Why is Hess's Law Generally Not Obeyed?

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In summary, Hess's Law is generally not obeyed in cases where the reaction conditions and/or paths vary.
  • #1
BrianC12
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Had a recent midterm and got this question wrong: why is Hess's Law generally not obeyed? Any ideas?

Thanks.
 
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  • #2
What do you understand by Hess' law?
 
  • #3
That enthalpy is a state function and when chemical reactions are added, their enthalpies must be added as well. Is the answer the fact that it only accounts for the energy in the chemical bonds and not the heat or work exchanged with the surroundings?
 
  • #4
Can you give an example where it is violated? Since enthalpy is a state function I don't see why it would not be obeyed.
 
  • #5
mishrashubham said:
Can you give an example where it is violated? Since enthalpy is a state function I don't see why it would not be obeyed.

If what I said above is right, then I'm guessing the professor meant why is Hess's Law not obeyed in real world examples where conditions and paths can vary. I feel like he should have included that in the question though...oh well. If what I said isn't right then I wouldn't know under what circumstances Hess's law wouldn't be obeyed either.
 
  • #6
Well, as has been said, Hess's law relates to the difference of state functions so is obeyed under any conditions which these hold.

So you need to enquire what is there about most chemical reactions that disobeys these conditons.

I suggest the answer is not in the reactions themselves or the sequence of reactions that make up a Hess chain, but the quantities of reactants themselves.

The ΔHs only hold good for reactions where the reagents are in the correct molar proportions.
 
  • #7
But even if you don't have enough of one type of reagent, it's sort of intuitive that the resulting change in enthalpy would just be a fraction of the number of moles that reacted, which wouldn't make Hess's Law wrong...right?
 

FAQ: Why is Hess's Law Generally Not Obeyed?

Why is Hess's Law not applicable in certain situations?

Hess's Law is based on the principle of conservation of energy, which states that energy can neither be created nor destroyed. This means that the total energy change in a reaction should be the same, regardless of the pathway taken. However, this assumption does not always hold true in real-life scenarios, leading to situations where Hess's Law is not applicable.

In which circumstances does Hess's Law not apply?

Hess's Law is not applicable in reactions where there is a change in the state of matter, such as solid to liquid or gas to liquid. This is because the energy required for these state changes is not accounted for in the Hess's Law equation.

Can Hess's Law be used for reactions involving complex molecules?

No, Hess's Law is not applicable for reactions involving complex molecules. This is because the enthalpy of formation of these molecules cannot be accurately determined, making it impossible to calculate the total energy change using Hess's Law.

Why does Hess's Law not work for reactions that involve non-standard conditions?

Hess's Law is based on the assumption that the reactions are taking place under standard conditions, which include a constant temperature of 298K, pressure of 1 bar, and concentration of 1 mol/L. If these conditions are not met, the energy changes in the reaction may not follow the expected pattern, making Hess's Law not applicable.

Are there any other limitations of Hess's Law?

Yes, Hess's Law is also not valid for reactions that involve significant changes in temperature. This is because the enthalpy of a reaction is temperature-dependent, and for large changes in temperature, the heat capacity of the reactants and products must be taken into account, making Hess's Law not applicable.

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