We present a conceptual framework for adding molecular details of chain extension and force-coupled bond dissociation to the Lake Thomas model of tear energy in rubbery crack propagation. Incorporating data reported from single-molecule force spectroscopy experiments provides an estimate for the stored energy per bond at fracture of 60 kJ morl for typical hydrocarbon polymers, well below the typical carbon carbon bond dissociation energy in these systems. Opportunities to test and exploit the role of molecular extension and covalent bond scission in experimental systems are proposed.