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### Topic: Hess law question  (Read 3521 times)

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#### Zyzz

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##### Hess law question
« on: March 27, 2011, 01:04:53 PM »
Hey,

I have this question regarding Hess law but I haven't been able to wrap my head around it, hopefully someone can enlighten me on how to go about it

An investigation was performed to test Hess's law.  A measured mass of cycloheptane was completely burned in a bomb calorimeter.  The following data was generated.

7C(s) + 7H2(g) --> C7H14(l)            Hf = -156.6 kJ/mol
C(s) + O2(g) --> CO2(g)                    Hf = - 393.5 kJ/mol
H2(g)  ½ O2(g) --> H2O(l)                Hf = - 285.8 kJ/mol

­Using the information from the investigation, what is the predicted molar enthalpy of combustion of cycloheptane?

#### opti384

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##### Re: Hess law question
« Reply #1 on: March 28, 2011, 02:07:18 AM »
First of all do you fully understand Hess's Law?

The next step will be writing the equation of combustion of cycloheptane.

#### phoenixcelestius

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##### Re: Hess law question
« Reply #2 on: April 06, 2011, 10:57:17 PM »
Basically Hess's Law says that you can manipulate chemical equations and their Hf (a lot like solving algebraic systems of equations with the elimination method) so that you get the final reaction and Hf that you want. That's a clue.