A spontaneous reaction






Moderators: Chem_Mod, Chem_Admin

Jesus Rodriguez 1J
Posts: 53
Joined: Sat Jul 22, 2017 3:00 am

A spontaneous reaction

Postby Jesus Rodriguez 1J » Mon Jan 29, 2018 3:28 pm

Could someone explain to me what a spontaneous reaction is and possibly use the picture that the professor drew last friday I believe (the picture was of something going up a hill, if I remember correctly)

Qining Jin 1F
Posts: 53
Joined: Fri Sep 29, 2017 7:07 am

Re: A spontaneous reaction

Postby Qining Jin 1F » Mon Jan 29, 2018 3:31 pm

A spontaneous reaction is a reaction that is favorable (with a negative deltaG)

Andrea ORiordan 1L
Posts: 52
Joined: Fri Sep 29, 2017 7:04 am
Been upvoted: 2 times

Re: A spontaneous reaction

Postby Andrea ORiordan 1L » Mon Jan 29, 2018 3:50 pm

A spontaneous reaction is one that is energetically favorable. The diagram of the hill was meant to show that increases in entropy are energetically favorable, just as decreases in enthalpy are energetically favorable. Gibbs free energy is what we use to determine whether or not a reaction is spontaneous- the formula is ∆G=∆H-T∆S. As Dr. Lavelle showed in class, ∆S(universe)= -∆G(sys)/T. Therefore, when ∆G is negative, ∆S(univ) is positive. When ∆S(univ) is positive, the reaction is spontaneous, because as the second law of thermodynamics states, the entropy of an isolated system will never decrease overtime.

Therefore, when ∆G is negative, we have a spontaneous reaction. This can occur when:
∆H is negative and ∆S is positive
∆H is positive and ∆S is positive (at high temperatures)
∆H is negative and ∆S is negative (at low temperatures).
Note that a reaction is never spontaneous when ∆S is negative and ∆H is positive (because ∆G will always be positive).

*Non-spontaneous reactions can still occur, provided work is done to make them happen.
Hope this helps!

Tim Nguyen 2J
Posts: 31
Joined: Sat Jul 22, 2017 3:00 am

Re: A spontaneous reaction

Postby Tim Nguyen 2J » Tue Jan 30, 2018 7:34 am

Spontaneous reactions are the likely paths reactions or processes will take without an input of energy (ice melting for instance).

Jennie Fox 1D
Posts: 66
Joined: Sat Jul 22, 2017 3:01 am

Re: A spontaneous reaction

Postby Jennie Fox 1D » Tue Jan 30, 2018 8:58 am

Spontaneous reactions occur without any type of outside intervention/force. Spontaneous reactions favor the formation of products at the current conditions under which the reaction is occurring.

Alejandra Rios 1L
Posts: 31
Joined: Fri Sep 29, 2017 7:06 am
Been upvoted: 1 time

Re: A spontaneous reaction

Postby Alejandra Rios 1L » Wed Jan 31, 2018 1:54 pm

A spontaneous reaction is a reaction which is driven by the energy which is already present in the system. This means that it is the more natural reaction that can occur with the given amount of energy from the reactants. Spontaneous reactions have a -ΔG value since they give off energy.

I believe the diagram which Dr. Lavelle drew in class was a hill-like structure but the reactant side was higher than the product side, this is because the reaction has to reach its traditional state using activation energy (which is the peak of the diagram) but then declines to a free energy state which is lower than that of the reactants. This would result in a -ΔG demonstrating it is spontaneous.

Sally Nason - 1K
Posts: 40
Joined: Fri Sep 29, 2017 7:05 am

Re: A spontaneous reaction

Postby Sally Nason - 1K » Wed Jan 31, 2018 2:00 pm

When you need to go up the hill (low to high), an input of energy is required to do so, so therefore this motion cannot happen on its own and it is not spontaneous. However, if you go down the hill (high to low), the reaction can happen on its own because no energy needs to be put into the system and it is therefore spontaneous.

Amanda Wu 2C
Posts: 31
Joined: Sat Jul 22, 2017 3:00 am

Re: A spontaneous reaction

Postby Amanda Wu 2C » Wed Jan 31, 2018 3:43 pm

A spontaneous reaction is one that is favorable to proceed given the current conditions without an external input of energy or intervention. In a spontaneous reaction, delta G is negative because the free energy of the products is lower than that of the reactants, much like the picture Lavelle illustrated in class with the ball initially at a higher point on the hill (reactants with higher free energy) and the position of the ball at the bottom of the hill (products with lower free energy).


Return to “Gibbs Free Energy Concepts and Calculations”

Who is online

Users browsing this forum: No registered users and 3 guests