Basic question about Enthalpy/Stoichiometry

[BacardiGirl]

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I am trying to figure out why the answer to this problem is positive, when I calculated it to be a negative. This is a really simple question (with probably a simpler answer) so don't bash me for being a chemistry Noob!! 😳

Consider the following equation for the combustion of acetone (C3 H6 O), the main ingredient in nail polish remover.
C3 H6 O + 4O2 ----- 3CO2 + 3H2 O
Hrxn = -1790

I came out with -4.78 x 10 to the third (sorry not sure how to type power signs on a keyboard)
The answer is 4.78 x 10 to the third. Why is it positive?

Thank you!!!

 

[Transformer]

I am trying to figure out why the answer to this problem is positive, when I calculated it to be a negative. This is a really simple question (with probably a simpler answer) so don't bash me for being a chemistry Noob!! 😳

Consider the following equation for the combustion of acetone (C3 H6 O), the main ingredient in nail polish remover.
C3 H6 O + 4O2 ----- 3CO2 + 3H2 O
Hrxn = -1790

I came out with -4.78 x 10 to the third (sorry not sure how to type power signs on a keyboard)
The answer is 4.78 x 10 to the third. Why is it positive?

Thank you!!!

You have a combustion reaction.

something with CnHnO plus oxygen producing carbon dioxide, and water will always release heat.

A reaction releasing heat is exothermic; thus, Enthalpy of reaction = Hrxn is negative.

hope that makes sense....🙂

 

[BacardiGirl]

So, in my conversion formula, I assume that I have to change the -1790 to 1790 in order to get the correct answer?

 

[medicalCPA]

You have a combustion reaction.

something with CnHnO plus oxygen producing carbon dioxide, and water will always release heat.

A reaction releasing heat is exothermic; thus, Enthalpy of reaction = Hrxn is negative.

hope that makes sense....🙂

No, it doesn't. The enthalpy of reaction is negative, but the correct answer is positive. I believe the question is incomplete.

 

[BacardiGirl]

No, it doesn't. The enthalpy of reaction is negative, but the correct answer is positive. I believe the question is incomplete.

So...my textbook is wrong?

Sorry forgot to add the last part of the question:
If a bottle of nail polish remover contains 155 g of acetone, how much heat would be released by its complete combustion?

 

[Transformer]

No, it doesn't. The enthalpy of reaction is negative, but the correct answer is positive. I believe the question is incomplete.

Yes, the enthalpy of reaction is negative. we are saying the same thing.

Yes, the question is incomplete



For

C3H6O(l) + 4O2 ----> 3CO2 (g) + 3 H20(l)

I calculate

Hrxn = -1789 KJ

 

[Transformer]

So, in my conversion formula, I assume that I have to change the -1790 to 1790 in order to get the correct answer?

You should get an negative answer for Hrxn without having to change the sign at the end

Hrxn = Sum of Hf product - Sum of Hf reactant

Hrxn = [((3mol)(-393kJ/mol)) + ((3mol)(-285.5KJ/mol))] -

[((1mol)(-249KJ/mol)) - (4mol)(0 KJ/mol))]


[-1180.5 KJ - 857.4 KJ] + 249.4KJ

= -1788.5KJ

round to -1789 KJ (4 sig figs)


please check my work...thanks


Furthermore, the negative answer makes sense since combustion reaction releases heat or energy from the system into the surrounding.

 

[BacardiGirl]

You should get an negative answer for Hrxn without having to change the sign at the end

Hrxn = Sum of Hf product - Sum of Hf reactant

Hrxn = [((3mol)(-393kJ/mol)) + ((3mol)(-285.5KJ/mol))] -

[((1mol)(-249KJ/mol)) - (4mol)(0 KJ/mol))]


[-1180.5 KJ - 857.4 KJ] + 249.4KJ

= -1788.5KJ

round to -1789 KJ (4 sig figs)




please check my work...thanks


Furthermore, the negative answer makes sense since combustion reaction releases heat or energy from the system into the surrounding.

The -1790 was already a given in the problem, but it wanted to know how much heat would be released by the complete combustion. (with the textbook answer being 4.78 x 10 to the third)

 

[Transformer]

So...my textbook is wrong?

Sorry forgot to add the last part of the question:
If a bottle of nail polish remover contains 155 g of acetone, how much heat would be released by its complete combustion?

Show me how you got your answer

 

[BacardiGirl]

155 g C3 H6 O x 1 mol/58 g C3 H6 O x -1790/1 mol C3 H6 O = -4.78 x 10 to the third

 

[Transformer]

155 g C3 H6 O x 1 mol/58 g C3 H6 O x -1790/1 mol C3 H6 O = -4.78 x 10 to the third

I think it's the wording that's kinda tricky here

I believe the answer is -4.78 x 10^3 kJ
The negative sign already denotes release of energy

but when you write 4.78 x 10^3 kJ is released then you no longer need the negative sign

It's improper to write -4.78 x 10^3 KJ is released, because there is no such thing as negative energy released...

Positive sign only shows that energy is absorbed
Negative sign only shows that energy is released

🙂

p.s. 10^3 = raised to the third.

 

[payme2]

They are asking for the heat released. It is assumed it is already released so q = just a number.


If they asked for the heat absorbed you give them just a number.

Now that if they asked for the heat transfer, you would have to be make sure you have the right sign.


I remember my Pchem teacher telling us to worry about the sign at the end of the problem. Solve out the equation and understand what is going on to determine the sign.