How to use Hess's Law to calculate enthalpy for the reaction?

Can someone good at calculating reaction enthalpy changes help me with this problem?

• Usually I'm great with problem solving, math, chem, ect., but I can't figure it out for the life of me: Calculate the reaction enthalpy for the following reaction: 5CO2 + Si3N4 --> 3SiO + 2N20 + 5CO It gives me the following enthalpies and tells me to use Hess's law to figure it out: CO + SiO2 --> SiO + CO2, enthalpy change is +520.9 kJ 8CO2 + Si3N4 --> 3SiO2 + 2N2O + 8CO, enthalpy change of +461.05 kJ If you know how to do this one, can you kind of walk me through the steps and try to explain it to me? If you give a man a fish, he eats for a day; teach a man to fish, and he stops spamming yahoo! answers with the same type of question over and over

• Answer:

  1) multiply first reaction by 3 2) do nothing to second reaction The result: 3CO + 3SiO2 --> 3SiO + 3CO2, enthalpy change is +1562.7 kJ 8CO2 + Si3N4 --> 3SiO2 + 2N2O + 8CO, enthalpy change of +461.05 kJ Add the two reactions together and, in so doing, you cancel 3CO, 3SiO2 and 3 CO2. Add the two enthalpies together for the final answer: +2023.75 kJ You may round off as you see fit. More Hess' Law: http://www.chemteam.info/Thermochem/Thermochem.html Tutorials 11-15 "he stops spamming yahoo! answers with the same type of question over and over" Ha! Tell that to our spontaneous human combustion troll. Although, to tell the truth, I haven't seen him in quite a while.