- #1
blithe285
- 4
- 0
When doing constant volume calorimetry, the enthalpy can be calculated as follows:
ΔH = ΔU + Δ(PV)
ΔH = w + q + Δ(PV)
ΔH = PΔV + q + Δ(PV)
and at constant volume:
ΔH = q + VΔP
which I've then see people rewrite using the ideal gas law as follows:
ΔH = q + (Δn)RT
where Δn is the change in the moles of gas and T is constant.
This is what I don't understand. Why is T constant? If you're doing calorimetry, the temperature is changing. Why are we now assuming that it is constant? Before looking it up, I originally had the following:
ΔH = q + nRΔT
Why isn't it this? Or even ΔH = q + ΔnRΔT
How do you know when to keep moles constant and when to keep the temperature constant?
Thanks!
ΔH = ΔU + Δ(PV)
ΔH = w + q + Δ(PV)
ΔH = PΔV + q + Δ(PV)
and at constant volume:
ΔH = q + VΔP
which I've then see people rewrite using the ideal gas law as follows:
ΔH = q + (Δn)RT
where Δn is the change in the moles of gas and T is constant.
This is what I don't understand. Why is T constant? If you're doing calorimetry, the temperature is changing. Why are we now assuming that it is constant? Before looking it up, I originally had the following:
ΔH = q + nRΔT
Why isn't it this? Or even ΔH = q + ΔnRΔT
How do you know when to keep moles constant and when to keep the temperature constant?
Thanks!