The heating value of combustible fuels is evaluated based on the quantities known as the higher heating value (HHV) and the lower heating value (LHV). The HHV has a higher absolute value and assumes that the water formed in the combustion reaction is formed in the liquid state. The LHV has a lower absolute value and assumes that the water formed in the combustion reaction is formed in the gaseous state. The LHV is therefore the sum of the HHV (which is negative) and the heat of vaporization of water for the number of moles of water formed in the reaction (which is positive). The table on the right lists the enthalpy of combustion-which is equivalent to the HHV-for several closely related hydrocarbons. Use the information in the table at right to answer the following questions:
a. Write two balanced equations for the combustion of C3H8;= one equation assuming the formation of liquid water and the other equation assuming the formation of gaseous water.
b. Given that the heat of vaporization of water is 44.0 kJ/mol, what is Hrxn for each of the reactions in part a? Which quantity is the HHV? The LLV?
c. When propane is used to cook in an outdoor grill, is the amount of heat released the HHV or the LLV? What amount of heat is released upon combustion of 1.00 kg of propane in an outdoor grill?
d. For each CH2 unit added to a linear alkane, what is the average increase in the absolute value of Hcomb?