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### Why are exothermic reactions generally spontaneous?

Posted: Tue Feb 11, 2020 4:17 pm
I get that exothermic means a -delta H but idk how to interpret the delta S part.

Thanks

### Re: Why are exothermic reactions generally spontaneous?

Posted: Tue Feb 11, 2020 4:32 pm
When a reaction is exothermic, the delta G is negative. In order for delta G to be negative, the change in enthalpy must be smaller and the change in entropy must be greater. This is because the term delta H does not have a negative sign in front of it which means that if it is increased, delta G increases as well. In order to have a negative delta G, the second term T delta S must be larger. This is because it has a negative sign in front that increases the spontaneity of the overall reaction as it increases. S stands for entropy which is the number off states the atoms or molecules of a system can be found in, or, in other words, the randomness of the system, the uncertainty of the states of its parts. According to the second law of thermodynamics, the entropy of the universe constantly increases and is a favorable process. Increasing the randomness of the system means that there are more available states for its atoms or molecules and they have less constrains. In a spontaneous reaction, the change in entropy is usually positive because there is more available energy that is released from the atoms or molecules as they are more free to move around and occupy different states and due to other factors as well. Overall, if a reaction is spontaneous, this means it is favorable, and since entropy always increases and particles prefer randomness as in the case of gases compared to being fixed as in the case of solids, a negative delta G usually but not always implies an increase in entropy. An example of particles favoring randomness is the expansion of a gas in a larger container than its previous container which is a spontaneous process even though the energy of the atoms does not change. The reason it is spontaneous is that the entropy increases as the gas occupies a larger volume and the atoms have more space to move around.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Wed Feb 12, 2020 12:51 pm
Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction?

### Re: Why are exothermic reactions generally spontaneous?

Posted: Thu Feb 13, 2020 9:24 am
Looking at the equation delta G = delta H - T*delta S, delta S must simply be greater than delta H/T for delta G to be less than 0 (spontaneous reaction). For spontaneous exothermic reactions (delta H and delta G < 0), this means that delta S can be negative as long as it is greater than delta H/T.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Thu Feb 13, 2020 10:29 am
$\Delta G=\Delta H - T\Delta S$

If delta G is negative then the reaction is spontaneous.

So, using the formula above you can see that in most cases when $\Delta H$ is negative and $T\Delta S$ is positive or $T\Delta S< \Delta H$
then delta G will be a negative value. There are generally more scenarios of delta G being negative when delta H is also negative. However, keep in mind that a negative delta H is not always spontaneous, consider T Delta S as well.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Thu Feb 13, 2020 10:44 am
A very general explanation is that systems tend to want to be in the lowest energy state and when a reaction is exothermic it loses energy as heat.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Thu Feb 13, 2020 3:15 pm
LBacker_2E wrote:Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction?

Yes, a spontaneous reaction will always have a negative Gibbs value and proceed in the forward direction. If the reaction has a positive Gibbs value, then the reverse process is spontaneous.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sat Feb 15, 2020 7:18 am
There is a grid on one of the pages in the chapter about Delta G that could be helpful to explain this!

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 11:42 am
Typically if delta H is negative, delta S is positive and delta G is negative this indicates spontaneity

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 1:10 pm
Reactions will usually want to go towards a state that they are, in a sense, "comfortable" in, so they want to go towards favorable reactions. Usually, these reactions release heat. And I think an easy way to think about spontaneous reactions is that they can happen randomly (spontaneously) and that would release something.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 3:10 pm
Exothermic reactions are generally spontaneous, because unlike endothermic reactions, they do not require heat input

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 3:16 pm
This is mainly because a reaction that is spontaneous would not require heat input. They release energy upon completion.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 8:10 pm
Yes, a spontaneous reaction has a negative delta G.

LBacker_2E wrote:Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction?

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 8:45 pm
If delta H is negative, then to determine if the reaction is spontaneous, you have to look at T*delta S. In order to make G positive given a negative delta H, entropy would have to be negative and overpower the delta H. This is unlikely because the release of energy from an exothermic reaction is unlikely to result in a reduced (negative) entropy. Hence, delta s is usually positive, making delta G more negative, and hence making the reaction spontaneous.

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 8:50 pm
LBacker_2E wrote:Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction?

Yes, when a reaction is spontaneous, it has a negative delta G

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 16, 2020 9:04 pm
Here you must consider the equation for Gibbs Free Energy, which is deltaG = deltaH - TdeltaS. A negative deltaG denotes a spontaneous reaction, while a negative deltaH denotes an exothermic reaction. You can see that in order to make deltaG negative, having a negative deltaH can help, as then the deltaS does not necessarily have to be positive, as long as the absolute value of the negative deltaH is larger than the TdeltaS term (if deltaS is negative).

### Re: Why are exothermic reactions generally spontaneous?

Posted: Sun Feb 23, 2020 8:57 pm
According to the laws of thermodynamics, the entropy of a system always tends to increase. This is because when entropy increases, the number of states a particle can be found in increases which removes the constrains on that particle that stores potential energy. Imagine a gas in a small container. Due to being in a small container and being constrained by the walls of this container, the gas molecules will have to be packed together, forced into a limited amount of space that they can occupy and therefore will build up potential energy because of compression. When you open a way to a larger container, all the gas molecules will diffuse and move as far away as possible, and they will have more freedom in which place they occupy. Although this diffusion does not change the energy of the system, it increases the entropy which causes the reaction to be spontaneous and occur unaided.

An exothermic reaction releases heat to its surroundings, meaning that the bond energy of its products will be less than the bond energy of its reactants. This causes the products to be more stable since the energy contained in their bonds is less than the energy contained in the bonds of the reactants. Because it is a more stable form compared to the reactants, the reaction occurs spontaneously due to principle that elements always prefer to be in their most stable form (a reactive element will always be unstable). Another way to look at it would be to look at Gibbs free energy equation where the enthalpy plays a role. If the bond energy of the products is less than the bond energy of the reactants, this energy has been released to the surroundings. Because delta H is found by final-initial and this yields a negative value for an exothermic reaction, and the sign of enthalpy is positive in the equation, the more negative this value becomes, the more negative the Gibbs free energy will become, which will increase the spontaneity of the reaction.