Molar heat of combustion (1 Viewer)

[~ ReNcH ~]

Should molar heat of combustion be expressed as a positive or a negative value?

 

[Dreamerish*~]

When the reaction is endothermic, ΔH is positive, and when the reaction is exothermic, ΔH is negative.

However, in the molar heat of combustion of fuels, ΔH always comes out as being positive in the calculations. It's implied in that section that the result (ΔH) represents the amount of heat released upon burning an amount of a certain alcohol.

 

[~ ReNcH ~]

Yea. I've always taken molar heat to be positive, however in the CSSA paper the solutions quoted the molar heat as being -ve which doesn't quite make sense.

 

[s_t_a_r1234]

so would there be any specific rule concerning endothemric and exothermic, say for example endothermic is all combustion and exothermic is all decomposition reactions

 

[Dreamerish*~]

so would there be any specific rule concerning endothemric and exothermic, say for example endothermic is all combustion and exothermic is all decomposition reactions

Combustion is exothermic, that is, the products of the reaction include heat.

Basically, when the reaction breaks bonds, heat is required and therefore it is endothermic. For example, the dehydration of ethanol:

C₂H₅OH Conc. H₂SO₄ > C₂H₄ + H₂O

When a reaction forms a bond (or bonds), heat is released as the bond is formed, therefore the reaction is exothermic. For example, the hydration of ethene:

C₂H₄ + H₂O Dil. H₂SO₄ > C₂H₅OH

 

[s_t_a_r1234]

ah k cheerz, one thing, wth decomposition reactions that would be endothermic because bonds are broken right? say calcium carbonate

 

[Dreamerish*~]

ah k cheerz, one thing, wth decomposition reactions that would be endothermic because bonds are broken right? say calcium carbonate

I would imagine so. If during decomposition reactions, bonds are broken, then they are endothermic.