Question

a. Identify/circle which of the following substances are defined to have an enthalpy of formation of exactly zero because they are an element in its standard, most stable state at room temperature. (Hint: there are only three, try to not consult Appendix C, use your knowledge of the physical states to identify the elements in their standard states.). Cl2 (g) H2O (g) C (s, diamond) Ozone, O3 (g) O2 (l) Fe (s) F2 (g) K (g) b. Write the enthalpy of formation reaction for H2O (g). Look up the bond enthalpies of the H–H, O=O, and O–H bonds in Table 5.4 in your textbook or in the lecture noets to estimate the ∆H of the formation reaction of water. Compare this value to the enthalpy of formation value found in Appendix C for gaseous water.

Answer 1

a) enthalpy of formation of exactly zero :   Cl2 (g) , Fe (s), F2 (g)

the reason is that these are in their standard states.

b)   H₂ (g) + 1/ 2 O₂ (g) ------------------> H₂O (g)

c) ∆H   = bond energy of reactants - bond energy of products

           = (436 + 1/2 x 498) - ( 2 x 460 )

           = - 235 kJ / mol

for gaseous water   ∆H = –241.8 kJ / mol . so the above value somewhat close to this value