heat and enthalpy

[superduper12]

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my books says that q = m * change in H

q is heat
change in H is enthalpy ....units of energy per mass

I thought heat and enthalpy were the same thing..could someone explain the difference?

 

[sansrufino]

Enthalpy, H, can be thought of as the energy stored in bonds or configurations of a system. More stable systems have lower enthalpies, usually more negative. It's units are usually given in joules/mol. I think the "m" may represent mol instead of mass. Otherwise you may need to convert.

Heat, q, is the transfer of energy through heat flow, units are in joules. Hence, per the equation, multiplying joules/mol by mol will yield a value in joules, which is the units of heat.

So if the heat flow is positive, or into the system, the system may become less stable, or have higher enthalpy. If heat flows out, the system would become more stable, having lower enthalpy.

Hope this helps.

 

[kupakid]

Enthalpy can simply be thought of as an (heat) energy stored in a system. The heat (which is a form of energy) released/acquired by the system is simply the difference in the final and initial enthalpy times its mass units (whether it's grams or moles doesn't matter as they both describe the "amount" of matter.)

 

[superduper12]

H = U + PV

ok first of all my ek book has three dashes for the equal sign in this equation, why?

and second it says that H is measured in Joules...and according to this
q = m * change in H ...it would be Joules per mass...so which is it?

 

[engineeredout]

H = U + PV

ok first of all my ek book has three dashes for the equal sign in this equation, why?

Three dashes mean "equivalent to". It is generally used for proofs.

and second it says that H is measured in Joules...and according to this
q = m * change in H ...it would be Joules per mass...so which is it?

H can be measured in joules, or joules per mol, or joules per unit mass, depending on what you're doing. If you have 5g of a substance with an enthalpy of 10J/g, you can multiply to get 50J for the enthalpy.

This is pretty basic stuff here... You better get good at manipulating units.

 

[StayingSteady]

EDIT: I'm stupid

 

[engineeredout]

Oh yeah did I mention I'm the incarnate of the devil? True story.



Bah you changed your post. Everyone is going to look at my post and be like "wtf is he talking about?"