- #1
kotchenski
- 16
- 0
Homework Statement
Assume that a wax candle with mass m at temperature T1 undergoes constant heat admission dQ/dt=Kp
Assume:
m=50g
T1=25 celsius
Kp=10W
Calculate how long it will take for the wax to melt and vaporise during constant heat admission
without any loss.
Data:
ρ=791 kg/m3
M=310 g/mol
Tm=317.15 K (melting temperature)
Lf=252 kJ/kg
Tb=641.8 K (boiling temperature)
Lv=105 kJ/kg
Cp(s)=598.1 J/mol K
Cp(l)=739 J/mol K
Cp(g)=1193 J/mol K
Homework Equations
P=Q/Δt
Q=mL
Q=mcΔT
The Attempt at a Solution
Well the posibilities overwhelmes me.
I know that it all boils down to using Δt=Q/P the question is though which Q are they asking for, I have only made an attempt at the boiling question though.
For boiling
If I apply what I know from heat of transformation:
Q=mL
Q=0.05kg*252 kJ/kg -> Q=12600J
Thereby:
Δt=12600J/10W -> Δt=1260s
In other words it would take 21mins.
However if that's the case then it would be even faster to boil it away when it's in liquid.
Am I doing something wrong or is there something blatantly obvious that I don't seem to get?