Exothermic Rxns & Temperature

[PagingDr.F]

For a reaction that is exothermic and proceeds with an increase in entropy, how will increasing temperature affect ΔG?

ΔG = ΔH - TΔS

ΔH is negative because it's exothermic and the question states ΔS is positive. ΔG is negative.

If you increase T, ΔG will become more negative and the rxn is more spontaneous (this is the correct answer)

But if Heat is a product (the rxn is exothermic), wouldn't increasing the T (i.e, adding more heat) shift the rxn towards the endothermic?

ΔG becoming more negative but the rxn shifting to the reverse is contradictory. Which happens?

---

Members don't see this ad.

 

[Nugester]

Gibbs free energy is the maximal energy available to to do work from a chemical reaction. A positive or negative sign indicates whether the reaction will proceed spontaneously ( ie requires an input of energy or not).

Chemical equilibrium in a reaction is reached when the forward and reverse reaction rates occur at equal rates so that the concentrations don't change with time. At chemical equilibrium, Gibbs free energy is at its lowest.

So...for your example above, for an exothermic reaction and you increase temperature, equilibrium constant will decrease as you shift the reaction back to the left (reactants). You don't make the reaction endothermic though.

 

[PaulTownMD]

Entropy is state dependent and thus a function of both temperature and matter phase (s/l/g), and matter phase is a function of pressure and temperature. Therefore entropy is a function of temperature (second order) and pressure. In this process, ΔH does not change because ΔH is a function of system pressure and volume, though less heat is released in a stoichiometric sense because of Le Chatelier. The higher temperature guarantees an increase in TΔS, thus making ΔG more negative. The MCAT won't require knowledge of thermodynamic or kinetic functions beyond gen chem/orgo level, but skimming the basics of physical chemistry (thermodynamics and kinetics, no need for quantum* or statistical mechanics) can help explain these processes on a more fundamental level.
*It's useful to know spin states/Pauli exclusion/quantum numbers