In: Chemistry
In this week's experiment, the heat of vaporization of liquid
nitrogen is determined by measuring the change in temperature of a
known sample of warm water when liquid nitrogen is added.
In one experiment, the mass of water is 104 grams,
the initial temperature of the water is
69.3oC, the mass of liquid nitrogen
added to the water is 60.6 grams, and the final
temperature of the water, after the liquid nitrogen has vaporized,
is 41.3oC.
Specific heat of water = 4.184 J K-1g-1
How much heat is lost by the warm water?
Heat lost = J
What is the heat of vaporization of nitrogen in J
g-1?
Heat of vaporization
= J
g-1
What is the molar heat of vaporization of nitrogen?
Molar heat of vaporization
=
J mol-1
Trouton's constant is the ratio of the enthalpy (heat) of
vaporization of a substance to its boiling point (in K). The
constant is actually equal to the entropy change for the
vaporization process and is most often a measure of the entropy in
the liquid state. The value of the constant usually lies within the
range 70 to 90 J K-1mol-1, with a value
toward the lower end indicating high entropy in the liquid
state.
The normal boiling point of liquid nitrogen is -196oC.
Based upon your results above, what is the value of Trouton's
constant?
Trouton's constant
= J
K-1mol-1
1. Heat lost
Heat Lost = ms(T1-T2) = 104 X 4.184 X (41.3-69.3) = -12.183 KJ
2. What is the heat of vaporization in J g^-1?
A: Just take the first answer and divide it by the mass of the
liquid nitrogen
Mass of nitrogen = 60.6 g
heat = -12183/ 60.6 =- 217.54 J / g
3. What is the molar heat of vaporization of nitrogen?
A: Well, we have found the heat of vaporization of nitrogen in J/g,
so now we're just multiplying the second answer by 28 (molar mass
of N2) = -217.54 X 28 = - 6091.12 J / mole
The normal boiling point of liquid nitrogen is -196C. Based upon
your results above, what is the value of Trouton's constant?
A: Trouton's constant (from above) is the heat of vaporization (the
molar heat, not the grams) divided by the boiling point. But,
Trouton's equation is in units of K (Kelvin), not degrees Celsius,
so add 273 to - 196, so the boiling point of Nitrogen is 77K.
Therefore, the answer to this question is the answer to the third
question,
divided by 77.
- 6091.12 / 77 = 79.10 J/K mol