O level Chemistry Questions

• dkcx

  20 Oct 09, 22:28

  Originally posted by qdtimes2:

  oh i see...thanks everyone

  the formula of aqueous ammonia is NH4OH right?

  but my teacher says that in the chemical equation where the question states only aqueous ammonia, we write only NH3 (aq).

  Normally we will call NH4OH ammonium hydroxide

• UltimaOnline

  21 Oct 09, 00:21

  Ammonia has the capacity to act as a Bronsted-Lowry base, due to the availability of a lone pair on N to accept a proton, for instance, from water solvent, thereby generating the hydroxide ion; hence where the discriminating student decides the question requires the involvement of the hydroxide ion (where "aqueous ammonia" is stated"), the student should first write the equation for the hydrolysis of the ammonia :
  
  NH3 (aq) + H2O (l) (insert equilibrium double half arrow here) NH4+ (aq) + OH- (aq) or simply NH4OH (aq)
  
  Subsequently, the remaining required equation(s), eg for the formation of a blue ppt when aqueous ammonia is added to an aqueous solution containing Cu2+ ions :
  
  Cu2+ (aq) + OH- (aq) (insert equilibrium double half arrow here) Cu(OH)2 (s)
  
  and in excess aqueous ammonia :
  NH3 (aq) + [Cu(H2O)6]2+ (aq) (insert equilibrium double half arrow here) [Cu(NH3)4(H2O)2]2+ (aq)
  
  Furthermore, there are instances, in which the student may optionally write the equation in which ammonia is directly the Bronsted-Lowry base, OR the equation in which ammonia first undergoes hydrolysis to generate a separate Bronsted-Lowry base - the hydroxide ion, which accepts the proton from the Bronsted-Lowry acid; as an example, both of the following are acceptable at 'O' levels and 'A' levels :
  
  Either : NH3 (aq) + HCl (aq) (insert equation arrow here) NH4Cl (aq) Or :
  NH4OH (aq) + HCl (aq) (insert equation arrow here) NH4Cl (aq) + H2O (l)
  
  Technically though, the former is more accurate. The reason being, because the hydroxide ion OH- is a stronger base than ammonia NH3 (or put in another way, NH4+ is a stronger acid than H2O), the position of equilibrium lies strongly to the left :
  
  NH3 (aq) + H2O (l) (insert equilibrium double half arrow here) NH4+ (aq) + OH- (aq)
• yiha093

  21 Oct 09, 18:22

  Originally posted by qdtimes2:

  oh i see...thanks everyone

  the formula of aqueous ammonia is NH4OH right?

  but my teacher says that in the chemical equation where the question states only aqueous ammonia, we write only NH3 (aq).

  hi, for me i will write nh4oh.

  because it is aqueous and it is

  nh4+ oh -

   

• ViewtyKU990

  21 Oct 09, 20:29

  i heard from my teacher that we do not write NH4OH cause it 'does not exist' as it exists as its ion in NH4+ and OH- rather than a compound of ammonium hydroxide.

  I aint very sure about that

• dkcx

  21 Oct 09, 20:35

  Originally posted by ViewtyKU990:

  i heard from my teacher that we do not write NH4OH cause it 'does not exist' as it exists as its ion in NH4+ and OH- rather than a compound of ammonium hydroxide.

  I aint very sure about that

  If u wan say it that way, there are quite a number of compounds that cannot be written as a compound since they only exist in ionic form. However for O's its not really that important but i do not know for A's. In uni,

• ViewtyKU990

  21 Oct 09, 21:09

  i also o this year, my teacher very 'niao' about this small things de. lol

• dkcx

  21 Oct 09, 21:12

  Originally posted by ViewtyKU990:

  i also o this year, my teacher very 'niao' about this small things de. lol

  Don't worry, ur teacher not marking O's paper.

• ViewtyKU990

  21 Oct 09, 21:14

  just hope cambridge marker not that niao lo. Now i struggling over English and Combined Humanities

• dkcx

  21 Oct 09, 21:16

  Originally posted by ViewtyKU990:

  just hope cambridge marker not that niao lo. Now i struggling over English and Combined Humanities

  Even if they so niao, as long as whole singapore most ppl got it wrong, the moderation will still make it fair for everyone

• ViewtyKU990

  21 Oct 09, 21:45

  yeah the bell shape curve, but this will be kind of question to allow cambridge to recognise the best of the best 

• dkcx

  21 Oct 09, 21:48

  Originally posted by ViewtyKU990:

  yeah the bell shape curve, but this will be kind of question to allow cambridge to recognise the best of the best 

  Since its not mentioned in the syllabus, i don't think its an essential knowledge.

  UltimaOnline, your thoughts?

• UltimaOnline

  21 Oct 09, 23:08

  Yes, 'O' level students should be able to give a reasonable answer to this question, which to be fair, already comes with hints. When oxygen is blown over the molten iron containing carbon impurities, these carbon impurities are combusted (and combustion is obviously exothermic) away in the form of carbon dioxide. This is a reasonable deduction 'O' level students should be able to make. It's not 'essential knowledge' in the sense that it's found in all 'O' level textbooks or 'O' level notes, and 'O' level students must memorize about it; because much (or ideally, all) of chemistry, including this question, is all about (chemistry-type) common sense and reasonable deductions, not blind memorization.