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Topic: Hess's Law with Three Equations  (Read 1382 times)

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Offline MilkyCar

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Hess's Law with Three Equations
« on: April 23, 2023, 02:20:32 PM »
I'm confused and I've been trying to solve this problem.

So it states that given the following data:
C2H4 (g) + 3O2 (g) -> 2CO2 (g) + 2H2O (l)  H= -1411.0 kJ
2C2H6 (g) + 7O2 (g) -> 4CO2 (g) + 6H2O (l) H = -3119.8 kJ
2H2 (g) + O2 (g) -> 2H2O (l)  H = -571.7 kJ

And I have to calculate H for the reaction:
C2H4 (g) + H2 (g) -> C2H6 (g)

I didn't change anything for equation one and then I did -1411.0 - -3119.8 + (-571.7) and then I divided the answer by 2 to get 568.6 kJ. I am stuck and I'm unsure how to get the right answer.

Offline Hunter2

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Re: Hess's Law with Three Equations
« Reply #1 on: April 23, 2023, 03:44:11 PM »
Sum of rectants minus sum of products.
Substitut C2H4 + H2 => C2H6 to the given equations.
You take the first equation how it is for C2H4.
For H2 you have to take the building of water by half and for C2H6 you also have to subtract the second equation by half.
« Last Edit: April 23, 2023, 04:11:09 PM by Hunter2 »

Offline MilkyCar

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Re: Hess's Law with Three Equations
« Reply #2 on: April 23, 2023, 05:01:26 PM »
Okay, I tried that and I got -137.0 kJ as a result. Is this correct?

Offline Hunter2

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Re: Hess's Law with Three Equations
« Reply #3 on: April 23, 2023, 05:02:13 PM »
Its correct

Online Borek

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Re: Hess's Law with Three Equations
« Reply #4 on: April 23, 2023, 05:21:17 PM »
Assuming I made no typos, you took first equation, subtracted second and added third, yes?

  C2H4 + 3O2  :rarrow: 2CO2 + 2H2O
- (2C2H6 + 7O2  :rarrow: 4CO2 + 6H2O)
+ (2H2 + O2  :rarrow: 2H2O)
--------------------------------------------
C2H4 + 3O2 - 2C2H6 - 7O2 + 2H2 + O2  :rarrow: 2CO2 + 2H2O - 4CO2 - 6H2O + 2H2O

sorting
C2H4 - 2C2H6 + 3O2 - 7O2 + O2 + 2H2  :rarrow: 2CO2 - 4CO2 + 2H2O - 6H2O + 2H2O

combining
C2H4 - 2C2H6 - 3O2 + 2H2  :rarrow: -2O2 - 2H2O

moving things with negative coefficients to other side
C2H4 + 2H2 + 2CO2 + 2H2:rarrow: 2C2H6 + 3O2

Is the final equation the one you are looking for?
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Offline Hunter2

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Re: Hess's Law with Three Equations
« Reply #5 on: April 23, 2023, 06:02:56 PM »
No
C2H4 + 3 O2 => 2 CO2 + 2 H2O -1411 kJ
Convert first
C2H6 + 3,5 O2 => 2 CO2 + 3 H2O -1559,9 kJ
C2H6 = - 3,5 O2 + 2 CO2 + 3 H2O
and H2 + 0,5 O2 => H2O  - 285,85 kJ
H2 => H2O - 0,5 O2

C2H4 + 3 O2 + H2 - 3,5 O2 + 2 CO2 + 3 H2O => 2 CO2 + 2 H2O + H2O - 0,5 O2 + C2H6
C2H4 + H2 => C2H6
-1411 kJ + (-285,85 kJ) -(-1559,9kJ) = 136,95 kJ

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