Hess's Law

[Ashley Lopez 3J]

Whenever there's a problem regarding Hess's Law, I have trouble knowing which equations I am suppose to flip in order to get the final equation. Are there any right ways to do it or is it more of a trial and error?

[jessicaosuna_1F]

I usually just start with knowing what goes on the product side and how to manipulate the given equations from there to cancel out, but basically, it is just a little more logical trial and error.

[Stuti Pradhan 2J]

The best way to figure it out is to look at which compound appears as a product vs a reactants. Try to begin with "unique" compounds that are only in one of the equations and get those on the correct side. Once all the compounds are on the correct sides, you can figure out what needs to be removed and what equations can be used to do so.

Hope this helps!

[Christine Ma 3L]

I've found it helpful for these types of problems to write everything out. Like the previous posters, I like to start with the equations that have species that only appear in one equation, so I'll know exactly which way to flip the equation and what coefficient it needs. After doing all the equations that are like that, I'll see what still needs to be balanced/cancelled out at that point and use the remaining equations accordingly. Personally, this way is faster for me, but straight trial and error will probably still give you the right answer.

[Lauren Mungo 1K]

In regards to flipping an equation, you will first be able to tell when there is an equation that has a final product on the side of the reactants or vice versa. Then you know you have to flip it in order to get the substance on the correct side. Another way to know when to flip is if you need to switch the side a substance is on in order to cancel it out to get the correct compounds in your final equation. You just have to start trying to cancel things out and see what you have left!

[IshanModiDis2L]

Like mentioned above, I agree that Hess's Law problems is pretty much like a puzzle and you just have to try and put the info together. What I like to do first personally is get all the individual components on their correct sides , starting with the ones that only appear once which makes it less confusing if there is cancelling. Then I would add coefficients to get the proper ratios. I would keep track of what I did on the side in terms of manipulating and then conduct my changes to the heat values after.

[Daniela Santana 2L]

Hi! The way that I solve problems involving Hess's Law is by noticing what compounds the target reaction contains and then looking for those same compounds in the available reactions given. Then I think about how I can manipulate these reactions into giving me the target reaction in the end. A way that helps me determine whether a reaction needs to be flipped or not is by seeing which compounds that you don't need in a target reaction. A rule when adding reactions is that if a compound is on the reactant and product side of two reactions, the compound is cancelled. For example say I have an O2 on the products side in a reaction that I don't need for my final reaction but I need the other compounds that that reaction has. Say I also have another reaction that has another O2 on the products side. I can flip one of these reactions to get one of the O2 compounds on the reactants side so that when I add all of my reactions together, the unnecessary O2s will cancel. I hope this helped!

[Javier Perez M 1H]

Definitely, things on both sides are to be canceled.
However, how do we add the energy values and how do the coefficient ratios relate to them?

[Diana Aguilar 3H]

Thank you everyone for the explanations and tips, I also was having trouble with problems involving Hess's Law and this was really helpful!

[Charmaine Ng 2D]

I think part of it is definitely some trial and error, but it also helps to find the specific compounds in the given chemical equation, and to flip the equations according to the compounds you're looking for. Because you must end up with certain compounds, you also know that those specific ones must end up on a certain side. Therefore, you can start by flipping according to those! From there, you can use other equations to cancel out the compounds that don't belong in the final equation :))

[Sara Sandri 2B]

I like to look at Hess Law problems as just really big algebra problems. The biggest thing I like to do is just try stuff and see if it works. I also found that practicing the concept with problems (perhaps even ones from the textbook) is a really good way to get a grasp on things. the biggest tip I found is that if you have a compound in your final equation and it's present in all the other equations you need to manipulate, save it for last. Start handling the compounds in the final equation that appear only once or twice in the other chemical equations you have to adjust.

[Crystal Hsueh 2L]

I think it's usually a mix of trial and error and having a certain way about going at it. I tend to first check what is on the reactant and product side of the desired equation and then flipping the ones I need accordingly. Then I tackle the coefficients. It's really easy for everything to get messy once you start cancelling things out so I think the most important part is to stay organized and manipulate the delta H or K (depending on what the question is asking) as you're manipulating the equations.

[Ashley Kim 3H]

I think for Hess's Law there is some trial and error you have to do when solving a problem. I think I always try to cancel things out in a way so that the products will end up in the products side and reactants will end up in the reactants side.