Untitled - Kelly Walsh High School

Thermochemistry 101
Don’t Forget!
Total heat absorbed by the calorimeter and water
Total heat released by the glucose
Converting the grams of glucose to moles then allows us to calculate the molar
heat capacity:
Heat released by the glucose/moles glucose molar heat of reaction
You will need to change this value to a negative value because the reaction gave
off this quantity of heat.
Don’t confuse the heat capacity, specific heat, and the molar heat capacity.
6-2 Enthalpy (H)
Many of the reactions which chemists study are reactions that occur at constant
pressure. Because this constant pressure situation is so common in chemistry,
scientists use a special thermodynamic term to describe this energy, enthalpy.
The enthalpy change, H, is equal to the heat gained or lost by the system during
constant pressure conditions. The following sign conventions apply:
If H > 0 the reaction is endothermic
If H < 0 the reaction is exothermic
We sometimes call it the H reaction (H rxn). The H is normally associated with
a specific reaction. For example, the enthalpy change associated with the formation
of hydrogen and oxygen gases from water is:
2 H 2O(g) l 2 H 2(g) O 2(g) H 483.6 kJ
The positive sign indicates that this reaction is endothermic. This value of H
is for the decomposition of 2 mol of water. If 4 mol were decomposed, the H
would be 2 483.6 kJ.
If we reverse the reaction for the decomposition of water above, the sign of the H
reverses, 483.6 kJ. That would indicate that the reaction releases 483.6 kJ of
energy in forming 2 mol of water. This would now become an exothermic process.