# Why would a reaction be nonspontaneous at higher temperatures?

• Jaccobtw
In summary, at higher temperatures, a reaction with a negative enthalpy and entropy will have a positive Gibbs free energy, making it nonspontaneous. This is because the increase in entropy of the surroundings is not enough to compensate for the decrease in entropy of the system. While this is a general rule, it is important to note that all reactions will occur spontaneously to some extent and in both directions when pure reactants or products are mixed.
Jaccobtw
TL;DR Summary
Typically we think of a higher temperature speeding up the reaction rate and/or supplying the activation energy of a reaction. So why is it the case that some reactions are only spontaneous at lower temperatures?
Using the gibbs free energy equation ## \Delta G = \Delta H - T \Delta S ##, If I have a reaction where ##\Delta H## is negative (exothermic?) and ## \Delta S## is negative it makes ##\Delta G## positive at higher temperatures which means the reaction is nonspontaneous at higher temperatures. Why would this be the case?

Jaccobtw said:
Summary: Typically we think of a higher temperature speeding up the reaction rate and/or supplying the activation energy of a reaction. So why is it the case that some reactions are only spontaneous at lower temperatures?

Using the gibbs free energy equation ## \Delta G = \Delta H - T \Delta S ##, If I have a reaction where ##\Delta H## is negative (exothermic?) and ## \Delta S## is negative it makes ##\Delta G## positive at higher temperatures which means the reaction is nonspontaneous at higher temperatures. Why would this be the case?
Did you show this with the van't Hoff equation? What does the equilibrium constant look like as a function of temperature. What is the value of K at 25C?

Jaccobtw and Lord Jestocost
Chestermiller said:
Did you show this with the van't Hoff equation? What does the equilibrium constant look like as a function of temperature. What is the value of K at 25C?
The way I understand it is if a reaction in nonspontaneous at a high temperature, then its opposite reaction is spontaneous at that same high temperature. So a reaction is still spontaneous if the parts are there.

As far as why can't an exothermic reaction that reduces entropy of its system (rather than surroundings) occur at a high temperature (## \Delta H ## is negative, ##T## is high, ##\Delta S## is negative) is because (my guess) it would break the second law of thermodynamics. An exothermic reaction at high temperatures barely increases the relative entropy of the surroundings while decreasing the entropy of the system more, leading to an overall decrease in entropy of the universe.

Jaccobtw said:
The way I understand it is if a reaction in nonspontaneous at a high temperature, then its opposite reaction is spontaneous at that same high temperature. So a reaction is still spontaneous if the parts are there.

As far as why can't an exothermic reaction that reduces entropy of its system (rather than surroundings) occur at a high temperature (## \Delta H ## is negative, ##T## is high, ##\Delta S## is negative) is because (my guess) it would break the second law of thermodynamics. An exothermic reaction at high temperatures barely increases the relative entropy of the surroundings while decreasing the entropy of the system more, leading to an overall decrease in entropy of the universe.
You realize that this thing about ##\Delta G^0## having to be negative in order for a reaction to be spontaneous is just a rule of thumb, right? If simply means that the equilibrium constant is > 1. All reactions will occur spontaneously to some extent if you mix pure reactants, and all reactions will occur spontaneously in reverse to some extent if you mix pure products.

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## 1. Why does temperature affect the spontaneity of a reaction?

Temperature affects the spontaneity of a reaction because it changes the amount of energy available for the reaction to occur. At higher temperatures, the molecules have more kinetic energy and are more likely to collide with enough force to overcome the activation energy barrier and initiate the reaction. However, at lower temperatures, the molecules have less energy and are less likely to collide with enough force, making the reaction less spontaneous.

## 2. How does the change in enthalpy affect the spontaneity of a reaction at higher temperatures?

The change in enthalpy, or heat content, of a reaction can also affect the spontaneity at higher temperatures. If the reaction has a positive change in enthalpy, meaning it absorbs heat, then increasing the temperature will make the reaction less spontaneous. This is because the added heat energy will counteract the decrease in Gibbs free energy caused by the higher temperature, making the reaction less favorable.

## 3. Can a reaction be nonspontaneous at higher temperatures but spontaneous at lower temperatures?

Yes, it is possible for a reaction to be nonspontaneous at higher temperatures but spontaneous at lower temperatures. This is because the change in enthalpy and entropy of a reaction can have opposite effects on the spontaneity. While an increase in temperature may favor a reaction based on its change in entropy, it may also have a negative effect on the change in enthalpy, making the reaction nonspontaneous.

## 4. How does the concentration of reactants and products affect the spontaneity of a reaction at higher temperatures?

The concentration of reactants and products can also play a role in the spontaneity of a reaction at higher temperatures. If the concentration of reactants is high, the reaction may be more spontaneous at higher temperatures because there are more molecules present to collide and initiate the reaction. On the other hand, a high concentration of products may make the reaction less spontaneous at higher temperatures because the equilibrium of the reaction is already shifted towards the products.

## 5. Are there any other factors besides temperature that can affect the spontaneity of a reaction?

Yes, there are other factors besides temperature that can affect the spontaneity of a reaction. These include the nature of the reactants and products, the presence of a catalyst, and the pressure of the system. Additionally, the change in Gibbs free energy, which takes into account the change in enthalpy, entropy, and temperature, is the most accurate measure of spontaneity and can be affected by a combination of these factors.

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