Consider gas here as idea gas.
The gas expands from state 1: P1, V1 and T1 to state 2: P2, V2, and T1 using two different paths.
Path A: reversible expansion at constant T
Path B: irreversible expansion by releasing the gas to a vacuum to achieve V2 at adiabatic condition.

Thing I confuse:
consider that enthalpy as a state function: H1 to H2 from state 1 to state 2.
for reversible path A:　dU = q + w. dU = 0. so q=-w. The system needs some q from surrounding to perform w. and dH = q. H2 = H1 + dH = H1 + q.
but for Path B: w=0, q=0, dU=0. dH=0.
So, H2 = H1.
Why? enthalpy can'

Question

Consider gas here as idea gas. 
The gas expands from state 1: P1, V1 and T1 to state 2: P2, V2, and T1 using two different paths. 
Path A: reversible expansion at constant T
Path B: irreversible expansion by releasing the gas to a vacuum to achieve V2 at adiabatic condition. 

Thing I confuse: 
consider that enthalpy as a state function: H1 to H2 from state 1 to state 2. 
for reversible path A:　dU = q + w. dU = 0. so q=-w. The system needs some q from surrounding to perform w. and dH = q. H2 = H1 + dH = H1 + q.  
but for Path B: w=0, q=0, dU=0. dH=0.
So, H2 = H1.  
Why? enthalpy can'

anonymous · Answer

my question is enthalpy can't be same for path B, because enthalpy is a state function which is independent from its path.