Enthalpy Change

Key Concepts

The heat content of a chemical system is called the enthalpy (symbol: H)
The enthalpy change (H) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.
H = H_(products) - H_(reactants)
H is specified per mole of substance as in the balanced chemical equation for the reaction
The units are usually given as kJ mol^-1 (kJ/mol) or sometimes as kcal mol^-1 (kcal/mol)
1 calorie (1 cal) = 4.184 joules (4.184 J)
Energy changes are measured under standard laboratory conditions (S.L.C.)
25^oC (298K) & 101.3kPa (1 atmosphere)

	Exothermic Reaction	Endothermic Reaction
Energy absorbed or released	Energy is released. Energy is a product of the reaction. Reaction vessel becomes warmer. Temperature inside reaction vessel increases.	Energy is absorbed. Energy is a reactant of the reaction. Reaction vessel becomes cooler. Temperature inside reaction vessel decreases.
Relative Energy of reactants & products	Energy of the reactants is greater than the energy of the products H_(reactants) > H_(products)	Energy of the reactants is less than the energy of the products H_(reactants) < H_(products)
Sign of H	H = H_(products) - H_(reactants) = negative (-ve)	H = H_(products) - H_(reactants) = positive (+ve)
Writing the equation	N₂(g)+ 3H₂(g) ---> 2NH₃(g)+ 92.4 kJ N₂(g)+3H₂(g)--->2NH₃(g) H=-92.4 kJ mol^-1	2NH₃(g)+ 92.4 kJ ---> N₂(g)+ 3H₂(g) 2NH₃g--->N₂(g)+3H₂(g) H=+92.4 kJ mol^-1
Energy Profile	Energy of reactants (N₂ & H₂) is greater than the energy of the products (NH₃). Energy is released. H is negative. H is measured from the energy of reactants to the energy of products on the Energy Profile diagram.	Energy of reactants (NH₃) is less than the energy of the products (N₂ & H₂). Energy is absorbed. H is positive. H is measured from the energy of reactants to the energy of products on the Energy Profile diagram.