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# Gibbs free energy and spontaneity

Intuition behind why spontaneity is driven by enthalpy, entropy and temperature. Introduction to Gibbs free energy. Created by Sal Khan.

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• This is kind of weird, I thought that usually at an higher temperature a reaction would be more spontaneous than at a lower temperature...
• the high temperature changes things only if you have negative entropy, rather than an increase in entropy.
• Why do you refer to entropy as disorder? Entropy in the thermodynamic sense is not disorder,it is a measure of the diffusion of energy. Often disorder can accompany entropy, but this is not always the case.
It sounds as if I am nitpicking, but I truly appreciate your videos.
I just think that the entropy as disorder confuses so many people. Entropy as disorder in information theory has some meaning, but in thermodynamics or physical chemistry it does not.
• In one of his earlier videos on entropy he actually talks about how disorder is a bad synonym for entropy and tells why.
• Is a reaction spontaneous if the equation equals 0?
• It is at equilibrium. It is neither spontaneous nor non-spontaneous. The reaction goes back and forth without changing the actual contents.
• At 12.14 when temperature increases and the reactant molecules collide into each other so fast that some of the constituent element atom is knocked off, wouldn't that be a spontaneous reaction as there would be an increase in particles which results in an increased in states and entropy?
• The change in entropy was assumed to be negative, so a decrease in the amount of particles would not be spontaneous.
• As I remember, the 2nd Law of Thermodynamics says that Del(S)>0 in the Universe. So what should be different in a chemical reaction so that we could have the second case where Del(S)<0?

Doesn't that contradict the 2nd law?
(1 vote)
• The 2nd law states that Entropy will increase or stay the same in a closed system. When you have a localized decrease in entropy you always have an increase in entropy if you consider a larger system that includes where the energy is coming from.
• So is this saying anything could react with anything under the right conditions of enthalpy and entropy?
• This is saying anything could react with anything spontaneously under the right conditions (for many reactions these conditions would be hard to meet, and the substances would have to be at the right temperature too). Basically, under the right conditions, if you put two substances together they react without any outside help (i.e. without any added energy).

I'd recommend watching the other Gibbs Free Energy videos, though - this is only an intuition/introduction video.
• sal says Del(H)-TDel(S)=-ve then it is spontaneous but Del(S)=Q/T therefore TDel(S)=Q but as sal said at the starting this is at constant pressure the Q must be equal to Del(H) therefore it becomes zero then how does it become -ve please ans quickly
• Delta S=Q/T only when it is a reversible process. No spontaneous reactions are reversible. (or realist reactions) So you can't use this definition of entropy with Gibbs
• if a reaction has a positive enthalpy change and positive entropy change can it be spontaneous?
• Great question! Have a look at the Gibbs free energy formula (where d denotes delta):
dG = dH - T*dS
as long as G remains negative, the reaction will be spontaneous. If H is positive, and S is positive, then the T*dS term must simply be larger than dH to make dG negative.