A student places a mixture of plastic beads consisting of polypropylene (PP) and polyvinyl chloride — Thermodynamics Chemistry Question
Problem Context
A student places a mixture of plastic beads consisting of polypropylene (PP) and polyvinyl chloride (PVC) in a 1.0 L beaker containing distilled water. After stirring the contents of the beaker vigorously, the student observes that the beads of one type of plastic sink to the bottom of the beaker and the beads of the other type of plastic float on the water. The chemical structures of PP and PVC are represented by the diagrams below, which show segments of each polymer.
[VISUAL]
Model Answer
The PVC beads sink. The spacing between chains is similar, but a Cl atom has a greater mass than CH3.
1 point is earned for the correct polymer with a correct explanation.
Model Answer
Both substances have dipole-dipole interactions and London dispersion forces (or propene is essentially nonpolar with only LDFs while vinyl chloride has both LDFs and dipole-dipole forces). Propene contains a CH3 group, but vinyl chloride contains a Cl atom. Vinyl chloride thus has a larger electron cloud, is more polarizable, and has a larger dipole moment. Thus intermolecular attractions are stronger in vinyl chloride, which results in it having the higher boiling point.
1 point is earned for a discussion of intermolecular forces and for a comparison of their relative strengths.
Model Answer
ΔH° = 6(−394) + 6(−242) − 2(21) = −3858 kJ/molrxn
The combustion of 2.00 mol of propene releases more energy.
1 point is earned for the calculation of the enthalpy of combustion of propene.
1 point is earned for the comparison of propene to vinyl chloride that is consistent with the calculated value.