When to use which?

[Sulaimanshah Syed 3E]

I get how there are multiple ways of finding the enthalpies, but when do we know to use which one? Or does it not matter and we can use whichever one?

[Shimona_Gupta_3I]

I believe that there are three main ways to calculate enthalpies. The first method is using Hess's Law, this when we know the enthalpies of each step of a reaction from a multistep reaction and then add all of the enthalpies to get the total enthalpy change. This method would be used when in the problem we are given multiple reactions with their enthalpies of reactions and we manipulate and add the enthalpies to get to our final reaction. The second way to calculate the enthalpy of a reaction is by using bond enthalpies. In this, you look at the bonds broken on the reactant side and the bond formed on the product's side. Then you add the bond enthalpies together. The bond enthalpies will either be given to you or you can find them in a table of bond enthalpies. Something to keep in mind with this method is that to break bonds on the reactants side energy is required so the bond enthalpies will be positive however on the products side, bonds are formed that releases energy and therefore the bond enthalpies will be negative. Also keep in mind that when adding the enthalpies, if taking them from the table you are given the enthalpies of breaking bonds in the gas form, so if you have a different phase you have to add the phase change enthalpy as well. You will use this second method when given the bond enthalpies for different bonds used in the reaction, then you have to determine which bonds are broken and formed to calculate the enthalpies. The third way to calculate enthalpies is through the standard enthalpy of formation. The standard enthalpy of formation is the standard reaction enthalpy for one mol of a substance from its elements. If given a chemical reaction then you can calculate the enthalpy of a reaction by using the standard enthalpy of the formation of the products and subtracting the standard enthalpy of the formation of the reactants. You would use this method when given the enthalpy of formation for the products and reactants as opposed to being given the bond enthalpy.

Hope this helped!

[Karsten Wise 3D]

As always I think the best bet is just to look for constants given by the question and use whichever one makes sense to use based upon the information given. I always sort of panic when I know there is multiple ways to find a solution but focusing on what's given has always helped me tremendously!

[dsmith1E]

The majority of the time in this class, I imagine we'll be asked to calculate the enthalpy and be given the measurements/units for one particular method of finding the enthalpy (ex. we'll be given the bond enthalpies), so it should be pretty clear. If we have access to all of the necessary measurements, I'm sure the question will specify which method they'd like us to use.

[Katherine Phan 1J]

To decide which method to use, you should consider what is given to you in the problem (reaction enthalpies, bond enthalpies, standard enthalpies of formation, etc.) as well as which way is most convenient. Because enthalpy is a state function, it does not matter which path or method you use to obtain it, so you should always look for the shortest/easiest way to find it based on the given information.

[Devin Ma 3A]

I would go along with Katherine, reread the question and identify what information has already been given to you and what you must find! With the given information related to enthalpy, you will then be able to identify the appropriate method to approach answering the question!

[306022612]

To find enthalpy, you do what you would do for any other chemistry problem: use what is given to you. Based on what's provided it should be pretty apparent what they're asking you to find and what's given. From there all you do is find what you're trying to solve for!

[Neha Humbe 3E]

It really depends on what information you are given. If you are given the enthalpies of multiple reactions you can use Hess's method by adding the delta H's together. If you are given bond enthalpies, you can use that to calculate the delta H of a reaction. And if no bond enthalpies are given you can calculate the standard reaction enthalpy by using the standard enthalpy of formation of all products and subtract the standard enthalpy of formation of all reactants. Hope this helps!

[Graciela Ayala 2I]

I agree with what everyone else was mentioning, based on what information you were given. But to also add to that is to include which equation would get you the answer in the shortest way. Some might have the same given information they take in but you should also pay attention to what the question is asking and what is the easiest way to get there.

[Eden Burkett 2C]

I was confused by this as well but the way I approach it now is to write down what is given first and choose a method second. The givens should help narrow down which method to use!

[Lilian Doan 1c]

Hi! Similar to what everyone else said, we have to look at what is given and the context of the problem. For example, if we are trying to find the enthalpy of a multistep equation and are given many enthalpies that can be manipulated, we are most likely using Hess' law. Another example would be using bond enthalpies when given and we know which bonds are being formed or broken.