Another Random Chem Question (2 Viewers)

[kloudsurfer]

Hey everyone,
My chemistry teacher has gone on holidays for three weeks before the exams!!!!!!!!! And hes left us practically the entire energy topic to do in that time!!!!!!!!!!!!

Anyway, I have abosulutely no idea how to do this question. Ahhhhhh!!!!!!!. Could someone please explain to me how to do it?

Q. Calculate the heat energy released from the complete combustion of 500 grams of propane, given that the heat combustion of propane is 2220 kJ moL^-1

Any help would be much appreciated. Thanks

 

[~shinigami~]

1. Write a balanced Chemical equation i.e. C₃H₈ + 5O₂ -> 3CO₂ + 4H₂O

2. moles of propane = 500 (mass) / 44 (molar mass)
= 11.36 mol

3. Because 2220 kJ is for ONE mole but we have 11.36 moles then,
2220 x 11.36 = 25219.2 kJ

4. Heat energy released is 25219.2 kJ

Well this is what I think, I could be wrong so you could wait for pLuvia, Riviet or SoulSearcher to confirm it. They're awesome.

 

[Forbidden.]

1. Write a balanced Chemical equation i.e. C₃H₈ + 5O₂ -> 3CO₂ + 4H₂O

2. moles of propane = 500 (mass) / 44 (molar mass)
= 11.36 mol

3. Because 2220 kJ is for ONE mole but we have 11.36 moles then,
2220 x 11.36 = 25219.2 kJ

4. Heat energy released is 25219.2 kJ

Well this is what I think, I could be wrong so you could wait for pLuvia, Rivet or SoulSearcher to confirm it. They're awesome.

Impressive, although you written the balanced chemical equation of the products and reactants, I don't think the question specifically targets that ...
Your calculations are correct since it is a complete reaction, if the question states it is an incomplete reaction, things will become complicated and then your euqation will need to be changed.
Carbon Monoxide or just Carbon will be formed and there will simply be more water formed as a result of less oxygen for the carbon atom(s)

 

[~shinigami~]

That's correct, but with some science staff, the might want you to round it off to 3 significant figures (In this case). It's really pedantic, but the rest is correct.

Thanks for the advice, I'll try to remember that. I guess that's why the answers in the Chemistry textbooks are in scientific notation all the time.

I'm curious though, is this also the case for the HSC? Would I be marked down for my answer if it wasn't rounded off to significant figures?

Impressive, although you written the balanced chemical equation of the products and reactants, I don't think the question specifically targets that ...
Your calculations are correct since it is a complete reaction, if the question states it is an incomplete reaction, things will become complicated and then your euqation will need to be changed.
Carbon Monoxide or just Carbon will be formed and there will simply be more water formed as a result of less oxygen for the carbon atom(s)

Yeah, I'm just used to it and luckily it's not an incomplete combustion because I didn't read the question properly and would've missed out on that point.

 

[Riviet]

I'm curious though, is this also the case for the HSC? Would I be marked down for my answer if it wasn't rounded off to significant figures?

It depends on what range of answers the markers will accept. Strictly speaking, you should round to the least accurate info (least significant figures) that you're given in the question, and if you're unsure, then 2 decimal places seems to be a safe bet that you won't be marked down.

 

[~shinigami~]

It depends on what range of answers the markers will accept. Strictly speaking, you should round to the least accurate info (least significant figures) that you're given in the question, and if you're unsure, then 2 decimal places seems to be a safe bet that you won't be marked down.

Sorry, I'm still a bit confused.

So in the case of the combustion above, would the answer be 2.5x10⁴ kJ? Is that what you mean?

Anyway, sorry for asking again but I'm just confused.

 

[SoulSearcher]

Well this is what I think, I could be wrong so you could wait for pLuvia, Riviet or SoulSearcher to confirm it. They're awesome. ;P

I don't do chemistry, so I'm useless here

 

[~shinigami~]

I don't do chemistry, so I'm useless here

You, pLuvia and Riviet have become synoymous with science because when I ask a question; it's mostly you three who come and answer them.

So yeah, my thought process was, "hmm...a chemistry question...chemistry's a science subject...ah...those three must know the answer."

In this case it would be 3 significant figures therefore 2.52 x104 kJ. But you don't have to do that.

Oh damn, yeah thanks for correcting my answer again.

At least now I won't ever forget to round off correctly.

 

[Riviet]

Sorry, I'm still a bit confused.

So in the case of the combustion above, would the answer be 2.5x10⁴ kJ? Is that what you mean?

Anyway, sorry for asking again but I'm just confused.

500 is accurate to 3 significant figures and is least precise so therefore your answer should be rounded to 3 sig fig. Therefore your answer should be 2.52 x 10⁴ KJ. Hope that clears it up.

 

[Forbidden.]

If the charge of an electron is 1.609x10^-19, then must I write 1.61x10^-19 ? ... Correct me if the value is incorrect or if it is the charge of a proton not an electron...

 

[~shinigami~]

500 is accurate to 3 significant figures and is least precise so therefore your answer should be rounded to 3 sig fig. Therefore your answer should be 2.52 x 10⁴ KJ. Hope that clears it up.

Thank you once again Riviet.

You 06'ers are near your HSC already, aye? I can just imagine you smashing everyone in Physics, they'll never know what hit 'em.

If the charge of an electron is 1.609x10^-19, then must I write 1.61x10^-19 ? ... Correct me if the value is incorrect or if it is the charge of a proton not an electron...

I don't know if this is correct or not but my teacher always has q = 1.6x10^-19 when calculating problems.

 

[SoulSearcher]

Whereas my teacher always uses 1.609x10^-19. I personally would use what they gave us in the paper. By the way, the charge f3nr15 has got is the charge of a proton, just make that charge negative for the electron

 

[kloudsurfer]

1. Write a balanced Chemical equation i.e. C₃H₈ + 5O₂ -> 3CO₂ + 4H₂O

2. moles of propane = 500 (mass) / 44 (molar mass)
= 11.36 mol

3. Because 2220 kJ is for ONE mole but we have 11.36 moles then,
2220 x 11.36 = 25219.2 kJ

4. Heat energy released is 25219.2 kJ

Well this is what I think, I could be wrong so you could wait for pLuvia, Riviet or SoulSearcher to confirm it. They're awesome.

Ahhhh I see...thats really simple actually.
Thanks heaps everyone

 

[Forbidden.]

Whereas my teacher always uses 1.609x10^-19. I personally would use what they gave us in the paper. By the way, the charge f3nr15 has got is the charge of a proton, just make that charge negative for the electron

just yayness ...