A pure substance initially at a uniform temperature of −25 °C is subjected to a…
PHYSICS · Multi-part · A Level · Physics · Further mechanics and thermal physics · Further mechanics and thermal physics
Problem
A pure substance initially at a uniform temperature of −25 °C is subjected to a steady supply of thermal energy at constant pressure. The process takes the substance through each of the following macroscopic stages in order:
• the solid warms from −25 °C to its melting point of 4 °C;
• the solid completely melts at 4 °C;
• the resulting liquid warms to its normal boiling point of 118 °C;
• the liquid completely vaporizes at 118 °C.
For each of the four stages listed above, the amount of energy transferred as heat has been measured. The total energy absorbed by the system for the entire process is 1.15×105J.
(a) Describe, with molecular-level reasoning, why the temperature of the substance remains constant during the two phase-change stages even though energy is still being added.
(b) The energy absorbed during the melting stage is 7.8×103J. The energy absorbed during the vaporization stage is 9.4×104J. Using only these data, determine the combined energy absorbed in the two temperature-change stages, and explain why that quantity must equal the difference between the total energy and the sum of the two phase-change energies.
(c) The same sample is now cooled back to −25 °C at constant pressure. Assuming no energy is lost to the surroundings other than as heat, state whether the total energy released during cooling is greater than, less than, or equal to 1.15×105J. Justify your answer in terms of thermodynamic quantities.