Enthalpy, Entropy, Gibbs Free Energy, Spontaneity

unfavorable

works against spontaneity

the reaction is less likely to occur without extra input

favorable

supports spontaneity

It is more likely the reaction will occur on its own

enthalpy favorable

when a reaction releases energy, -H

enthalpy unfavorable

when a reaction required energy to be put in, +H

entropy favorable

when disorder increases, +S

entropy unfavorable

when disorder decreases, -S

How to determine if a reaction is enthalpy favorable

• if the reaction forms stronger bonds (more bonds) in the products compared to the reactants

  • because stronger bonds release more energy when formed

• if phase changes to more condensed phase

  • ex: gas to liquid, liquid to solid

• if heat is a product

• nonpolar bonds to polar bonds

How to determine if a reaction is entropy favorable

• if products are more disordered than reactants

  • breaking apart large molecule into small molecules

  • forming gas molecules from liquids or solids

• if reaction produces gases

• if a substance dissolves/mixes

Spontaneity

-G = spontaneous

+G - non-spontaneous

-H, +S = spontaneous at all temps.

+H, - S = nonspontaneous at all temps

-H, -S, spontaneous at low temps.

+H, +S, spontaneous at high temps.