One mole of magnesium is heated in a closed vessel in the presence of one mole of chlorine, Cl2 and one mole of bromine Br2. If the solid compound remaining is completely soluble in water and has a mass of 139.5g, what could be deduced?
[Ar: Mg 24; Cl, 35.5; Br,80]
A all of the chlorine has reacted
B all of the bromine has reacted
C the mass of halogen left is 160g
D the solid is the compound MgBrCl
Second question
Water for drinking is treated in the following way
Stage 1 Solid material is removed (filtration)
Stage 2 Unpleasant smells are removed (use of carbon)
Stage 3 Harmful microbes are removed (chlorination)
For stage 2, how does using carbon remove unpleasant smells? I wrote that chlorination removed the smells instead. Haha
Originally posted by bonkysleuth:One mole of magnesium is heated in a closed vessel in the presence of one mole of chlorine, Cl2 and one mole of bromine Br2. If the solid compound remaining is completely soluble in water and has a mass of 139.5g, what could be deduced?
[Ar: Mg 24; Cl, 35.5; Br,80]
A all of the chlorine has reacted
B all of the bromine has reacted
C the mass of halogen left is 160g
D the solid is the compound MgBrCl
I guess D cos the rest all seems wrong...
Originally posted by bonkysleuth:Second question
Water for drinking is treated in the following way
Stage 1 Solid material is removed (filtration)
Stage 2 Unpleasant smells are removed (use of carbon)
Stage 3 Harmful microbes are removed (chlorination)
For stage 2, how does using carbon remove unpleasant smells? I wrote that chlorination removed the smells instead. Haha
Erm whats the question? Activated carbon is used to remove unpleasant smells. that is correct. Cl2 is a disinfectant and it can't remove unpleasant smells.
Originally posted by dkcx:Erm whats the question? Activated carbon is used to remove unpleasant smells. that is correct. Cl2 is a disinfectant and it can't remove unpleasant smells.
Ok. Thanks, I just thought of the activated carbon in the water filter! O, the question just lists the 3 stages of water treatment and I am suppose to choose which process they are under. (which I have specified in brackets () )
And hello to whoever is here, I still have a few more MCQ questions to ask
1. When testing for a chloride ion using silver nitrate, the solution must be acidified with dilute nitric acid. What is the purpose of the nitric acid? (won't type out the whole options, just the one I chose and the one given by the marking scheme. the underlined choice is the one provided by TYS)
A to prevent precipitation of silver carbonate
B to oxidise the chloride ion.
2. Which statement about the ions of the group VII elements is correct
I chose the option of
All the ions contain more electrons than protons. (Of course, they gain e- to form the ions)
Ans gave all the ions have 7 electrons in outer shell (thought this's talking about the atoms not ions)
3. Which gas is evolved when an alkaline solution of NaNO3 is warmed with Al
I chose ammonia (well isnt this test for NO3- ions?) answer is a crappy NO2.
4 These reactions are used in the manufacture of H2SO4
P S+O2 -> SO2
Q 2SO2 + O2 (reversible rxn) -> 2SO3
R SO3 + H2O -> HSO4
Which reactions are speeded up by using a catalyst? (not very sure about this. can someone tell me how to identify if a reaction requires a catalyst or not?)
anyway only reaction B requires a catalyst according to the answer given
Lastly
2 strips of different metal strips are fastened together and immersed in a beaker of HCl acid. The strips are of the same size. After 5 mins, which beaker contains the least amount of Zn ions
Mg and Zn strip
Pb and Zn strip
Fe and Zn strip
Ag and Zn strip
I chose Mg and Zn strip because Mg will react in place of Zn and hence prevent Zn from being oxidised to Zn2+. But the answer is also another crappy and unexplanable Fe and Zn strip.
Not sure if TYS answers these days are reliable. Cuz my teacher said there's one physics (or chemistry, can't remember) TYS MCQ that contains 10 wrong answers. Shocking. Haha. Anyway, I am not very sure of question 1 and 4. The rest of the answers look fine to me.
Whatever the case, please help me check if i got the right choices. Thank you.
Originally posted by bonkysleuth:And hello to whoever is here, I still have a few more MCQ questions to ask
1. When testing for a chloride ion using silver nitrate, the solution must be acidified with dilute nitric acid. What is the purpose of the nitric acid? (won't type out the whole options, just the one I chose and the one given by the marking scheme. the underlined choice is the one provided by TYS)
A to prevent precipitation of silver carbonate
B to oxidise the chloride ion.
2. Which statement about the ions of the group VII elements is correct
I chose the option of
All the ions contain more electrons than protons. (Of course, they gain e- to form the ions)
Ans gave all the ions have 7 electrons in outer shell (thought this's talking about the atoms not ions)
3. Which gas is evolved when an alkaline solution of NaNO3 is warmed with Al
I chose ammonia (well isnt this test for NO3- ions?) answer is a crappy NO2.
