Ch. 16 Section 3
Writing Thermochemical Equations
� Thermochemical equation: a balanced chemical equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy, change in H.
� The nature of the reaction or process described by a thermochemical equation is written as a subscript of change in H.
� In the highly exothermic combustion equation for glucose, the thermochemical equation is
o C6H12O6(s)+6O2(g) ?6CO2 (g)+6H2)(l) change in Hcomb=-2808 kJ
o The energy released (-2808 kJ) is the enthalpy of combustion
� Enthalpy (heat) of combustion:(change in Hcomb) of a substance is the enthalpy change for the complete burning of one mole of the substance
o Standard enthalpy changes have the symbol change in H degrees
o The zero subscript tells you that the reactions were carried out under normal conditions
o Standard enthalpies of combustion for several common substances are given in the table below
Standard enthalpies of combustion chart
Standard Rules for Writing Thermochemical Equations
Changes of State
� Molar enthalpy (heat) of vaporization:(change in Hvap) the heat required to vaporize one mole of a liquid
o When you step out of a hot shower, you shiver as water evaporates off of your skin. This is because your skin provides the heat needed to vaporize water.As heat is taken from your skin to vaporize the water, you cool down. This is an example of molar enthalpy of vaporization
� Molar enthalpy (heat) of fusion: (change in Hfus) the heat required to melt one mole of a solid
o If you want a glass of cold water, you may put ice into it. The water cools as it provides heat to melt the ice. This is an example of molar enthalpy of fusion.
o Because vaporizing a liquid and melting a solid are endothermic processes, their change in H values are positive
o Standard molar enthalpies of vaprization and fusion for 4 common compounds are shown in the table.
Standard molar enthalpies of vaporization
oIn changes of state equations, such as the vaporization of water and then the melting of ice the same amounts of energy are released in the exothermic processes as are absorbed in the endothermic processes of vaporization and melting
so, the molar enthalpy of condesations and the molar enthalpy of vaporization have the same numerial value but opposite signs
also, the molar enthalpy of solidifications and the molar enthalpy of fusion have the same numerical value but have different signs
Practice Problem Explained for Solving Equations
