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Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:05 pm
by Matt_Fontila_2L_Chem14B
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?

Re: Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:07 pm
by Matthew Lee 3L
I believe you would just take the negative of the enthalpy if you are taking the reverse reaction.

Re: Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:10 pm
by Payton Schwesinger 1J
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.

Re: Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:11 pm
by Hannah Chew 2A
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.

Re: Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:14 pm
by Sandhya Rajkumar 1C
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.

Re: Change in Enthalpy of the Reverse Reaction

Posted: Wed Jan 10, 2018 3:20 pm
by RyanS2J
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.