Change in Enthalpy of the Reverse Reaction

Matt_Fontila_2L_Chem14B
Posts: 39
Joined: Fri Sep 29, 2017 7:07 am

Change in Enthalpy of the Reverse Reaction

I remember in class today, Dr. Lavelle was calculating reaction enthalpies with us and he said something along the lines of the changes in enthalpies of the reverses of vaporization (condensation), melting (freezing), etc. were negative or inversely proportional or some other simple change from the original. I just missed what he explained.

If I were to calculate the enthalpy of freezing when given the enthalpy of fusion, do I make it negative or take the inverse or something else?

Matthew Lee 3L
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

Re: Change in Enthalpy of the Reverse Reaction

I believe you would just take the negative of the enthalpy if you are taking the reverse reaction.

Payton Schwesinger 1J
Posts: 45
Joined: Fri Sep 29, 2017 7:04 am

Re: Change in Enthalpy of the Reverse Reaction

If the reaction is reversed, then for the reversed equation you just switch the sign on the enthalpy from the original equation. To go from fusion to freezing, it would go from a positive to negative.

Hannah Chew 2A
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Joined: Fri Sep 29, 2017 7:05 am
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Re: Change in Enthalpy of the Reverse Reaction

Yes, you would just change the sign to indicate whether heat is being absorbed or released. When solid becomes liquid, the sign is positive since heat must be added to break bonds. If liquid froze to become solid, then the sign must be negative since heat must be released to form bonds.

Sandhya Rajkumar 1C
Posts: 50
Joined: Fri Jun 23, 2017 11:40 am

Re: Change in Enthalpy of the Reverse Reaction

You would make it negative, because for fusion, the process is endothermic and delta H is positive, which means the reverse reaction, freezing, would be exothermic and have a negative delta H.

RyanS2J
Posts: 32
Joined: Thu Jul 27, 2017 3:00 am

Re: Change in Enthalpy of the Reverse Reaction

With melting and vaporization (think melting an ice cube in your hand or boiling water on a stove), an input of heat is required to transform solid to liquid, and liquid to gas, respectively. Because heat is added to the system in order for these reactions to occur, the reaction is endothermic, with a positive change in enthalpy, or delta H. The exact same quantity of delta H applies for the opposites of these reactions (freezing for melting and condensation for vaporization), but since these reactions are proceeding in the opposite direction, we simply negate, or add a negative sign to, our previous positive, endothermic values of delta H.

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