Using Hess's Law

[105691548]

Could someone provide another example where the Hess Law would need to be used because I am still confused on when to apply this law.

[Nicole Ju 3H]

If you are given multiple reactions and asked to calculate the enthalpy change for the net overall reaction, you can add the individual enthalpy changes to get the overall enthalpy change.

This site provides a pretty good example: https://www.chemteam.info/Thermochem/He ... tro1a.html

[Tala Ayoub 1H]

Hess's law is pretty straightforward, it fundamentally builds up on the idea that enthalpy is a state function and therefore, it can be added or subtracted, therefore, if you have a multistep reaction you can either add or subtract the enthalpy values together to deduce the total enthalpy change.
Here is another detailed example: https://www.westfield.ma.edu/PersonalPa ... xample.htm

[Allen Mayo 1E]

Hess's Law is just the the previous idea mentioned in the previous lecture that enthalpy are state properties that can be added or subtracted. Therefore, it is why you see Prof. Lavelle add the multi-step reaction of the nitrogen oxide to form nitrogen dioxide example. He simply adds the two as soon as he figures which to cancel between the multi step reaction which he does in 2NO. He just then adds the two deltaH. Goes the same way in its reverse reaction, just flips the signs.

[Jordan Kaplan 2E]

If you remember in outline 2 when we had to calculate to overall K value when given multi-step chemical equations. We would add K1 and K2 according after adjusting the ratios to get the K value for the overall equation. The same goes for enthalpy and calculating H for the overall equation, it is just named Hess's law.

[Nishan Reddy 3K]

Hess's law should be applied when you want to know the delta h for an overall reaction but you are given the intermediate reactions where the reactants goes through multiple transformations before becoming the final products. Thus, since we do not care about this pathway because enthalpy is a state variable, we can simply add the enthalpies for the intermediate steps (with appropriate manipulations of the chemical equations/delta h) as long as the sum of the chemical equations equals the overall reaction we want to observe.