DAT Bio- An exergonic reaction is spontaneous?

[helloboys]

Hi, I am doing a BC practice exam and it said "Exergonic reactions release free energy, and therefore are spontaneous. Endergonic reactions require free energy input and are nonspontaneous." But like doesn't it depend on delta H and delta S. Like freezing of water is spontaneous only at low temperatures. I know Dr. Mike for general chem said you have to look at delta H and delta S. This is really confusing, please help!

---

Members don't see this ad.

 

[Auntymarkovnikov]

For entropy (delta S) think of it like this: gases are in the air and solids are on the ground, so when we go from gas to solid we go down... or we decrease (think negative). So when you go from gas to solid phase or gas to liquid phase we have a negative Delta S, or entropy. Conversely, going from a solid state to a liquid or gas state, you have positive entropy (Delta S), think "rising up" (solid to gas)

 

[Auntymarkovnikov]

Also, enthalpy (delta H) is negative in exothermic rxns and positive in endothermic rxns

 

[Auntymarkovnikov]

Then once you have the sign of Delta H and the sign Delta S, basically you just use basic quantitative reasoning to figure out if you need a big T or a low T to result in a negative Delta G (spontaneous) or positive Delta G (non spontaneous) .
I learned this from the destroyer book

 

[smilesandwaves]

So, you know that for a reaction to be spontaneous, it must have delta G less than 0. You have probably seen these reaction graphs before! so since Gfinal is less than Ginitial-->delta G is less than 0-->spontaneous!(and opposite for endergonic) i remember doing a double take on this exact issue before too. Hopefully visualizing it with something you've seen before helps.

 

[artist2022]

I think you know H and S, and you're right water freezing is spontaneous only at low temperatures. But what you posted: Exergonic reactions release free energy, and therefore are spontaneous. Endergonic reactions require free energy input and are nonspontaneous. The concept here is the exact same as it is in enthalpy, with the keyword being "free" energy. You're told that you're using free energy (Gibbs free energy). Just as an exothermic rxn releases heat, an exergonic reaction releases free energy. So here, you don't need to know anything about H or S to determine if it's spontaneous or not. Just know this: if it releases free energy, it means G must be less than 0 and is therefore spontaneous/exergonic. If it requires free energy, then G is greater than 0 and is therefore nonspontaneous/endergonic. Hope this helps 🙂

 

[DAT Destroyer]

Hi, I am doing a BC practice exam and it said "Exergonic reactions release free energy, and therefore are spontaneous. Endergonic reactions require free energy input and are nonspontaneous." But like doesn't it depend on delta H and delta S. Like freezing of water is spontaneous only at low temperatures. I know Dr. Mike for general chem said you have to look at delta H and delta S. This is really confusing, please help!

Most exothermic reactions are spontaneous and therefore have a negative Free Energy, BUT not all. Some exothermic reactions may not be in the same direction as the free energy change. If a reaction is exothermic AND spontaneous, the correct term will be exergonic. Referring to a reaction as exergonic also takes into account the entropy change.

Hope this helps.

Dr. Romano