Latent Heat Of Vaporisation Essay

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Determining the Latent Heat of Vaporisation of Nitrogen

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 [1] but the uncertainty is very large which could have been minimised
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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)
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This was to minimise heat transfer from the outside environment to the liquid nitrogen in the colorimeter. If the colorimeter was not immersed in liquid nitrogen, then there would be greater thermal energy transfer to it meaning the power supplied to it would be greater than only the power supplied by the resistor.
Before the power was switched on, the measuring cylinder was filled with water and placed upright above the behive. When the power was switched on, the liquid nitrogen was heated up by the resistor and vapourised. This gas traveled along the delivery tube and was bubbled through water into the measring cylinder. As the water at the top of the measuring cylinder was displaced, the time it took for the bottom of the meniscus to travel through a volume of 60cm³ was measured (this was equal to the time taken for 60cm³ of nitrogen gas to be

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