Question

Gases react explosively if the heat released when the reaction begins is sufficient to cause more reaction, which leads to a rapid expansion of the gases. Use bond energies to calculate ?H° of the following reactions, and predict which occurs explosively:

(a) H₂(g) + Cl₂(g) ? 2 HCl(g)

(b) H₂(g) + I₂(g) ? 2 HI(g)

(c) 2 H₂(g) + O₂(g) ? 2 H₂O(g)

Answer 1

dHrxn using bond enthalpies

(a) H2 + Cl2 --> 2HCl

dHrxn = dH(reactants) - dH(products)

           = (436 + 242) - (2 x 431)

           = -184 kJ/mol

(b) H2 + l2 --> 2Hl

dHrxn = dH(reactants) - dH(products)

           = (436 + 151) - (2 x 299)

           = -11 kJ/mol

(c) 2H2 + O2 --> 2H2O

dHrxn = dH(reactants) - dH(products)

           = (2 x 436 + 495) - (4 x 463)

           = -485 kJ/mol