Hello! I am a high school senior taking an advanced chemistry course. I have been working on this problem for two days straight, and cannot figure out our error. Here is the problem we are given:(adsbygoogle = window.adsbygoogle || []).push({});

Calculate the standard enthalpy of formation for the reaction HCL (g) + NH3 (g) ---> NH4CL (s), given the following thermochemical equations:

H2 (g) + Cl2 (g) ---> 2HCL (g), standard enthalpy of formation: -184kJ (-92kJ/mol)

N2 (g) + 3H2 (g) ---> 2NH3 (g), standard enthalpy of formation: -92kJ (-46kJ/mol)

N2 (g) + 4H2 (g) + Cl2 (g) ---> 2NH4CL (s), standard enthalpy of formation: -628kJ (-314kJ/mol)

It wants us to combine the equations using Hess's Law to arrive at the desired equation, and thus, the desired enthalpy of formation. I did so, and I arrive at the desired equation given above ( HCL (g) + NH3 (g) ---> NH4CL (s) ). However, the number I have arrived at time and time again (-176kJ/mol) is not the accepted standard enthalpy of formation for NH4CL, -314.43kJ/mol.

Someone please show me the error before I bash my head against the wall any further! Thanks!

Note, the book that gives the problem lists the answer I get (-176kJ/mol) as correct. I am wondering why this is not the accepted value. I'm sure it is something simple that I am missing, but I have been staring at it for much too long now and must admit I need assistance.

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# Homework Help: Hess's Law - Thermochemical Equations

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