HSC Chemistry Question Thread

[bluecookie]

Yes that's crucial.


As a rule of thumb, memorise as many equations as you can. However Jake made a list a while back.

Thank you ^^

[shreya_ajoshi]

THe amount o energy released in kJ per mole during cobustion of octance is 5460. How would you find the amount of energy release in kJ/mol of Carbon Dioxide.
The way I do it is to write Octane's combustion reaction and balance it.
C_{8H18 + O2 --> CO2 +H2O}
When you balance it, the ratio of octane to carbon dioxide is 1:8. Wouldn't that mean that the amount of energy carbon dioxide releases is 5460 x 8 = 43680. But in the answers, they do 5460/8 = 682.5.
Why do they divide instead of multiply

[jakesilove]

THe amount o energy released in kJ per mole during cobustion of octance is 5460. How would you find the amount of energy release in kJ/mol of Carbon Dioxide.
The way I do it is to write Octane's combustion reaction and balance it.
C_{8H18 + O2 --> CO2 +H2O}
When you balance it, the ratio of octane to carbon dioxide is 1:8. Wouldn't that mean that the amount of energy carbon dioxide releases is 5460 x 8 = 43680. But in the answers, they do 5460/8 = 682.5.
Why do they divide instead of multiply

Very interesting question! Never seen anything like it before. So, looking at the reaction, we have



As you've stated, the relationship between Octane and Carbon dioxide is clearly . We know that 5460kJ of energy is released in the combustion of one mole of Octane. How can we think about this question?

Well, if the forward reaction RELEASES 5460 kJ of energy, the backwards reaction (ie. Carbon dioxide and Water turning into Octane and Oxygen) must ABSORB 5460 kJ of energy (switch the arrow the other way around, and switch the sign of the heat of reaction). Therefore, we can say that the '8' moles of Carbon dioxide is ALSO associated with 5460 kJ of energy. Thus, to get ONE mole of Carbon dioxide, we divide the total energy (5460 kJ) by the number of moles ( to get our answer (682.5 kJ). I have the say, this is a really strange question, and I've never seen anything like it in the HSC. I also don't think I've explained in brilliantly, but that's my answer to the best of my knowledge. Does it sort of make sense? Don't worry too much if it doesn't; it's not very important in the HSC.

Great question though! Very hard, but you were on the right track!

(Mod edit: Fixing the LaTex)

[shreya_ajoshi]

Very interesting question! Never seen anything like it before. So, looking at the reaction, we have



As you've stated, the relationship between Octane and Carbon dioxide is clearly . We know that 5460kJ of energy is released in the combustion of one mole of Octane. How can we think about this question?

Well, if the forward reaction RELEASES 5460 kJ of energy, the backwards reaction (ie. Carbon dioxide and Water turning into Octane and Oxygen) must ABSORB 5460 kJ of energy (switch the arrow the other way around, and switch the sign of the heat of reaction). Therefore, we can say that the '8' moles of Carbon dioxide is ALSO associated with 5460 kJ of energy. Thus, to get ONE mole of Carbon dioxide, we divide the total energy (5460 kJ) by the number of moles ( to get our answer (682.5 kJ). I have the say, this is a really strange question, and I've never seen anything like it in the HSC. I also don't think I've explained in brilliantly, but that's my answer to the best of my knowledge. Does it sort of make sense? Don't worry too much if it doesn't; it's not very important in the HSC.

Great question though! Very hard, but you were on the right track!

(Mod edit: Fixing the LaTex)

Ahhh, i get it now! Thank you very much

[shreya_ajoshi]

Identify the species that is oxidised, and the species that is reduced

I2O5 + 3CO --> I2 + 3CO2

[RuiAce]

Identify the species that is oxidised, and the species that is reduced

I2O5 + 3CO --> I2 + 3CO2

Write out the relevant oxidation states.

Reactants:
O: -2 each
I: +5 each

O: -2 each
C: +2
________
Products:

I: 0

C: +4
O: -2 each


Hence, iodine is reduced whilst carbon is oxidised.

The important thing is if you understand where those numbers came from

[shreya_ajoshi]

Write out the relevant oxidation states.

Reactants:
O: -2 each
I: +5 each

O: -2 each
C: +2
________
Products:

I: 0

C: +4
O: -2 each


Hence, iodine is reduced whilst carbon is oxidised.

The important thing is if you understand where those numbers came from

I understand the oxidation states for the reactants and the iodine product. I don't understand how you got the oxidation states for the carbon dioxide product.

[RuiAce]

I understand the oxidation states for the reactants and the iodine product. I don't understand how you got the oxidation states for the carbon dioxide product.

Each oxygen always has an oxidation state of -2 unless it's the oxygen element or a member of the peroxide ion.

Carbon dioxide consists of 2 oxygens. Hence, in total, the two oxygens amount to an oxidation state of -4.
But carbon dioxide has no charge. Hence, the sum of oxidation states of all the atoms in carbon dioxide should be 0.

To ensure that happens, the oxidation state of carbon must be +4 to cancel it out

[Iminschool]

For this question, what does 'destructive and non-destructive testing' mean?

[jakesilove]

For this question, what does 'destructive and non-destructive testing' mean?

A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).

Does that make sense?

Specifically, the pH probe is non destructive, as it doesn't do anything to the substance. However, adding an indicator will change the composition of the liquid (ie. raise or lower the pH, react, etc.) and thus is considered destructive.

[RuiAce]

A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).

Does that make sense?

Never even heard of these before Jake... are they actually in the course or is this your own knowledge?

[jakesilove]

Never even heard of these before Jake... are they actually in the course or is this your own knowledge?

Definitely in the course; eg. AAS is a destructive test, which is a disadvantage of it

[RuiAce]

Definitely in the course; eg. AAS is a destructive test, which is a disadvantage of it

Man. Unless my memory is actually that bad, I feel like I just learnt something way too late...

[Iminschool]

A destructive test is one in which the sample is destroyed (eg. if I were to test water by vapourising it, then the water is destroyed!). A non-destructive test is one in which the sample is NOT destroyed (eg. if I tested the reflectivity of a mirror by shining a light onto it, the mirror remains intact!).

Does that make sense?

Specifically, the pH probe is non destructive, as it doesn't do anything to the substance. However, adding an indicator will change the composition of the liquid (ie. raise or lower the pH, react, etc.) and thus is considered destructive.

Yea it does make sense, great use of analogies. Thanks

[Iminschool]

I get C as my answer, requesting confirmation please