When does the change in enthelpy=change in internal energy?

AI Thread Summary
The discussion centers on the conditions under which the change in enthalpy (ΔH) equals the change in internal energy (ΔU). It highlights that this equality holds true in specific scenarios, such as combustion in a bomb calorimeter, where the number of moles of gas remains constant. However, in cases like irreversible expansion of a perfect gas, ΔH can differ from ΔU due to temperature changes, leading to the equation ΔH = ΔU + nRΔT. The conversation questions the applicability of the first justification and whether it is limited to bomb calorimeters or can be extended to other reactions with unchanged moles of gas. Ultimately, understanding the relationship between ΔH and ΔU requires careful consideration of the system's conditions, including temperature and pressure changes.
LogicX
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Sometimes in my book, a problem justifies ΔU=ΔH for a process, such as combustion in a bomb calorimeter, by saying that since the number of moles of gas doesn't change, they are equal.

In other questions, the number of moles doesn't change (such as an irreversible expansion of a perfect gas) but still, ΔH is different from ΔU because there is a change in temperature, so ΔH= ΔU + Δ(nRT)= ΔU + nRΔT

When do you use the first justification? Only in a bomb calorimeter? Any time I am given a reaction and the change in molar internal energy for that reaction where the moles of gas is the same on both sides of the equation? Does this mean that the temperature of a sample in a bomb calorimeter is constant?
 
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H = U + PV, so ΔH = ΔU whenever Δ(PV) = 0.
 

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