Hess's Law Help?

[thisguy23]

---

Members don't see this ad.

I'm usually able to work Hess's law problem fairly easily, but only when given the overall reaction (therefore being able to manipulate the individual reactions to match the overall reaction). But how do you derive the overall reaction if it is not given to you?

i.e. "How much heat is lost or gained in the following system of reaction?"

S(s) + O2(g) => SO2(g) delta H = -297 kJ
2SO3(g) => 2SO2 + O2 (g) delta H = 198 kJ

Thanks for any help!

 

[Thanhn]

I'm usually able to work Hess's law problem fairly easily, but only when given the overall reaction (therefore being able to manipulate the individual reactions to match the overall reaction). But how do you derive the overall reaction if it is not given to you?

i.e. "How much heat is lost or gained in the following system of reaction?"

S(s) + O2(g) => SO2(g) delta H = -297 kJ
2SO3(g) => 2SO2 + O2 (g) delta H = 198 kJ

Thanks for any help!

I think the problem is letting you assume that you're just combining the reaction and cancelling similar reactants/products.

S(s) + O2(g) + 2SO3(g) => 3SO2(g) + O2(g)
yields
S(s) + 2SO3(g) => 3SO2(g) delta H = -99 kJ

 

[cmF]

I'm usually able to work Hess's law problem fairly easily, but only when given the overall reaction (therefore being able to manipulate the individual reactions to match the overall reaction). But how do you derive the overall reaction if it is not given to you?

i.e. "How much heat is lost or gained in the following system of reaction?"

S(s) + O2(g) => SO2(g) delta H = -297 kJ
2SO3(g) => 2SO2 + O2 (g) delta H = 198 kJ

Thanks for any help!

I think we can derive the overall reaction just by looking at what the products are for each reaction.

S(s) + O2(g) => SO2(g) delta H = -297 kJ Sulfur Dioxide Formed
2SO3(g) => 2SO2 + O2 (g) delta H = 198 kJ Sulfur Dioxide Formed again

In the second reaction, we don't have SO3 to begin with so that's what we must be forming, thus reverse the equation. After reversing the second equation, multiply the first equation by 2 so the coefficients of SO2 in the first and second equation match. Don't forget to multiply Delta H in first equation by 2, and change sign on the second equation because we reversed it. Finally, just add everything up (and cancel out "in red") to get the final equation.

2S(s) + 2O2(g) =====> 2SO2(g)--------delta H = -594kJ
2SO2(g) + O2 (g) ===> 2SO3(g)--------delta H = -198 kJ
--------------------------------------------------------------------------------
2S(s) + 3O2(g) =====> 2SO3 (g)-------delta H = -792 kJ

I'm just went over this last week but already hazy! Hopefully this is correct. 🙄

 

[thisguy23]

That is the correct answer. "In the second reaction, we don't have SO3 to begin with so that's what we must be forming, thus reverse the equation" was the answer I was looking for. I knew it was something simple, just couldn't find it anywhere. Thanks a bunch!