product of combustion, while the product of equation (4) is peroxide. Therefore, using equation (3) with the correct coefficients cancels the hydrogen.
Equations (5), (6), and (7) can then be combined. Substances that appear in equal amounts on the right of one equation and the left of a subsequent equation can be cancelled from both. The resulting reaction is then
Since the enthalpies for each step were adjusted for any changes in direction or coefficients, the net enthalpy change is given by the sum of the enthalpy changes of equations (5), (6), and (7). This total is -1650 kJ/mol, a highly exothermic reaction.
Enthalpies of physical change can be read off of a graph of temperature vs. heat supplied. As heat is supplied to a solid system, the temperature of the system increases. The relationship between heat supplied and temperature is approximately linear in this region of the graph. As the melting point of the substance is reached, additional heat no longer causes an increase in temperature. At this point, the heat
