Molar Heat of Combustion (1 Viewer)

[Web Addict]

Hi.
Is the value for molar heat of combustion positive or negative? I've read different books and they're all different. Some don't put a negative sign for the molar heat of combustion, while some do. Which one is correct?
Thank you.

 

[someth1ng]

Heat of combustion is exothermic, therefore, deltaH is negative.

 

[Web Addict]

I know that the enthalpy change is negative, but what about the heat of combustion or molar heat of combustion?

 

[Galapagos]

When asked for heat of combustion it is assumed that heat is released so write it down as a positive value; if on the other hand the question calls for 'change in enthalpy' then write it as a negative number (as the answer could be positive or negative). It all depends on the question and context.

 

[someth1ng]

Usually, I don't worry about it too much. I just use the formula /\H=-mC/\T to determine heat of combustion.

I believe enthalpy of combustion and heat of combustion are the same thing and hence, negative. Sometimes, you see the molar heat of combustion of say ethanol to be 1367kJ/mol of heat released - you also see -1367kJ/mol. It really doesn't matter too much.

 

[golgo13]

I see where you're coming from and i agree when your being asked for a value the sign doesn't matter to much but at the same time if the question asks for the kind of reaction then it would but i think to simplify the solution us /\H=mC/\T just to get a value and when it asks for what kind then /\H=-mC/\T. Generally tho you would be able to work it out from /\T since it is (Final T - Initial T) when its endo the temperature drops so its gonna come out a negative and ultimately turn your /\H positive and vica versa for exo
Hope that kind of helps