4 These reactions are used in the manufacture of H2SO4
P S+O2 -> SO2
Q 2SO2 + O2 (reversible rxn) -> 2SO3
R SO3 + H2O -> HSO4
Which reactions are speeded up by using a catalyst? (not very sure about this. can someone tell me how to identify if a reaction requires a catalyst or not?)
anyway only reaction B requires a catalyst according to the answer given
Lastly
2 strips of different metal strips are fastened together and immersed in a beaker of HCl acid. The strips are of the same size. After 5 mins, which beaker contains the least amount of Zn ions
Mg and Zn strip
Pb and Zn strip
Fe and Zn strip
Ag and Zn strip
I chose Mg and Zn strip because Mg will react in place of Zn and hence prevent Zn from being oxidised to Zn2+. But the answer is also another crappy and unexplanable Fe and Zn strip.
Not sure if TYS answers these days are reliable. Cuz my teacher said there's one physics (or chemistry, can't remember) TYS MCQ that contains 10 wrong answers. Shocking. Haha. Anyway, I am not very sure of question 1 and 4. The rest of the answers look fine to me.
Whatever the case, please help me check if i got the right choices. Thank you.
1) Where do u even get the carbonate ion from?
2) Unsure about this but the answer would be right if its atom as u say.
3) Only NH4 compounds will give off NH3 gas. Just look at the structure of the compound and see what is the only gas possible to be given out from it.
4) B? Q is the 1 that needs a catalyst since its a reversible reaction. If no catalyst is used to increase the rate of the reaction, your entire process is held up in this reaction as part of the SO3 will go back to SO2 and O2.
5) For such experiments, the closer the 2 metals are in the reactivity series, the less is the potential difference between them and thus the slower the reaction.
Points to add :
1) Any carbonate ions that may be present (by design or contamination) would tend to give an insoluble ppt. Hence, acidifying the test solution would protonate the carbonate ion, allowing it to decompose into water and carbon dioxide (which being a gas, would leave the reaction mixture thus as predicted by Le Chatelier's principle, pulls the position of equilibrium significantly over to the gaseous products).
Therefore, any ppt observed would more likely be that of AgCl(s) meaning Ag+(aq) ions are indeed present, rather than the ppe being Ag2CO3(s), which may act as a false positive ppt, fooling pple into thinking "Ag+(aq) is present!" when in reality it may be "that's just CO3 2-(aq) present, no Ag+(aq) actually present".
2) Activated carbon works by adsorbing organic covalent molecules responsible for smells (since ionic compounds are non-volatile). See http://en.wikipedia.org/wiki/Activated_carbon
3) Al is oxidized to Al3+ as NO3- nitrate(V) anion is reduced to NH4+ ammonium cation. O.S. of N decreases from +5 to -3, as it accepts electrons from Al. To balance the reduction half-equation, add water molecules, protons and electrons. What occurs next is a Bronsted-Lowry acid-base reaction in which the proton is transferred from NH4+ to OH- (a stronger base than NH3), forming NH3 and H2O. The NH3(aq) is easily vaporized (warming is sufficient to overcome the hydrogen bonds of aqueous ammonia). Using moist or damp litmus paper to test for its presence, the NH3(g) undergoes hydrolysis to generate OH- ions, in a reaction the exact reverse of what occurred earlier (this time the proton is transferred from water to ammonia; the extent of ionization is slight, but sufficient for what occurs next). The presence of the OH- ions results in a chemical reaction involving compounds in litmus to cause a blue coloration.
4) Some students (particularly 'O' levels) don't realize that a catalyst lowers activation energy for both the forward and backward reactions, thus increasing rate of both forward and backward reactions. However, by controlling temperature (endothermic vs exothermic) and by carring out fractional distillation or other mechanical means of separation, to remove the product species of interest, thereby causing position of equilibrium to continue shifting towards the product side, as predicted by Le Chatelier's principle; catalysts are regardless still a very necessary component of industrial processes, where time = money.
For the Contact Process, see http://en.wikipedia.org/wiki/Contact_process
5) Check out the reduction and oxidation potentials for the various metals for yourself, and reach your own conclusion. The more positive the value, the more position of equilibrium lies to the right. The equations are given as reduction half equations, hence the values given are reduction potentials. To obtain oxidation potentials, reverse the direction of the equation and reverse the sign on the reduction potential. These values hold true for standard conditions and at 298K.
List of Standard Redox Potentials :
http://infinity.usanethosting.com/Tuition/StandardRedoxPotentials.gif
Hmm, understood.
