Question

Calculate the enthalpy of formation of sodium chloride from its constituent elements, given the following thermochemical equations:2Na (s) + 1/2 O2 (g) -> Na2O (s)     ΔH° = -414.1 kJ/mol Na2O (s) + 1/2 Cl2 (g) -> NaCl (s)     ΔH° = -411.2 kJ/mol

Answer 1

To calculate the enthalpy of formation of sodium chloride  NaCl  from its constituent elements, we need to manipulate the given thermochemical equations to obtain the desired reaction:Na  s  + 1/2 Cl2  g  -> NaCl  s First, we need to divide the first equation by 2 to obtain 1 mol of Na2O:Na  s  + 1/4 O2  g  -> 1/2 Na2O  s      H = -207.05 kJ/molNext, we need to multiply the second equation by 2 to obtain 2 mol of NaCl:2Na2O  s  + Cl2  g  -> 2NaCl  s      H = -822.4 kJ/molNow, we can add the modified first equation and the modified second equation:Na  s  + 1/4 O2  g  -> 1/2 Na2O  s      H = -207.05 kJ/mol2Na2O  s  + Cl2  g  -> 2NaCl  s      H = -822.4 kJ/mol----------------------------------------------Na  s  + 1/2 Cl2  g  -> NaCl  s      H = -1029.45 kJ/molHowever, we made an error in the manipulation of the equations. We should have subtracted the modified first equation from the modified second equation:2Na2O  s  + Cl2  g  -> 2NaCl  s      H = -822.4 kJ/mol-  Na  s  + 1/4 O2  g  -> 1/2 Na2O  s      H = -207.05 kJ/mol ----------------------------------------------Na  s  + 1/2 Cl2  g  -> NaCl  s      H = -615.35 kJ/molSo, the enthalpy of formation of sodium chloride from its constituent elements is -615.35 kJ/mol.