# Latent Heat Of Vaporisation Essay

2122 Words 9 Pages
Determining the Latent Heat of Vaporisation of Nitrogen

Abstract
The aim of this experiment was to determine the latent heat of vaporisation of nitrogen. To do this, a resistor connected to a power supply was used to heat up liquid nitrogen. Measuring the current and resistance in the circuit gave the power supplied to the liquid nitrogen and measuring how long it took for a fixed amount of nitrogen gas to be produced gave the rate of evolution of nitrogen gas. Plotting the power against the rate of evolution and determining the gradient yielded the value of 188 ± 22 kJkg¯¹ for the latent heat of vaporisation of nitrogen. This is within the accepted value of 199 kJkg¯¹  but the uncertainty is very large which could have been minimised
The energy required to vaporise a liquid is therefore given by:
Q = mLv (1) where Q is the energy required and m is the mass of the substance that has been vaporised.
If we supply power, P, to the liquid over a time, t, then the energy supplied to it is Q = Pt. Also, we can write the mass of the vaporised substance, m, in terms of its volume, V, and density ρ (m=ρV) so that (1) becomes:
Pt = ρVLv (2) which rearranges to:
P = ρ(V/t)Lv (3)
We now have an expression which includes V/t - called the volume evolution rate of the gas produced. This helps us because we can now measure the volume of gas produced when heating the liquid to help determine the latent heat of vaporisation.
To supply energy to the liquid, we can use the thermal energy produced by a resistor connected to a power supply. If we assume all the power supplied to the resistor is supplied thermally (through conduction) to the liquid, then we can write the power supplied in terms of the resistance of the resistor, R, and the current through it, I, so that P = I²R. Substituting this into (3)