Experiment #2 / Unit 6
Energy and Nitrogen
Introduction:
Molecular nitrogen (N2) is one of the most prevalent substances on Earth.
As you take in your next breath, almost 80% of the molecules will be nitrogen.
Nitrogen is a gas at normal conditions, but it can be condensed at temperatures
below its boiling point of -196*C. You may have seen liquid nitrogen used in a
science class, at a dermatologist's office, or even in a movie. Due to its low
boiling point, liquid nitrogen must be kept away from sources of kinetic energy
(remember that heat always transfers from hotter to colder). Liquid nitrogen can
be stored in a Dewar flask which is like an especially well insulated thermos
bottle. One of the layers of the flask is a vacuum. Without molecules present,
heat transfer is greatly reduced. Once in contact with warmer substances, liquid
nitrogen will readily gain heat and undergo rapid evaporation (boiling). Say, for
instance, that your hand was placed in a beaker of liquid N 2. The liquid N2 would
violently boil in the area around your hand as heat is transferred from your warm
skin to the N2 molecules. With a small amount of kinetic energy (> -196*C), the N2
molecules will have enough energy to overcome their weak intermolecular
attractions. Your skin will quickly lose its energy followed by the underlying
tissue. The temperature of that tissue will drop below its freezing point and the
tissue and nerves can be damaged. This phenomenon is known as frostbite.
Many substances can be quickly frozen by placing them in the low energy
environment of liquid N2.
In this experiment, we are going to study heat transfer involving N 2
molecules and H2O molecules (not your hand!). We are going to determine the
heat of vaporization for nitrogen. Hvap is the amount of energy required to boil a
given amount of a substance. Many important concepts will be illustrated such as
kinetic energy and temperature, heat transfer, conservation of energy,
intermolecular forces and state change, specific heat capacity, and calorimetry.
Procedure:
1. Obtain a container to use as a calorimeter and measure its mass.
2. Add about 1 L of warm water (~40*C) from the tap in the storeroom to the
calorimeter and measure the total mass. We can also heat this water on a hot
plate to start warmer.
3. Begin measuring the temperature of the warm water with a probe.
4. Tare a styrofoam cup and add liquid N2 until you have about 85.0 g. Make
note of the mass immediately before you add the liquid N2.
5. Pour (at a moderate pace) the liquid N2 into the cup with the hot water. You
might have to fan the fog away from the surface in order to see what is
happening.
6. Measure the final mass of the calorimeter and the H2O molecules.
Chemistry II
Cary Academy
W.G. Rushin
1
Data:
 mass of calorimeter
 mass of calorimeter and hot water
 mass of liquid nitrogen
 mass of calorimeter and cool water
 graph of the temperature readings for the water
Calculations and Questions:
1. Calculate the (a) mass of hot water (b) mass of cool water (c) average mass
2. Calculate the change in temperature for the water molecules.
3. Calculate the energy lost by the H2O molecules in joules. Use the average
mass.
4. So how much energy will we assume was gained by the N2 molecules?
5. Calculate the heat of vaporization (Hvap) for nitrogen in J/g.
6. Recalculate Hvap in KJ/mol (molar heat of vaporization).
7. Calculate your percent error for the experiment (Hvap = 5.58 KJ/mol).
8. What would be the heat of condensation for nitrogen?
9. Look up Hvap for water. Explain why the values are so different.
10. Write thermochemical equations for both the (a) vaporization and (b)
condensation of nitrogen.
11. Draw a Lewis structure for nitrogen and explain what forces had to be
overcome to allow the nitrogen molecules to move freely from their neighbors.
12. Calculate the volume of N2 gas at STP that was produced in the vaporization
of your liquid N2.
13. Explain the “fog” that was produced when the liquid N2 was added to the
water.
14. Draw heating and cooling curves to show the heat transfer, state changes,
and temperature changes involved in the experiment.
15. Explain (in terms of energy transfer) what occurred in this experiment. Include
what type of energy (potential or kinetic) was involved in the energy transfer
from the standpoint of N2 vs. H2O.
Lab Report #6.2:
 title page
 abstract
 procedure sheet
 data
 calculations and questions
 results and discussion
Chemistry II
Cary Academy
W.G. Rushin
2