Which of the following is true?

Question

A. The equation “dH = MCPdT” can be used to find the change in enthalpy when liquid boils into vapor, as long as the boiling occurs at constant pressure.
B. All compounds have the same normal boiling point, since the normal boiling point is always defined at P = 101.325 kPa.
C. If you compress a gas isothermally, it will condense into a liquid.
D. The heat capacity of a substance in the liquid phase need not be the same as the heat capacity of that same substance in the vapor phase.
E. None of the above are true.

Answer 1

A. Incorrect. The equation dH = MCPdT accounts for the effect of changes in temperature upon enthalpy. A phase change is a different process than a temperature change and dH = MCPdT is “blind” to the concept of phase changing.
B. Incorrect. The boiling point is defined as the saturation temperature that corresponds to a particular vapor pressure. The “normal boiling point” is defined as this temperature at a pressure of 1 atm, but every compound has a different boiling temperature at this pressure.
C. Incorrect. If you compress a gas isothermally until its pressure is above the critical pressure, it becomes a supercritical fluid. This statement would be true if the word “vapor” was used instead of “gas” and if the compression reaches the vapor pressure.
D. Correct. There is no reason to expect a compound will have the same heat capacity in the liquid and vapor phases. Changes in temperature and changes in phase both effect the heat capacity.
E. Incorrect. One of these statements is true.