Clarification on molar heat of combustion (1 Viewer)

[RachelGreen]

Can someone clarify clarify something for me?
My teacher said molar heat of combustion must be positive, as it -delta(h), but isnt combustion reactions exothermic, so its enthalpy change is negative..? I have a practical exam, and am confused on what to use? Delta H = -MC(delta.t) or without the negative?

 

[HecticLad]

It is negative for an exothermic reaction to indicate it has lost energy into the surroundings, while it is positive for an endothermic indicating an gain of energy from the the surroundings
the formula is always ΔH=-mCΔT as an increase in temperature (exothermic reaction) will give a negative value, while a decrease in temperature (endothermic reaction) will give a positive value

 

[RachelGreen]

thank you!!

 

[InteGrand]

Can someone clarify clarify something for me?
My teacher said molar heat of combustion must be positive, as it -delta(h), but isnt combustion reactions exothermic, so its enthalpy change is negative..? I have a practical exam, and am confused on what to use? Delta H = -MC(delta.t) or without the negative?

I think technically the heat of combustion is positive, but the enthalpy change is negative.

 

[RachelGreen]

So when we're calculating the molar heat of combustion, for ethanol. We know its enthalpy change is negative, but instead of taking the negative value, we write the positive value instead for the amount of heat liberated? I don't know if my final value should be positive or negative??

 

[InteGrand]

So when we're calculating the molar heat of combustion, for ethanol. We know its enthalpy change is negative, but instead of taking the negative value, we write the positive value instead for the amount of heat liberated? I don't know if my final value should be positive or negative??

The heat liberated in an exothermic reaction is a positive amount, since heat is indeed liberated to the surroundings. If heat liberated were negative, that'd imply that actually heat was being taken in from the surroundings (endothermic).