# 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. What is the meaning of a "nonspontaneous" reaction at higher temperatures?

A nonspontaneous reaction at higher temperatures means that the reaction will not occur on its own without the input of external energy. In other words, the reaction requires energy to be added in order to proceed.

## 2. Why does a reaction become nonspontaneous at higher temperatures?

This is because at higher temperatures, the particles in the reaction have more kinetic energy and are moving faster. As a result, they are less likely to interact with each other and form new bonds, making the reaction less likely to occur spontaneously.

## 3. Can a reaction that is spontaneous at lower temperatures become nonspontaneous at higher temperatures?

Yes, it is possible for a reaction to be spontaneous at one temperature but nonspontaneous at another. This depends on the specific thermodynamic parameters of the reaction, such as enthalpy and entropy.

## 4. How does the activation energy of a reaction affect its spontaneity at different temperatures?

The activation energy of a reaction is the minimum amount of energy required for the reaction to occur. At higher temperatures, the activation energy barrier may be overcome by the increased kinetic energy of the particles, making the reaction more likely to occur spontaneously.

## 5. Are there any exceptions to the rule that reactions are nonspontaneous at higher temperatures?

Yes, there are some reactions, such as endothermic reactions, where an increase in temperature can actually increase the spontaneity of the reaction. This is because the energy input needed to overcome the endothermic reaction may become more readily available at higher temperatures.