HSC Chemistry Question Thread

[winstondarmawan]

Would appreciate help with this Q (2007 HSC):
[ https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23023624_1365754790216707_196707540_n.png?oh=679c254c90b7ff9a64ffcdf9fec28b28&oe=59F9F75E
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23113784_1365754990216687_594565583_n.png?oh=8d23c08682923b5849368a2267c647dc&oe=59F9CD8F ]
Part b).
TIA!

[aryak]

Hi is it ok if you could explain how to answer question 19 of the 2015 hsc multiple choice? I dont get why the answer is C.

[bsdfjnlkasn]

Why is ozone good in the stratosphere?

Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?

[Mathew587]

Why is ozone good in the stratosphere?

Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?

Yeah.
Ozone assists in the maintenance of the ozone layer. CFCs result in the depletion of the ozone layer not oxygen.

[ACCC]

Just double checking, is the Q the same as delta H (in DeltaH = mc deltaT) or completely different? Thank you!

Quick question, for this equation
C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
how is there 8 hydrogens in citric acid in reactants but only 7 in the products (combined)?

Mod edit: Merged

[Natasha.97]

Hi is it ok if you could explain how to answer question 19 of the 2015 hsc multiple choice? I dont get why the answer is C.

toasted has answered this here

Just double checking, is the Q the same as delta H (in DeltaH = mc deltaT) or completely different? Thank you!

Both Qs are the same

Quick question, for this equation
C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
how is there 8 hydrogens in citric acid in reactants but only 7 in the products (combined)?

On the reactant side, there are 11 Hydrogens (8 from citric acid, 3 from sodium hydroxide), and on the product side, there are also 11 Hydrogens (5 in sodium citrate and 6 in water)

[ACCC]

Oh! Thank you can't believe I missed that. But in citric acid the H8 changed to H5, does that always happen?

[MisterNeo]

Would appreciate help with this Q (2007 HSC):
[ https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23023624_1365754790216707_196707540_n.png?oh=679c254c90b7ff9a64ffcdf9fec28b28&oe=59F9F75E
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23113784_1365754990216687_594565583_n.png?oh=8d23c08682923b5849368a2267c647dc&oe=59F9CD8F ]
Part b).
TIA!

Hi is it ok if you could explain how to answer question 19 of the 2015 hsc multiple choice? I dont get why the answer is C.

Why is ozone good in the stratosphere?
Is it because U.V is absorbed in the formation of oxygen? Doesn't that deplete ozone though? Don't we need ozone in the stratosphere?

Oh! Thank you can't believe I missed that. But in citric acid the H8 changed to H5, does that always happen?

Citric acid can donate one proton at a time like how sulfuric acid does. In the HSC, we just assume full protonation whenever it reacts with something, so all 3 hydrogens are used

[bsdfjnlkasn]

Why can temperature only affect the equilibrium constant? thanks!

[angelahchan]

Hi, can someone please help me with Q17 of the  2014 paper? thanks
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf

[erinyyyyy]

For the production of sulfuric acid why can’t water be added straight to SO3, why is oleum produced first?

[MisterNeo]

Why can temperature only affect the equilibrium constant? thanks!

The K constant is the ratio of products to reactants at a specific temperature.
When concentration/pressure changes, the ratio is out of balance, so the equilibrium will shift to minimise the change and re-establish that special ratio of products to reactants. When temperature changes, the equilibrium will shift to minimise that change in temperature and it will stay there, hence the K ratio is now changed as long as that temperature remains constant.
For example:
I have an exothermic reaction, and I increase the temperature from 400'C to 700'C and keep it there. The equilibrium will shift to the left to minimise the change but the position will stay there because it cannot forward react to the right since that's exothermic. hence, K is now changed.
Hope this explanation helps

Hi, can someone please help me with Q17 of the  2014 paper? thanks
https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-chemistry.pdf

For the production of sulfuric acid why can’t water be added straight to SO3, why is oleum produced first?

The reaction between sulfur trioxide and water is very exothermic and will create a fine mist of sulfuric acid everywhere, which is both dangerous and hard to clean up. The reaction between oleum and water is less exothermic and is safer to use.

[liya1234]

when doing calculations involving heats of combustion and specific heat capacities, should we always use J/kg/K because that is what is given? and how do you convert something that is in kJ/g/K to J/kg/K? thanks so much in advance, I really appreciate all the help here!

[ellybubble]

For the production of sulfuric acid why can’t water be added straight to SO3, why is oleum produced first?

if water is added directly, its a very exothermic reaction, and risks the production of acid mist instead of liquid sulfuric acid, which is what we want. Acid mist is firstly very dangerous and secondly hard to convert back to liquid sulfuric acid. Thus, oleum is produced first, and then diluted to form sulfuric acid (l)

[winstondarmawan]

Hello, I'm struggling with knowing when exactly a line of best fit should be linear or a curve.
For example:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23146248_1365590483566471_2141030131_n.jpg?oh=25754963e74fcebb116b007aaf0d0367&oe=59F9EB63

Bump.