A) 1. Enthalpy ( delta H) is the amount of heat content.
1. Heat content is accounted for by a change in “heat flow” or enthalpy of the reaction system.
1. Endothermic reaction: delta H > 0
(i.e., H products >H reactants).
Heat absorbed goes to increase the enthalpy of the reaction system.
2. Exothermic reaction: delta H < 0
(i.e., H products < H reactants).
Heat is evolved at the expense of the reaction system.
2. Thermochemical Equation: specify delta H in kilojoules/mole.
1. CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.3 kJ
delta H = -890 kJ

6.00kJ + H2O(s) –> H2O(l)
delta H = +6.00kJ

! In some textbooks delta H is written as a product or reactant !

The preceding is based upon the Law of Conservation of Energy (James Joule, 1818-1889, Joule also developed the First Law of Thermodynamics): energy is neither created nor destroyed in ordinary chemical or physical changes.
2. Quantitative delta H
delta H = qreaction mixture (at constant temperature only)

q = (m)( delta t)(Cp)

q = heat absorbed by the water in joules (J)
m = mass of substance
delta t = tfinal – tinitial
Cp = specific heat of water = 4.184 J/g oC

When using moles, molar heat capacity is used. The units are kJ/mol K

1 cal = 4.184 J
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