The determination of the heat of evaporation can be accomplished using a liquid–gaseous phase transition calorimeter. In this experiment, a liquid in equilibrium with its vapor at the boiling point is subjected to a known amount of electric energy. Consequently, a certain mass of the liquid under goes evaporation. The specific latent heat of evaporation of the utilized liquid is then determined by analyzing the ratio between the supplied energy and the mass that evaporates.
A device for measuring heats of evaporation was introduced by Jamin (1870) and is illustrated in Figure-A. The experimental setup involves a vessel containing the test liquid and a heating resistance, situated within a second vessel containing the same liquid in a boiling state.
During a specific time interval, a steady state emerges, resulting in a constant rate of evaporation from the inner vessel, which is heated to the boiling point. The condensate formed within the given time frame is measured.
Upon activating the electric heater, an additional mass of liquid evaporates during the same period, and this mass is also determined through weighing.
Nevertheless, this type of calorimeter is plagued by a systematic error: as the measurement progresses, the liquid level in the boiling vessel decreases, leading to an increase in gas volume byΔV. Consequently, a portion of the obtained vapor remains within the calorimeter vessel. The vapor mass contained in the volume ΔV is,
Δmvap = ρvap . ΔV
Δmvap = ρvap . ΔV
where ρvap is the vapor density.
The acquisition of this vapor mass was
facilitated through the application of energy equivalent to qvap = Δmvap (where qvap denotes
the specific heat of evaporation). A portion of the electric energy supplied
was utilized for this purpose. Accordingly,
The voltage, U(t), the electric
current, I(t), time, t, mass of condensate, Δmcond, and the initiation and
conclusion times of electric heating, tini and tfin,
respectively, are involved in the process.
For water,
the computation of the second term in the aforementioned equation can be easily
carried out: given that 1 g of water occupies a volume of 1 cm3, the
steam density directly offers the correction term's magnitude.
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