

positive electrode= anode= oxidisation occurs

umm. isnt the positive electrode = cathode?
also the answer at the back is KOH or NaOH

[teletubbies_95]

Sorry! Yes , the positive electrode is the cathode.
Yep! So the electrolyte has to be an ionic compound which has a  hydroxide anion!
Hope that helps!

[lolipopper]

another question guys:

Two platinum electrodes were placed in 1.00L of a 0.207 M lead (II) nitrate solution. An electric current was passed through the solution until the concentration of lead ions in the solution was 0.201 M. If the volume of the solution remained unchanged, the amount of electricity that passed through the cell was:

A) 290 C
B) 579 C
C) 1160 C
D) 38800 C

i got D. but the answer is C. why?

[thushan]

1) work out how much Pb is deposited on cathode
2) work out how many electrons passed into cathode
3) work out charge passed to cathode

does this help?

[lolipopper]

1) work out how much Pb is deposited on cathode
2) work out how many electrons passed into cathode
3) work out charge passed to cathode

does this help?

thank you. lol it was the first step i was getting wrong.

[chemdeath]

I always manage to stuff up these simple questions  can some please clarify how to find concentration of
sodium thiosulfate in solution (m) in:

10ml Na2S2O3, 35ml water and 5m,l HCl mixed together

Thanks

[Homer]

I always manage to stuff up these simple questions  can some please clarify how to find concentration of
sodium thiosulfate in solution (m) in:

10ml Na2S2O3, 35ml water and 5m,l HCl mixed together

Thanks

don't we need the moles too, to calculate the concentration? I shall just wait for someone else to answer :s