enthalpy and entropy

Moderators: Chem_Mod, Chem_Admin

Posts: 93
Joined: Thu Jul 11, 2019 12:17 am

enthalpy and entropy

Postby AMahadi » Thu Feb 13, 2020 3:07 pm

how can the entropy change for a reaction be positive if the enthalpy change is negative?

Alicia Lin 2F
Posts: 83
Joined: Wed Sep 18, 2019 12:17 am

Re: enthalpy and entropy

Postby Alicia Lin 2F » Thu Feb 13, 2020 4:58 pm

Rearranging the equation for Gibbs free energy (G=H-TS), we can get TS=H-G. Using this equation, we can see that entropy (S) change can be positive if the change in Gibbs free energy (G) is more negative than H. If G is more negative than H, we are adding on a value that is greater than H, therefore making TdeltaS positive.

Amy Pham 1D
Posts: 103
Joined: Fri Aug 09, 2019 12:15 am

Re: enthalpy and entropy

Postby Amy Pham 1D » Fri Feb 14, 2020 10:33 am

For exothermic spontaneous reactions, we know that the exothermic property indicates that the change in enthalpy must be negative, and the fact that it is spontaneous implies that the change in entropy must be positive according to the second law of thermodynamics. Enthalpy and entropy are not directly correlated in that one has to follow the sign of the other; they may have different signs.

Posts: 105
Joined: Sat Aug 17, 2019 12:16 am

Re: enthalpy and entropy

Postby Mulin_Li_2J » Fri Feb 14, 2020 12:02 pm

You probably see the equation ΔS = ΔH / T and ask this question. To explain this, first remember that we have two different forms of entropy: thermal entropy and residual entropy. For an exothermic reaction, its thermal entropy does decrease according to the equation. However, for some chemical reactions that produce a large amount of gas molecules, increase in residual entropy supersedes the decrease in thermal entropy. As a result, some reactions have the combination of a positive ΔS and a negative ΔH.

Hope this can help! -_+

Posts: 47
Joined: Mon Nov 18, 2019 12:18 am

Re: enthalpy and entropy

Postby WGaines_2E » Mon Mar 09, 2020 10:09 pm

basically if dS is very negative (very disordered) and |TdS|>|dH| then the dG becomes positive

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 2 guests