### Hess's Law

Posted:

**Wed Jan 10, 2018 8:25 pm**When using Hess's Law, is there ever a chance that none of parts of the involved reactions will cancel each other out? If so, how would you write the net reaction?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=25249

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Posted: **Wed Jan 10, 2018 8:25 pm**

When using Hess's Law, is there ever a chance that none of parts of the involved reactions will cancel each other out? If so, how would you write the net reaction?

Posted: **Wed Jan 10, 2018 8:41 pm**

You should be able to change the equations so that you can cancel things. For example, you could reverse the direction of one equation. Or you could multiply an equation's coefficients by 2. These changes would make it possible to cancel something that needs to be cancelled. I do not think there will be a situation where you are completely unable to cancel anything. But I could be wrong.

Posted: **Wed Jan 10, 2018 9:14 pm**

If you choose to reverse the reaction or multiply by coefficients then you would have to change the enthalpy of the reaction as well. If the reaction is reversed, you multiply the enthalpy by -1 or by the coefficient that you multiplied the equation by.

Posted: **Wed Jan 10, 2018 9:25 pm**

They will always cancel each other out because hess’s Law is just a representation of steps that give the final output. Therefore, if some didn’t cancel out, there would be a different number of products

Posted: **Thu Jan 11, 2018 1:00 pm**

I found in question 8.55 that the O2 term was multiplied by 1.5 to cancel out but the rest of the equation remained the same. How is this allowed?

Posted: **Fri Jan 12, 2018 12:40 pm**

In regards to multiplying by 1.5 to cancel out the reactants, you can do this because enthalpy is a state function, which means it is additive. By multiplying the coefficients in the equation you must also multiply the delta H by the same equation. Doing this makes the equation still the same, just a larger multiple of the base equation, and therefore helpful for us in the problem. Reflect on Chem 14A when we did this to make net equations as well.

Posted: **Fri Jan 12, 2018 12:51 pm**

I think in Hess's Law problems, the stoichiometric coefficients are often fractions to make the math easier. Multiplying by fractions just means you multiply the enthalpy by the same fraction as well.

Posted: **Sun Jan 14, 2018 1:00 pm**

The only situation in which you wouldn't be able to cancel out intermediate compounds is if you haven't fixed the coefficients at all or there was a mathematical mistake in doing so. Otherwise, you can be confident in knowing that a Hess's Law problem is solvable. Be sure to double check coefficients if you're having trouble as not all problems will require a simple 'multiply the equation by 2'. You may need more complex fractions in order to make intermediate steps cancel out, like the problem Prof. Lavelle did in lecture requiring a factor of 3/2.

Posted: **Sun Jan 14, 2018 7:21 pm**

From my understanding, the reactions that are given will always be able to be added (whether the direction needs to be reversed or not) to yield the desired reaction. Hope this helps!