Well, what other information do you have to work with?nmi13 said:Homework Statement
How many kilocalories of heat are needed to change 143N of ethyl alcohol at 65°C to vapour?
Homework Equations
Lv = Q/m
The Attempt at a Solution
I'm really stuck. I have no idea how to work the units of N with heat problems. I've looked online and in my book and have no idea how to get to the solution but I know the final answer is 3090kcal. A nudge in the right direction is greatly appreciated!
Chestermiller said:If it weighs 143N, what is its mass? At what temperature is that 204 kcal/kg? Have you heard of Hess's Law?
Chet
Hi SteamKing,SteamKing said:Chet, it's not clear why Hess's Law would be helpful here. There's no chemical reaction occurring, AFAIK, just vaporizing some ethanol.
Chet, I appreciate the explanation, but I think that Hess's Law is a little advanced for the OP. He appears to be self-studying physics for the first time (See Post #3).Chestermiller said:Hi SteamKing,
Thanks. Even though hess's law appears to officially apply only to heats of reaction, the basic principle is still the same for phase change: The change in enthalpy between two thermodynamic equilibrium states of a system is independent of path. I guess I thought that this was a more general statement of hess's law. The basic principle can still be applied to determining the heat of vaporization at one temperature if you know the heat of vaporization at another temperature and the heat capacities of the vapor and liquid. Of course, you may have to make a small correction for the effect of pressure on enthalpy of the liquid phase, even if the gas phase is treated as an ideal gas. But this shouldn't be any big deal.
Chet
Yeah, I can see that. But there doesn't seem to be an easier way of doing it than that (at least not to me).SteamKing said:Chet, I appreciate the explanation, but I think that Hess's Law is a little advanced for the OP. He appears to be self-studying physics for the first time (See Post #3).