Does phase matter in Hess's Law?

[Jenaye Brelland 2I]

Does phase matter in Hess's Law? Can I cancel something out even if they have different phases?

[Aaron Akhavan-Dis1B]

You can't cancel if the phases of the same compound are different. They have to be the exact same phase and compound in order to be able to cancel if they are on opposite sides of different equations.

[Emily_Stenzler_2H]

You can't cancel if the phases of the same compound are different. They have to be the exact same phase and compound in order to be able to cancel if they are on opposite sides of different equations.

Do you know why this is? Is this because certain values change based on the phase? If so, what values change?

[Aaron Akhavan-Dis1B]

I think the way that each deltaH value is given is based on the standard enthalpy formation method, meaning each compound and its different phases have their own value. So we just manipulate the equations in a way that we can find the overall equation we want even if we don't know the specific numbers for each compound.

[Edwin Liang 1I]

Phases matter in Hess's Law. Like in the sapling homework for weeks 3/4, you can use a given reaction to change the phases.

[AlbertGu_2C]

For Hess's Law, it is useful to think of the same elements that have different phases as completely different elements when you add and manipulate equations

[Adrienne Chan 1G]

Yes, phase does matter in Hess's law. It takes energy to go from one state to another, so you need to take that into account for Hess's law.

[rita_debbaneh2G]

Yes. It takes different amounts of energy to break the bonds of a compound that is, say, a gas as opposed to it's liquid counterpart.

[Siwa Hwang 3G]

Phase matters in all aspects of chemistry because heat moves in and out whenever there is a heat change

[305405193]

does Lavelle speak over this? by any chance specifically in his lectures?

[Kaihan_Danesh_2J]

The phase of the compounds do matter because often times heat is absorbed or released to break or form bonds when changing the compounds to their different phases in order to allow them to react. The heat from these processes must be considered when applying Hess's Law.

[Gabriel Nitro 1E]

Hi,

Yes, it does!

You always have to make sure one portion of the chemical equation is equivalent to the other side if you were to combine two equations into an overall equation (i.e. He(g) on the left and right side for an arbitrary reaction). However, if you had He(s) on the left side and He(g) on the right side, the phases do matter and you could not cancel them out right away!

Hope this helps! :)

[Eileen Quach Dis 2A]

Yes, phase does matter in Hess's law. It's not enough to just look for the change in enthalpy of the compound name.

[Sara Sandri 2B]

Yes!! It does matter. If two compounds that chemically look the same are on opposite sides of a reaction but their phases are different, you can not cancel them out since they are not in the same physical state.

[Crystal Hsueh 2L]

Yes phases matter in Hess's law. If you remember for some of the Sapling questions when the delta H index list was provided for us, The delta H values for the same compound was different with each phase it is in. For example, delta H of H2O(l) is -285.8 while delta H of H2O(g) is -241.8. So this is why you cannot cancel out substances of different phases even if they have the same molecular formula. It throws off the entire calculation.

[Ashley Kim 3H]

Yes, they do matter! Two compounds would have to be the same phase to "cancel out" when using Hess's law.

[Jenaye Brelland 2I]

ahhh okay thank you!

[Adam Bustamante 1I]

I believe that the states have to match when using Hess' Law and cancelling them out because they have to be the same coefficient and phase for them to truly cancel each other out.

[Michelle Nguyen 3F]

Yes, phases matter in Hess’s Law! It takes energy to go from one phase to another

[Jeremy Wei 2C]

Yes phases do matter in Hess's Law since the change from one phase to another phase involves releasing/absorbing energy.

[LarisaAssadourian2K]

I agree with the previous posts! Phase does matter in Hess's law, because it takes energy to change phases, so it's important to take them into account.

[OmarArafat_2K]

Remember they have to be the exact same phase and coefficient for them to cancel each other out. The phase is really important in Hess's Law.