Heat of vaporization conceptual problem

[Natjack]

When I'm doing the Physics homework the problem for heat of vaporization uses Q=mcΔT and Q=mLv. How do I know when to use which equation through out the problem?

 

[DrClaude]

The first formula is related to a change in temperature (hence the presence of $\Delta T$), when a single phase is heated, while the second is related to the phase transition at constant temperature.

 

[Natjack]

So I use that one just when the temperature has changed?

 

[DrClaude]

Problems usually ask to make calculations going from $T_1$ to $T_2$, with a phase transition somewhere in there, $T_1 < T_t < T_2$. You then need to calculate for instance the heat in the liquid phase from $T_1$ to $T_t$, then the latent heat of vaporization, then heat in the gas phase from $T_t$ to $T_2$.

 

[Natjack]

so use the mLv for phase changes?

 

[DrClaude]

so use the mLv for phase changes?

Yes.

 

[Natjack]

Thank you so much for your help