4D.15

[Sara Richmond 2K]

Hey guys I had a question on a home problem. I’ve included the textbook, my work, and the solution manuals work. As you can see, I get a positive answer while the solution manual gets a negative one. The solution manual uses equations and I am not sure where they found them. Please help/ explain what I am doing wrong.

[Sara Richmond 2K]

Photos

[Junwei Sun 4I]

The equations the solution manual uses are combustion reactions. Because we are told the substances’ enthalpy of combustion we can first write out those combustion reactions as the solution manual did. A combustion reaction happens when a substance reacts with water and produces CO2 and H2O. You got a different answer because you have the wrong signs. After writing out three combustion reactions and flipping one of the equations to make things cancel out, enthalpy should be +1560, -1300 and - 286 KJ

[Junwei Sun 4I]

The final enthalpy should be +1560-1300-2(286)=-312kJ

[Sara Richmond 2K]

The equations the solution manual uses are combustion reactions. Because we are told the substances’ enthalpy of combustion we can first write out those combustion reactions as the solution manual did. A combustion reaction happens when a substance reacts with water and produces CO2 and H2O. You got a different answer because you have the wrong signs. After writing out three combustion reactions and flipping one of the equations to make things cancel out, enthalpy should be +1560, -1300 and - 286 KJ

Wow thank you!