Non-spontaneous exothermic reaction

[Fiona Huang 3C]

How can an exothermic reaction be nonspontaneous? What would be an example of this?

[Josh Chou 3K]

Combustion reactions are good examples of exothermic reactions that are also nonspontaneous. For example, the combustion of methane with oxygen gas to produce carbon dioxide and water (CH₄(g) + 2O₂(g) --> CO₂(g) + 2H₂O(g)) is exothermic because it releases heat, but it is nonspontaneous because it requires an external heat source to proceed with the forward reaction. Using the ΔG_f° values in Appendix 2A of the textbook, the sum of the ΔG_f° values of the products and reactants is [-50.72 + 2(0)] - [-394.36 + 2(-228.57)] = 800.78. Because ΔG_r°is positive, it is nonspontaneous. However, the combustion of methane releases ~890 kJ/mol as heat, and you could calculate the temperature at which this process becomes spontaneous by setting ΔG to 0 and then plugging the ΔH° and ΔS°values of the reaction into the Gibbs free energy equation

[Kiran Marla]

This would occur when the entropy is negative, and the temperature is very high.

[Jenny Lee 2L]

Based on the equation ∆G = ∆H-T∆S, we see there are two factors that make a reaction spontaneous or nonspontaneous, ∆H and ∆S. So if ∆H is negative (in other words the reaction is exothermic), a negative ∆S could make the overall ∆G positive (a nonspontaneous reaction even though exothermic).

[Keon Amirazodi 3H]

An exothermic reaction can be non-spontaneous when deltaS is negative and the temperature is high.

[Earl Garrovillo 2L]

Spontaneous processes are when ∆G = ∆H-T∆S is negative. In an exothermic process, ∆H is negative but this doesn't always mean ∆G is also negative. If ∆S is negative and the temperature is high enough, ∆G can become positive. I think this is called an entropy-driven reaction or entropy is dominant. In any case, despite the process being exothermic, at the end of the day if ∆G is positive, the process is nonspontaneous.

[Mari Williams 1K]

An exothermic reaction can be nonspontaneous if the change in entropy is negative and the temperature very high, making it a entropy dominant reaction. Spontaneity of a reaction depends on the sign of delta G, not delta H

[Emily Vu 1L]

i know it's counter-intuitive and i remember dr. lavelle saying non-spontaneous exothermic reactions are sometimes difficult to conceptualize but i would just look at the equation for gibb's free energy and remember that (as what people said above) for delta G < 0, we have to examine enthalpy AND entropy. so a negative delta H for an exothermic reaction cannot be the sole factor for determining spontaneity.

[MCalcagnie_ 1D]

I was a little confused about this as well, but spontaneity is determined by delta G. So non-spontaneous is when the delta G is positive, but the entropy needs to be negative and temperature is high.

[Ansh Patel 2I]

Referencing the ∆G = ∆H-T∆S equation, an exothermic reaction can be non-spontaneous when entropy is negative and the temperature is high.

[SashaAnand2J]

An exothermic reaction may be non-spontaneous if the reaction also leads to a decrease in entropy. Once temperature is above a certain point, this means the reaction becomes entropy-driven, causing the reaction overall to have a positive deltaGo. An online example I found of this is the combustion of methane. Hope that helps!

[Ranen_Chang_2G]

Combustion is a great example. Because it is exothermic, is negative. To offset this and make , it would require a high T and a negative

[Lesly Lopez 3A]

It is possible for which an exothermic reaction could be non-spontaneous/ This could happen when S is negative. Another reason would be that the temperature of the reaction is high.

[Gian Boco 2G]

Exothermic reactions only tell the delta H. If the delta S is negative and the temperature value high enough, the delta G can be positive.

[Grace_Remphrey_2J]

An exothermic reaction can be non-spontaneous when the temperature is high and entropy is negative, as shown in the ∆G = ∆H-T∆S equation.