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Chemistry Problem...Q=mc(delta)t

Q=mct problem...Chemistry Enthalpy Change!?

Heres the question:
In the experiment, 200g of water were used. The temperature of the water changed from 18 degrees to 68.3 degrees when 1.00g of propane was burnt.

Calculate the energy produced in kJ. The specific heat capacity is 4.18Jg-1K-1.

I did 200x4.18x50.3 and got 42.1 kJ. Then I added a negative sign because it looks exothermic to me (as all combustion reactions are exothermic and temp of surroundings increased), but the mark scheme has no negative sign. Have I got it wrong? Can anyone help me? :smile:

Thanks
Reply 1
Avatar for Lrilol
Lrilol
1
mark scheme is wrong, all exothermic reactions have a negative enthalpy change value.


Probably an error in print?
Yeah there should be a negative sign there.

But shouldn't you trying to figure out kj mol-1?

In whic case it would be -1850 kj mol-1
Reply 3
Avatar for Altocirrus
Altocirrus
2
Exothermic reactions give out heat, so there will be a positive output of energy from the system to the surroundings (the water). Hence, no minus sign.
Altocirrus
Exothermic reactions give out heat, so there will be a positive output of energy from the system to the surroundings (the water). Hence, no minus sign.


This - the question is not asking about the enthalpy change.
Reply 5
Avatar for jacko0892
jacko0892
OP
2
Ok thanks...it seems most people are saying the negative sign should be there! Cheers!
Reply 6
Avatar for AviK
AviK
1
No the mark scheme is right because they are not asking about the overall energy produced by input and output; they only ask for the output of energy created by burning propane not the energy level change.
Reply 7
Avatar for Pigster
Pigster
20
Original post by AviK
No the mark scheme is right because they are not asking about the overall energy produced by input and output; they only ask for the output of energy created by burning propane not the energy level change.


Excellent addition to a thread, but in the future I would suggest that you pay attention to the date that the conversation took place.

I doubt that after eight years most of the people involved care much about A level chemistry.
Reply 8
Avatar for Hshdj
Hshdj
1
Firstly the mass must be measured in kg so instead of 200g it should have been 0.2 and the question does not ask if it is exothermic or endothermic.the question is asking how much energy is required to raise the water by 1degree Celsius

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