Water exhibits both physical and chemical transformations in everyday settings
CHEMISTRY · Multi-part · AP · Chemistry · Chemical Reactions · Chemical Reactions
Problem
Water exhibits both physical and chemical transformations in everyday settings. The molar enthalpy of vaporization of liquid water at its normal boiling point is 40.7kJmol−1. In contrast, the molar enthalpy change for decomposing liquid water at 25∘C into hydrogen gas and oxygen gas is +286kJmol−1. Dry ice (solid CO2) sublimes at −78.5∘C with a molar enthalpy of sublimation of 25.2kJmol−1.
(a) Describe, in terms of particle-level interactions, the fundamental distinction between a physical change and a chemical change.
(b) Using the enthalpy values provided, explain why vaporizing water requires significantly less energy per mole than decomposing water, even though both processes consume energy.
(c) Classify the sublimation of dry ice as a physical or a chemical change. Justify your classification by referring explicitly to the nature of the interactions altered and to the magnitude of the enthalpy change relative to those given for water.