You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin. A table of single bond energies should be used as a help (available in most general chemistry text books).
Estimate the change in enthalpy, DH, for the following reaction:
H2(g) + Cl2(g) --> 2 HCl(g)
To work this problem, think of the reaction in terms of simple steps:
First, the reactant molecules, H2 and Cl2, break down into their atoms
H2(g) --> 2 H(g)
Cl2(g) --> 2 Cl(g)
2. Next, these atoms combine to form HCl molecules
2 H(g) + 2 Cl(g) --> 2 HCl(g) ...