Need help w/ a Chem prob.

[WorkOnIt]

For the reaction

S + O2-----> SO2 DeltaH = -296 kj/mol


how much heat is evolved when 275 g sulfur is burned in excess O2

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[mws99]

Use the atomic weight of sulfur and convert the amount you have (275 g) to moles. Then multiply the number of moles by the delta H. That's it. Ignore the O2 because it is excess.

 

[WorkOnIt]

Thanks yo.

 

[WorkOnIt]

can anyone tell me why the answer in the book is positive, but when u calulate it urself its negative

when i did the math it comes out to -2536, when u multiply 8.57 mol S and -296 kj/mol

in the book the answer is 2.54 x 10 to the 3rd power.

it cant be a typo b/c there's another similar question and sign ,when i calulate, doesnt come out right

 

[14931]

heat evolved is already negative so i think the book is just saying that's the heat evolved , get?

its like they are not repeating themselves, anyway thats what i think i might be wrong.

 

[souljah1]

you can't evolve negative heat. the negative sign implies that it is exothermic and b/c of this, you know that the reaction gives off heat. it's been 5 years or so, but i think that i'm right on this one.

 

[WorkOnIt]

makes sense

 

[14931]

which one, mine or souljah's?

 

[CD]

AHHHHhhhh. I just did a problem like this one last Thursday with my class (I T.A. for general chem). First 8.57 moles is correct as is -2538 KJ. The sign seems to be the thing that gets people. (I had a rousing "discussion" over this very issue). Technically the sign MUST be negative because it is exothermic and exothermic reactions give OFF heat. By convention giving off heat is negative. The trick is that the sign is already implied in the question. You are told that heat is evolved. Meaning heat is given off. Therefore the sign is unneeded. Like writing a check. The amount on the check is positive but the effect on the checking account is negative. So did you write a negative check? Of course not! The negative is implied by the check just as the negative is implied in the way the question is stated.