Question

Heat of Vaporization Questions

We did a lab where we placed a graduated cylinder upside down in a beaker to observe the air bubble and how it was affected by temperature. There are a few questions I need help with:

1. You have assumed the vapor pressure of water below 5 degrees Celsius to be negligible. How would the inclusion of its actual vapor pressure affect your results?

2. Assume the graph of ln(P) versus 1/T results in a curved line instead of a straight line. What does this mean?

3. Other liquds may be examined by this experimental method to determine the heat of vaporization. What properties should these liquids exhibit?

4. Assume you make a mistake and plot ln(P) against 1/T and express T in degrees C instead of K. How does this affect your results?

           There would be no change right?

     

Answer 1

1. We assume that the vapor pressure of water is negligible, so the total volume of gas in the cylinder is due to the air alone. We use the ideal gas equation to calculate the number of moles of air so that the number of moles of air can be added to other calculations at higher temperatures. The equation is this:

, if you say that the vapor pressure of water is not zero then the pressure in the cylinder will be lower and the number of moles of air calculated will be slightly smaller.

Since, the total pressure for other data is the sum of air pressure and water vaopr pressure. Considering the water vapor pressure is not zero at 5 degrees will result in data consistently giving slightly smaller air pressure and slightly larger water vapor pressure. However, the fact is that the vapor pressure of water at 5 degrees is so small that the error is less than 1 % in the pressure calculation. So the effect on final results is negligible.

2. The straight line obtained when we plot lnP againt 1/T corresponds to the Clausius-Clapeyron equation. If we obtained a curve instead of a straight line, the data would not fit into the equation since this equation assumes a linear relationship between lnP and 1/T and calculation of vaporization enthalpy would not be possible.

3. For other liquids to be examined by this method, they should have negligible vapor pressure at cold temperatures so that the number of moles of air can be calculated.

4. Remember 1/T is a fraction and it is very much affected by the value of the denominator which is the temperature. If you took temperature in C instead of K, all of the work would have to be redone. This becomes especially important when you have temperatures close to zero celcius. Assume how would you plot a value of . It would be impossible.

If you need more information, please do not hesitate to ask.