PURPOSE

The object of this experiment is to measure the heat of vaporization of liquid nitrogen. Liquid nitrogen boils at 77 K at atmospheric pressure. It can cause serious injury if it is spilled on your body. BE CAREFUL!

EQUIPMENT  thermos bottle, liquid nitrogen, resistive heater, ammeter, voltmeter, balance, and stopwatch.

DISCUSSION

The latent heat of vaporization (L_V) is the amount of heat energy required per unit mass to change the phase of a substance from a liquid to a vapor. In mathematical terms:

The latent heat of vaporization is reversible: it is added when the substance is transformed from a liquid to a vapor (evaporation), and it released (removed) when the substance is transformed from a vapor to a liquid (condensation). Different substances exhibit different values for the latent heat because it is truly a measure of energy required to break the attractive bonds holding the liquid atoms or molecules together.

The method we will use to determine the latent heat of vaporization is to first measure the rate of boil-off due to normal heat leakage and then to compare it to the boil-off rate we observe when we add a known amount of heat. The latent heat will then be determined by subtracting the normal heat leakage rate from the total rate. Our known heat input comes from a resistor (an electrical heater). The power (in watts) dissipated by the resistor is equal to the electric current as measured (in amperes) multiplied by the voltage across the resistor (in volts):

P (watts) = I (amps) x V (volts)

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