Chemistry Question HSC 2007 (1 Viewer)

[Shoom]

Question 20, what is the correct answer.

http://www.boardofstudies.nsw.edu.au/hsc_exams/exam-papers-2007/pdf_doc/chemistry_07.pdf

Question 17, do we have to remember the solubility table is this recovered in the HSC?

 

[shanks27]

For Q20 the anode is PbO2 going ot PbSO4 and full equation follows pretty easily, jst take out the both 2e- and combine the two equations

and a benifit is that it means you don't have to replace them often, they last as long as other car parts, less Pb waste as less batteries used

For Q17 you have to know the baisc solubility rules for Pb, Cu, Ba, Ca, Fe2+, Fe3+, Cl, SO4, PO4, CO3 as far as i have been told by all my school teachers

hope that helps

 

[minijumbuk]

Sometimes they give you solubility table, sometimes they don't. It's best to remember it.
I think it's a syllabus dot-point anyway. It's really not too hard to remember.

20.a)
Anode is the electrode where oxidation takes place. Oxidation means loss of electrons.
Thus, anode is Pb(s).
To write an overall equation, just write out everything on the left of each equations, then the same for the right. Cross out anything that exists on both left and right side.
Pb(s) + SO4 2-(aq) + PbO2(s) + SO4 2-(aq) + 4H+(aq) + 2e --> PbSO4(s) + 2e + PbSO4(s) + 2H2O(l)
Therefore: Pb(s) + 2SO4 2-(aq) + PbO2(s) + 4H+(aq)--> 2PbSO4(s)+ 2H2O(l)

b) Since lead acid batteries can be recharged simply by adding more H2SO4, the battery can last several years. The ability to be recharged is seen as a benefit because it eliminates the need of frequent replacement of the battery over time. This means that it's more convenient for vehicle owners, and also eases economical stress on the production of more batteries.

 

[shanks27]

yeh sorry that was my bad, jst realised as i read over my post, good sign for the HSC :s

 

[Shoom]

Thats cool minijumbuk, how to write the overall equation.

Do you have any other easy tips for things related to chem?

Thanks for the help everyone.

 

[minijumbuk]

O___________O

I'm REALLY bad at answering half-existant questions, sorry xD

 

[Shoom]

I know the first topic we will be doing is Acidic Environment, so I still have holidays do you think I should make notes on the production of materials or acidic environment?

THe reason I ask is because theirs no point in me being taught the same thing twice ( I do acidic environment on the holidays and ill do it again term1, so if I start production of materials now I will be ahead of the class)

 

[Shoom]

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf

Question 17 b please.

 

[tommykins]

J (joules required) = 200*4.18*24

Molar heat of combustion/Joules = moles of ethanol

mass of ethanol = moles of ethanol*molar mass of ethanol.

 

[minijumbuk]

ΔH = mCΔT
= 200 x 4.18 x (45-21)
= 20064 J

So if only 20064 kJ of heat is required, and 1 mole of ethanol gives 1367000 J, then we need 1/1367000 x 20064 = 0.0147 mol

So mass required = 0.0147 x (2x12 + 6x1 + 16)
= 0.675 g

 

[Shoom]

Woot!

Thanks

Is Molar Heat of Combustion revisted in the HSC course at all?

 

[danz90]

Woot!

Thanks

Is Molar Heat of Combustion revisted in the HSC course at all?

yeah, its in production of materials.

i hope we get a heat of combustion question in short answers this year.. its easy marks... its likely since they didn't have it last year.

edit: actually, there was... although it was more theory based around alkanols etc. hopefully this year we get a short answer question on the first hand investigation of molar heat of combustion (ie validity, reliability, procedure, calculations) etc.

I also hope this year's paper has a lot of calculations

 

[danz90]

b) Since lead acid batteries can be recharged simply by adding more H2SO4, the battery can last several years. The ability to be recharged is seen as a benefit because it eliminates the need of frequent replacement of the battery over time. This means that it's more convenient for vehicle owners, and also eases economical stress on the production of more batteries.

