# Lecture Demonstrations

## The q/T Paradox: Which "Contains More Heat", a Cup of Coffee at 95 °C or a Liter of Icewater?

A small mass of water at 0oC is added to a measured mass of liquid nitrogen, and the amount that evaporates is compared to the mass that evaporates when a larger mass of water at 95oC is added to liquid nitrogen. This demonstration requires knowledge of both specific heat and heat capacity.

## Determination of the Enthalpy of Fusion of Water

Calculate Enthalpy of Fusion of Ice (assuming heat capacity prerequisite)

Dip a computer-interfaced thermistor probe in 100g of water in a styrofoam cup calorimeter. Add 3-5 g of ice to a paper towel on a balance, and record the total mass. Start temperature acquisition 1 sample/second, 3 minutes total, and after a few readings, remove ~2 g of ice from the balance and add it to the calorimeter. Record the final mass on the balance and calculate the mass of ice. Display the T vs. time plot . Record the final temperature.

q (cal) + q (water ) = q (water from ice) + q (ice)

14.4 J/oC (20.97-22.70oC) + 100 (4.18)(20.97-22.70oC) = -(q + 1.90 (4.18)(20.97-0oC))

q = 581 J

ΔH = 581J/1.90 g x (1kg/1000 J) x (18 g/mol) = 5.5 kJ/mol (6.07 kJ/mol true value)

## References

1. J. Chem. Educ., 2005, 82 (6), p 856
2. We use Vernier LoggerPro(R)software