Friday, September 7, 2007

Chemistry Questions

From the scan in the attachment,- How do you answer question 20? How do you know how the different indicators will behave when they are mixed together?-
What's the correct answer for Q21?XXXX22. Some potassium carbonate solution is added to a sample of tap water. The mixture then appears cloudy. Which of the following ions is probably present in the sample?A) Bromide ions B) Magnesium ions C) Ammonium ions D) Sulphate ionsYea, I have no clue how to do this questionXXXX-
What is observed when hydrogen sulphide gas is reacted with iron(III) chloride solution?XXXX-
What happens when iron is heated with sodium oxide?* Hmm, for this question, isn't there like no reaction. Cause iron is less reactive than sodium right, so it won't displace sodium & won't react? Evidently I'm wrong cause it's a 5 mark question though. How?


lijun(: said...

hey.. just checking if these are correct.

Q20 D

cos i don't think the indicators actually react with one another. and at pH5, both methyl orange & bromothymol blue are yellow, and phenolphthalein is colourless.

Q21 B

chlorine displace iodide ions.
ionic eqn: Cl2 + 2I- -> I2 + Cl2
so the solution will turn from yellowish-green to reddish brown.

Q22 B

cos MgCO3 is insoluble. other compounds such as KBr, NH4CO3 & K2SO4 are soluble.


lijun(: said...

What is observed when hydrogen sulphide gas is reacted with iron(III) chloride solution?

H2S(g) + 2FeCl3(aq)-> 2FeCl2(aq) + 2HCl(aq) + S(s)

so light yellow deposits will be seen. also, the reddish-brown solution changes colour to become a green solution.

What happens when iron is heated with sodium oxide?

actually, i think you're right that there's no reaction. but since it's a 5m question, i expect they want to to explain why there isn't a reaction.

for example, sodium is more higher in the reactivity series, it has a greater tendency to form its positive ions. Iron has less tendency to form iron(II) ions, hence cannot displace sodium from the oxide.

btw. is it correct, that sodium oxide is a very stable compound, as there's a stronger attraction of the nucleus on the electron(as valance electron quite near to nucleus)

please do point out my mistakes if i have any misconceptions.

lien said...

HAHAHAHAHA. IM a ALEVEL student! not O! LOL. Anyway, nice! Wish there was this blog 2 years ago. =). Any tips for a alevel student?

alwaysLovely said...

Q21) Answer is Yellow color. At ph5, Phenolphthalein will remain colorless, Methyl orange will be yellow and bromothymol blue will be yellow too. As such, with equal volume, the overal color will be yellow in color for the solution.

Q22) See below for all 4 choices and answer through elimination:
A) Sodium carbonate will be formed from carbon dioxide reacting with NaOH, which is soluble in water. As such, no Visible Change will be observed,
B) Chlorine gas will react with Potassium Iodide (Colorless solution) to form Iodine (Brown) and Potassium Chloride. In this reaction, Iodide ions are oxidized to form Iodine gas. Visible color change from colorless to Brownish solution.
C) Ammonia gas reacts with Hydrochloric acid to form ammonium salt, in this one -NH4Cl, which is soluble in water due to their ionic nature, As such, no visible change.
D) No known reaction mechanism between SO2 and Iron (II) Sulphate.
Thus answer should be (B).

Q23) For Potassium Carbonate solution reacting with Sample of Tap Water. Question is testing you on the ions that is present in the tap water.
Potassium carbonate are soluble in water and thus looks colorless, as in all solutions.
Do elimination again:
Bromide ions -> formed KBr which is soluble -> no visible change
Magnesium ions -> formed MgCO3 which is insoluble in water - Visible change with precipitation
Ammonium ions - > formed NH4CO3 which is soluble in water -> no visible change
Sulphate ions -> formed K2SO4 which is soluble in water -> no visible change
Thus answer should be (B).

Q24) For H2S reacting with FeCl3 solution,
It involves the reduction of Iron(III)Chloride with Hydrogen sulfide.
It reduces the yellow acidified Iron(III)Chloride to green Fe2+ with the precipitation of Sulfur.

Q25) On this question: Iron + Sodium Oxide, do you have more information as in the reaction paramaters, conditions, etc?? Bcos some reactions will occur in controlled high temp, pressure conditions.
Do let me know more details. Theoretically, at room temp and without any additives - i do not see them reacting.

Helen said...

Hey .... I have got my O level exams after tomorrow... and I am really worried coz em really bad in chemistry

Anyway ... I have got a question related to Q 22.
Always Lovely ...u said that ammonium chloride is formed by ammonia reacting with hydrochloric acid .. and that it dissolves... so no visible change

but doesn't ammonium chloride form in dense white fumes? I mean one of the means of identifying ammonia is by reacting it with hydrochloric acid and checking for the dense white fumes? that's quite a visible change ... why can't this be the answer?