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

  • Thread starter LogicX
  • Start date
  • #1
181
0
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?
 
Last edited:

Answers and Replies

  • #2
Bill_K
Science Advisor
Insights Author
4,157
202
H = U + PV, so ΔH = ΔU whenever Δ(PV) = 0.
 

Related Threads on When does the change in enthalpy=change in internal energy?

Replies
3
Views
887
Replies
8
Views
3K
  • Last Post
Replies
23
Views
45K
  • Last Post
Replies
7
Views
38K
  • Last Post
Replies
7
Views
5K
Replies
26
Views
25K
Replies
1
Views
2K
Replies
3
Views
3K
Replies
8
Views
2K
Replies
18
Views
33K
Top