delta u vs delta h

[805939635]

I am just a bit confused on the difference between delta u which as I understand is the change in internal energy and delta h which is the change in enthalpy. I get how they are related in equations I am just not sure what the difference is from a definition/theoretical standpoint. Thanks!

[Karina_Esqueda_2B]

Hi!

Delta U is the internal energy of a system, where as delta H is the heat. Delta U includes work and heat, whereas delta H does not. That is how I've understood it but let me know if I am wrong.

[Julianne Morris 2C]

We know that enthalpy(heat) is a state function, and it measures the heat transferred from a system to its surroundings, or vice versa. A negative Delta H means that heat from the system is transferring into its surroundings, otherwise known as an exothermic reaction. A positive Delta H means that heat is flowing into a system from its surroundings, also called an endothermic reaction. Delta U uses enthalpy to calculate its value, which represents the change in internal energy. Internal energy is the amount of energy a system has, and is calculated using Q (the heat transferred to the system) and W (the work done by the system). Both can be used to calculate the other, though they are conceptually different.

[Allie Wilcox]

Consider this relation: ΔH = ΔU + PΔV (constant pressure)

It might help to consider where this equation came from. ΔU = q + w --- this comes from the idea the internal energy of a closed system can be changed through heating(+q)/cooling(-q) or compression(+w)/expansion(-w). Under constant pressure, ΔH = q and w = -PΔV, thus under constant pressure ΔU = ΔH + -PΔV, which can be rearranged to give the equation above. It is also helpful to know why ΔH = q and w = -PΔV, but I think that is better explained by someone else.

[Jessica Li 3H]

I think of delta U as a more general term that describes the internal energy of a system, while delta H (which equals q at constant pressure) is a term contributing to the internal energy. Delta H represents the change in heat, and it is added to work (which also contributes to internal energy) in order to equal delta U.

[Zain Syed 1E]

Try to think of it like this. DeltaU is the internal energy, so total internal energy within the system where it considers both the heat that has been exchanged from the system to surroundings (vice versa) and the work done on the system. This is where we get the deltaU equation, deltaU= q + w. The q is the heat I just described and the w is the work. DeltaH comes into the equation when the pressure is constant. Then the equation become deltaU= deltaH + w.