sign of deltaH in sublimation

[dentalstudent2021]

what would deltaH be in a sublimation rxn ?

It seems to me that when energy is put into it (+ deltaH) the bonds are broken and energy is released (-deltaH).

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[Frank22]

In sublimation we are transitioning from a solid directly to a gas, which requires the system to absorb heat, and is therefore endothermic. Ergo, the enthalpy change is positive.

 

[DAT Destroyer]

what would deltaH be in a sublimation rxn ?

It seems to me that when energy is put into it (+ deltaH) the bonds are broken and energy is released (-deltaH).

This is an important question that you will see on the DAT... COVER ALL YOUR BASES.........solid------->liquid------->gas.......all these forward processes involve ADDING thermal energy, and will be ENDOTHERMIC +H......thus melting, boiling, and subliming will all be positive......The reverse reactions are exothermic..having negative enthalpy values........freezing... condensing, and deposition.

Hope this helps.

Dr. Romano

 

[nucshuc]

Are the magnitude of the enthalpy change exactly the same, but reversed (made up examples: sublimation is +500kJ, deposition is -500kJ)?

 

[Frank22]

Are the magnitude of the enthalpy change exactly the same, but reversed (made up examples: sublimation is +500kJ, deposition is -500kJ)?

Yes.

You can see this from the formula for enthalpy change, which is the sum of the enthalpies of the products minus the sum of the enthalpies of the reactants; if one reverses the phase change, then the reactants become the products and vice versa, so the sign is merely reversed.

An important note is that phase changes are never spontaneous or nonspontaneous at all temperatures; if the reaction is exothermic, then it is accompanied with a decrease in entropy, and vice versa. This is also a good way to determine if a phase-change is endothermic or exothermic; for example, if the phase change is accompanied with an increase in entropy (e.g., vaporization), then the reaction is endothermic (i.e., the phase change is only spontaneous at high temperatures).