January 29, 2025, 07:11:42 PM
Forum Rules: Read This Before Posting


Topic: Heat of Combustion  (Read 2903 times)

0 Members and 1 Guest are viewing this topic.

Offline LHM

  • Full Member
  • ****
  • Posts: 144
  • Mole Snacks: +1/-3
Heat of Combustion
« on: November 13, 2010, 10:05:10 PM »
I think I remember reading somewhere that the heat of combustion is the energy released during combustion, so in most cases such as methane it should be positive, right? And it's just (-1) times the enthalpy of reaction? But I was looking up something and on some sites it seems to contradict this and list the heat of combustion of methane as being negative (for example, here: http://pslc.ws/fire/cellulos/combcals.htm) so just to clarify is heat of combustion the exact same thing as enthalpy of reaction, or is it the -(enthalpy of reaction)?

Offline Grundalizer

  • Full Member
  • ****
  • Posts: 257
  • Mole Snacks: +19/-31
Re: Heat of Combustion
« Reply #1 on: November 13, 2010, 10:23:21 PM »
Enthalpy and "heat of reaction" are the same thing.  If it is EXOTHERMIC (meaning gives off heat) then it is NEGATIVE in sign.  If it TAKES ENERGY then it is Endothermic and absorbs heat from the environment

Offline LHM

  • Full Member
  • ****
  • Posts: 144
  • Mole Snacks: +1/-3
Re: Heat of Combustion
« Reply #2 on: November 14, 2010, 01:35:32 PM »
I just looked it up in my textbook though, and it says: "The heat released on combustion of a substance is called its heat of combustion. The heat of combustion is equal to - :delta: H° for the reaction written in the direction shown."

I know that for enthalpy, like you said, if it is exothermic, then it is negative. But then wouldn't it be the opposite for heat of combustion, and if it is exothermic, then it is positive? Sorry I'm just kind of confused as to why some sources seem to have them both as keeping the same exact sign while the textbook says they should be different signs - is there one that's more correct or something?

Offline rabolisk

  • Chemist
  • Full Member
  • *
  • Posts: 494
  • Mole Snacks: +45/-25
Re: Heat of Combustion
« Reply #3 on: November 14, 2010, 01:48:11 PM »
The answer is that you are most likely using a high school textbook. The heat of combustion at constant pressure is the same thing as the enthalpy of combustion. I really wouldn't worry too much about it. I suppose a bad teacher could ask the question, "how much heat is released when x amount of methane is burned?" and the answer choices contain both the positive and the negative answer. At which point you should argue that either answer is valid, and also punch your teacher for giving a terrible question...
« Last Edit: November 14, 2010, 02:08:36 PM by rabolisk »

Sponsored Links