[Jonie]

Hi, I'm a newbie, and I was wondering why is it that when the enthalpy change is calculated through Hess Cycles, the value is different from when it's calculated through Mean Bond Enthalpy Data?

I've just done one for a reaction, and my answers for both values are right; I just wanted to know why the theoretical value is higher than the experimental one.

Experimental value= -5762 kJmol^-1
Theoretical value = -4240 kJmol^-1

So... if the exp'l value is more negative then it's more exothermic, and releasing more heat. But why is it releasing more heat in the experiment; why doesn't the method for calculating the mean bond enthalpy through Bond Data account for this?

 

I'm really confused; could someone help, please?

[Borek]

What reaction?

[Jonie]

Oops, sorry:

3C₄H₁₀ + 13O₂ -------> 8 CO₂ + 10H₂


I've made a Hess Cycle: H₂ + 3O₂ + C

C₄ H₁₀ = -125.0
CO₂ = -394.0
H₂ = = -286.0

(teacher suggested we leave the Oxygen alone in the top equation as no data was given.)

I've calculated DeltaH = 2(125) + 8(-384) + 10(-286) = -5762kJmol-1


As for the Bond Enthalpy using the bond data given:

C-H = 413 kjmol-1
C-C = 347
O=O = 498
C=O = 736
O-H = 464

Which is...
Reactants: 10 (C-H) = 4130, 3 (C-C) = 1041 all x2
and 13 (O=O) 3474
(4130 + 1041) + 6474 = 16818kJmol -1


Products: 8(O=C=O) = 8(736x2) = 1176
10 (O-H) = 10(464x2) = 9280

1176 + 9280 = 21056 kJmol-1


Mean bond enthalpy therefore is 16816 - 21056 = -4240kj-mol



I don't understand why those two values are different- my Chemistry teacher suggested thinking about it, but I'm afraid I don't quite get it. Hope someone can explain this to me in simple terms- I don't think he's looking for a very complex reason either because we're only AS level Chemistry!

[BHAMirza]

Hi, I'm a newbie, and I was wondering why is it that when the enthalpy change is calculated through Hess Cycles, the value is different from when it's calculated through Mean Bond Enthalpy Data?

I've just done one for a reaction, and my answers for both values are right; I just wanted to know why the theoretical value is higher than the experimental one.

Experimental value= -5762 kJmol^-1
Theoretical value = -4240 kJmol^-1

So... if the exp'l value is more negative then it's more exothermic, and releasing more heat. But why is it releasing more heat in the experiment; why doesn't the method for calculating the mean bond enthalpy through Bond Data account for this?

 

I'm really confused; could someone help, please?

wait, if you say experimental, you mean you did the reaction, or you just took few reactions and made the hess cycle

whatever .. i guess the explanation for your problem is rather simple, the values which are taken from the data booklet are experimental.. maybe they are very precise, but none can measure anything perfectly  .. you know that errors are always possible (this may not be an error .. as it is too different.. .. lol :confused:)
what im trying to say is that the values which you get from an experiment are always different from the theoretical values ..  even scientists which are equipped with the top equipment, can not get 100% correct results, so if they make small mistakes, at every value, that mistakes can sum up to a bigger mistake ... and that way you get different experimental from theoretical results ..
i aint claiming anything .. this is just my oppinion .. mayb im wrong ..  :Dr_Evil:

[Jonie]

We were given the top equation, and we had to make a Hess cycle.


I got my bottom equation from C + O2 ---> CO2
and
H2 + 1/2O2 ----> H20


Is it the mistakes/apparatus error while doing the experiment? Because in a way it's still a theoretical value, since we never did the reaction. The numbers I used to calculate the 'experimental' value are ones provided to us.

[BHAMirza]

yes, but those who provide you with that values, may make mistakes .. soo ..

[Jonie]

I doubt the examining board will! It's on an external exam paper :p

[Borek]

3C₄H₁₀ + 13O₂ -------> 8 CO₂ + 10H₂

Perhaps before calculations you should balance the equation?

[Jonie]

I'm sorry, it is balanced, the 3 moles of C was a typo. It's actually 2.  

[Borek]

I'm sorry, it is balanced, the 3 moles of C was a typo. It's actually 2.  

Still it is not.

[plu]

Bond enthalpy values cannot be relied on to calculate the enthalpy change for a reaction.  They are merely averages of the energy needed to break the particular bonds and, as bond energies vary from compound to compound, large degrees of error will result if they are used for enthalpy calculations.  They should be used exclusively for estimates.