[bennym84]

Enthalply change help?
I have an assignment with this question and I'm struggling with it and not find much help so anyone able to answer and show me how would be a saviour.

Butane is the main ingredient in ''calor gas''. in an experiment to determine the enthalpy change of combustion 1.16g of the gas was completely burnt in oxygen and the energy used to heat 500cm3 of water by 27.4*C (the capacity is 4.2)

a) calculate the enthalpy of combustion? i think i worked this out fairly easily

q=mcAt so 500x4.2x27.4=57540J
The Ah=57540/ 0.02 moles (/1000) = -2897kjmol

does this seem okay???

These are the questions i have then struggled with-
b) a group of mountaineers are running short of gas for their cooker. They estimate that they have only 5g of butane left. Calculate the amount of heat in KJ their remaining gas will produce?
C)the mountaineers want to make some tea. calculate the mass of water it they could just boil and so use for their drinks if the initial temperature was 15*C.
d) give one reason why in practice they would only be able to boil half this mass of water?
e) A more efficient fuel would be a solid parrafin wax such as C20H42. calculate the enthalpy of combustion given that the enthalpies of formation c20h42, CO2 and H2O are -456, -393 and -285 kj mol respectively?


Please pleaseeeeee help. I need to work out how to do this stuff and the combustion enthalpy calculation seems easy enough just have no idea how to do this other stuff...
1 day ago - 3 days left to answer.
Answer QuestionReport Abuse

[DrCMS]

The first bit is the correct way to do it but the answer is wrong.

Ah=57540/ 0.02 moles (/1000) = -2897kjmol

Check your maths again.

Part b) just uses this calculate value from part a) with a new mass of butane.
Part c) uses the total energy from b) with a known temp change (15°C to boiling) you need to calculate a mass of water.
For part d) you need to consider the oxygen content of the air high up and also the reduced pressure
For part e) write out the combution equation, balance it and then use the heats of formation of the products and reactants to calculate the heat of combustion.

[vanklik]

I think I've done part e) but I'm not sure it's right. Could anyone correct me please if I'm wrong?
Thanks!