Enthalpy Change

Definitions

• 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
  25^oC (298K) & 101.3kPa (1 atmosphere)

Type of Reaction	Exothermic	Endothermic
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	Energy of the reactants is less than the energy of the 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.