However I still don't get the part on the gas evolved when NaNO3 is warmed with Al. Why NO2? Sorry to say, but I tried reading the explanation for the question numerous times but still dont quite understand.thanks
Originally posted by bonkysleuth:Hmm, understood.
However I still don't get the part on the gas evolved when NaNO3 is warmed with Al. Why NO2? Sorry to say, but I tried reading the explanation for the question numerous times but still dont quite understand.thanks
dude that is A lvl stuff u 'understood ' ?
waaa ?
hmm yea ultimaonline's explaination is 100% chimology for an o level student yet he "understood"! lol. who knows maybe ure looking at a prodigy here....
Originally posted by absol:hmm yea ultimaonline's explaination is 100% chimology for an o level student yet he "understood"! lol. who knows maybe ure looking at a prodigy here....
prodigy wouldnt be asking questions =p
Haha, don't say the O's boys, even when i see UltimaOnline's reply, i also dunno whether he means my answers are correct or wrong since i lazy to analyse his answers.
Contrary to most belief, by the time you go uni, you hardly study the stuff taught in O's or some of the A's content so my deductions are still using what i remember from sec sch...
If uni still studies focus on the knowledge O's teach, i won't have 4 chemistry majors debating on whether phenolthalein is pink in acid or alkaline since we have never touched that indicator for years. Its amazing how many wierd explainations we manage to came up with to explain our guess...
Originally posted by UltimaOnline:Here’s some fun Organic Chemistry for you guys :
By reference to the images provided on this Wikipedia URL ( http://en.wikipedia.org/wiki/Phenolthalein ), draw the mechanisms involved for the 4 structures of the phenolphthalein molecule ( IUPAC name 3,3-bis(4-hydroxyphenyl)isobenzofuran-1(3H)-one ) as the aqueous environment progresses with increasing pH from 0 to 12.
http://en.wikipedia.org/wiki/Phenolthalein
I’ll give some tips to help you along :
(but you should attempt to solve it by yourself first).1 to 2
deprotonation of carboxylic; lone pair on carboxylate attacks trigonal planar carbocation.2 to 3
deprotonation of both phenolic groups; lone pair on phenoxide forms pi bond; pi electrons in ring shifts down through ring to form pi bond with sp3 carbon; sigma bond cleaves to become lone pair on carboxylate.3 to 4
hydroxide nucleophile attacks sp2 carbon outside rings; pi bond outside ring shifts into ring to regenerate the phenoxide.
Mechanism? Thats uni level chemistry right? Next sem when i do my advance organic then i try. Now doing quantum... wanna help? :p
Originally posted by UltimaOnline:Nope ;Þ
Izzit, then y all the JC students in yr 1 all mostly did badly in mechanism? Alot of people all cannot do arrow pushing well.
Originally posted by UltimaOnline:I was replying to your other qn! :Þ
Heh, thats the 1 i wan u say YES!!!!!!
I got quantam midterm in 2 wks, 40% leh and i dunno what the hell to study cos i hardly understand the lecturer from day 1...
Hi guys, curious about the starting question posted by TS too. Why is the answer D? Not just based on gut instincts i mean. Any logical explanatioN?
Originally posted by anpanman:Hi guys, curious about the starting question posted by TS too. Why is the answer D? Not just based on gut instincts i mean. Any logical explanatioN?
Just look at the mass of the compound.
If all Cl has reacted, you should get MgCl2 and the mass is incorrect.
Likewise for b which will get you MgBr2
Option C is just like A meaning all the Cl has reacted since halogen mass of 160g is roughly the mass of 1 mole of Br2.
Option D is the only option that fits the mass of the compound obtained.
Originally posted by dkcx:Just look at the mass of the compound.
If all Cl has reacted, you should get MgCl2 and the mass is incorrect.
Likewise for b which will get you MgBr2
Option C is just like A meaning all the Cl has reacted since halogen mass of 160g is roughly the mass of 1 mole of Br2.
Option D is the only option that fits the mass of the compound obtained.
Ok. So the mass, (referring to option A), do you mean the mass of 139.5 for MgCl2 is incorrect? Likewise for option B(you are referring to MgBr2?)
And for option C, if the mass of halogen left is 160g, like what you said, all the chlorine will have reacted away since 80 + 80 = 160 (gives us the Mr of Br2).
Just paraphrasing to see if i got the correct idea. Thanks!
Originally posted by anpanman:
Ok. So the mass, (referring to option A), do you mean the mass of 139.5 for MgCl2 is incorrect? Likewise for option B(you are referring to MgBr2?)And for option C, if the mass of halogen left is 160g, like what you said, all the chlorine will have reacted away since 80 + 80 = 160 (gives us the Mr of Br2).
Just paraphrasing to see if i got the correct idea. Thanks!
yeah
Paraphrasing... thats what i learn for counselling...