But wouldn't the Pb(s) waste away since it is the reductant and is oxidised... hence will form PbSO4(s).. and so wouldn't you have to also replace the Lead anodes?

 

[minijumbuk]

But wouldn't the Pb(s) waste away since it is the reductant and is oxidised... hence will form PbSO4(s).. and so wouldn't you have to also replace the Lead anodes?

That's what I always thought. I'm assuming that the Pb electrode is the thing that gives the battery its shell life, but I'm not sure.

 

[Shoom]

btw in an exam does it matter if I write ethylene as ethene?
what if I wrote this H2C=CH2 like this H2C=H2C

Help please.
Stability of a nucleus if a atom has an atomic number of less then 20, it is stable if the n is 1:1 so why is a helium atom a radio isotope? shouldnt it be stable because it has 2 protons and 2 neutrons hence the ratio is 1:1?

 

[minijumbuk]

It doesn't matter if you write ethylene or ethene, but when it asks you for an IUPAC or a systematic name, it's ethene. If it says 'common name', it's ethylene.

You can't write H2C=CH2 as H2C=H2C, as that would indicate that it's a carbon-hydrogen double bond, which is highly incorrect xD

I'm not sure about the helium atom question though. But I think if a He isotope has a natio of 1:1, it should not be radioactive. Perhaps you were looking at some other isotopes of He which were radioactive?

 

[Shoom]

Hi
Thanks
Im assuming that the alpha particle is a normal He atom. We know alpha particles are radioactive so im wondering why.

By the looks of things sounds you will be getting a bad 6 for your chem exam.

so to test for unsaturation we put a hydrocarbon in bromine water which has hypobromius acid, do I have to remember the equations for this reaction
CH2Ch2+HOBr------ CH2BRCH2OH should I write this as H2CCH2BROH?

in your opinion whats the hardest part of the chemistry HSC course

But I can write ethylene as CH2CH2? thats what my book does but I think thats wrong thyey write this CH2Ch2+Br2-------CH2BrCH2Br isnt that wrong because the carbons should be next to each other or does it not matter because in this type of equation I dont need to show the bonds i.e H2C=CH2?

 

[Shoom]

CAn you explain what a condensation polymer is I dont really understand the definitions given to me.

 

[Darrow]

A condensation polymer is a type of polymer created by the addition of many monomer units where a small molecule (usually water) is eliminated from the polymer chain.
For example, our lovely Cellulose

 

[minijumbuk]

Condensation polymer is a polymer created by removing a H2O molecule to join two monomers, eg. Cellulose is a condensation polymer of glucose.

CH2CH2 is the 'condensed structural formula' for ethene. It's just a lazy way to make structural formulas. Like say 1-propanol. The condensed structural formula would be CH3CH2CH2OH. Although this looks like the 2nd C is joined to the first 3 H, it's not the case. That's the point of the condensed structural formula. It basically breaks up each molecule into several smaller bits.
The molecular formula of propanol could be C3H8O.
The full structural formula would be...

...H..H.H
...|..|..|
H-C-C-C-O-H
...|..|..|
...H..H.H
(Ignore the dots)
I hope that tells you enough about what the difference between each is.

Now, to answer you question about the bromine reaction:
They rarely ask you to draw condensed structural formula for reactions. Mostly, it'd be molecular or structural. In that case, just add up all the number of atoms in the reactants/products if they ask for a molecular formula. If not, then you must draw all the bonds in the correct order, with the atoms in the appropriate positions.

Also: Although they usually say "bromine water", which is technically HOBr, they actually mean Br2, though I guess either would be marked as correct. I just find it less troublesome to use Br2 instead. Just break up the double bond and put 2 new Br atoms on each C atom. Easy work xD

And I'd kill myself if I didn't get a band 6 in chemistry =P It's my best subject xD