The Russian chemist D. Mendeleev is known for his Periodic Table of elements. This discovery made hi — Physical Chemistry — Kinetics Chemistry Question
Dmitry Ivanovič Mendelejev: What besides the periodic table?
The Russian chemist D. Mendeleev is known for his Periodic Table of elements. This discovery made him famous worldwide. Dmitry Mendeleev has carried out some other interesting studies as well. Consider two of them.
Mendeleev was the first who state that every substance has “the temperature of the absolute boiling”. Above this temperature “the substance will stay in the gas phase no matter how high the pressure is”. According to Mendeleev “the temperature of the absolute boiling of water” is 543 °C.
In Russia many people believe that D. Mendeleev invented the recipe of the famous drink “Russian vodka”. We have a chance to check this legend.
The fact is that in his Ph.D. thesis Mendeleev characterized some properties of the binary system “ethanol-water”. He measured the density ρ of a series of binary solutions of various compositions w, where w(%) is the mass percent of ethanol in the mixture. The derivative dρ / dw is presented in Fig. 1 as a function of w. The curve markedly changes the slope three times. According to D. Mendeleev these three special points correspond to the compositions of the weakly bonded chemical compounds, “hydrates of ethanol”.
Fig. 1. Experimental results obtained by Mendeleev
What is “the temperature of the absolute boiling”?
Model Answer
At present temperature of the absolute boiling is called critical temperature.
D. Mendeleev introduced the «temperature of the absolute boiling» in 1860.
T. Andrews introduced his concepts of the critical temperature and the critical point in 1869.
Indicate the temperature of the absolute boiling in the P-T phase diagram of water.
Model Answer
On the phase diagram of water the line of phase equilibrium between liquid and vapor terminates at the critical point. The corresponding temperature is “the temperature of the absolute boiling” (see figure).
Calculate the temperature of the absolute boiling of water from the Van der Waals equation of state:
(p + a/V^2)(V - b) = RT,
For H2O, a = 5.464 l^2 atm mol–2, b = 0.03049 l mol–1.
Model Answer
Critical temperature Tc can be calculated from the parameters a and b of the Van-der-Waals equation of state:
Tc = 8a / 27Rb
For H2O this equation gives
Tc(H2O) = (8 × 5.464 × 101.3) / (27 × 8.314 × 0.03049) = 647 K = 374 °C
One can see that Mendeleev overestimated the temperature of absolute boiling of water significantly. His value was 170 degrees above the real one.
What are the chemical formulas of “the hydrates of the ethanol”?
Model Answer
From mass percent we calculate molar ratio:
n(C2H5OH) / n(H2O) = (w(%) / 46) / ((100 – w(%)) / 18) = 18w / (46(100 – w))
There are three break points in the figure, namely at w = 17.5, 46 and 88 %. They correspond to the molar ratios n(C2H5OH) : n(H2O) = 1:12; 1:3; 3:1
According to Mendeleev the binary solution consists of the weakly bonded associates of ethanol with water. The compositions of these “hydrates of ethanol” are given by the molar ratios mentioned above.
Does the composition of any of the “hydrates” resemble the recipe of vodka (40 volume percent of C2H5OH)? The density of ethanol is 0.794 g cm–3. Decide whether or not Dmitry Mendeleev took part in “the discovery of Russian vodka”.
Model Answer
However, the special compositions found by Mendeleev have nothing in common with the recipe of vodka. The volume percent V% of the ethanol in vodka is 40. The corresponding mass percent is:
w = (40 × 0.794 / ((40 × 0.794) + 60 × 1.000)) × 100% = 34.6%
There is nothing special in this part of the graph! From the point of view of physical chemistry there is nothing special in the recipe of vodka.