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Gibbs free energy and spontaneity _ Chemistry _ Khan Academy

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Kind: captions
Language: en

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In the last few videos we have learned that if we have
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a system which is under the influence of constant pressure, or
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in a constant pressure environment, change
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in enthalpy is equal to the heat added to the system.
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I will write a little p because everything is going on at constant pressure.
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So if we have one reaction, say A plus B gives C,
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and the change in enthalpy - i.e. our enthalpy in this state
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minus the change in enthalpy in that state -
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let's say our enthalpy change is less than 0,
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we know this is an exothermic reaction.
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Why is that?
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And once again, I assume that the pressure is constant.
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How can we know that the reaction is exothermic,
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with energy release?
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Because the change in enthalpy is when we have a constant pressure system
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is expressed in the added heat to the system.
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If this heat is negative, then you should
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to release heat.
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This is how we release heat or energy.
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So, plus energy.
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And in the last clip we learned, I think, or in the last clip,
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or one or two clips ago that we call such a reaction
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exothermic reaction.
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And, if there is a reaction that needs energy-
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let's say we have A plus B plus some energy gives C,
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then what does this mean?
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This means that the system has absorbed energy.
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The amount of energy we absorb
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represents our change in enthalpy.
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Accordingly, delta H will be positive.
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The change in enthalpy is positive.
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Energy is absorbed into the system.
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We call such reactions endothermic.
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Heat is absorbed.
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So, if we want to know if a reaction is happening
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just by itself - whether it is spontaneous -
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this change in enthalpy seems to be a good candidate.
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Obviously, if I release energy, I don't need any energy,
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for this reaction to take place, perhaps this reaction is spontaneous.
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And similarly, after I have to somehow add energy to the system,
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something inside me tells me that maybe there is no spontaneity here.
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But another small part of me tells me that
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if the particles are going around really fast,
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and have great kinetic energy that can be used
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to coalesce these particles, probably then suddenly
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it will become spontaneous.
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So the enthalpy by itself may not fully describe what will happen.
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And to gain a little intuition, and maybe build
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a sense of whether a reaction is spontaneous,
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let's look at the participants in it that matter.
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We already know that Delta H probably matters.
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If we release energy - we know that the delta H is less than 0,
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which makes me think there may be spontaneity.
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But what if Delta S, entropy, decreases?
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What would happen if things got a little tidy?
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We already learned from the second law of thermodynamics,
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that this is not the case here.
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And we know from personal experience that
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things by themselves do not go into a macro state,
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which has fewer micro-states, according to what we know.
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An egg does not collect itself, it does not jump fun on the floor alone,
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although there is some likelihood that this will happen.
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And as if entropy matters anyway.
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Now comes the temperature.
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We already talked about this when we were discussing energy.
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And as we know, if energy is required here,
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and probably the temperature is high enough, maybe
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I would actually collect some of these particles,
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generating energy to get here.
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So let's think and see.
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Let's think about the participants
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and what the reactions depending on would look like
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different combinations of participants.
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The participants I'm going to start with - Delta H looks like
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it definitely matters if it does
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energy absorption or not.
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We have delta S, which is the change in entropy.
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Are there more states or fewer states in the system?
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Is it getting tidier or not?
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Following is the temperature it represents
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average kinetic energy.
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Let us now consider a few possible situations.
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Let's take the first case first.
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This is a situation where the delta H is less than zero,
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and entropy is greater than 0.
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I mean, I already feel inside what will happen.
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This is a situation where the entropy will be greater
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after the reaction.
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One way to look at entropy is
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that there may be more conditions. Probably more particles.
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We have seen that entropy is related to the number of particles,
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that we have. And that may be the reaction where
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let's say we have ... we want to have more particles.
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And let's say I have this friend.
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He has another friend like him there, and I'm adding another,
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and for example, it has one such molecule.
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Let's say she is ... I won't say stable or not.
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But let's say that when these particles collide with each other,
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finally this remains.
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I do this very quickly.
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Maybe one of these molecules binds to that molecule,
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that's how we see one of the dark blue molecules ...
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i will make all the dark blue ...
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it binds to this light blue molecule,
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another of the dark blue bonds with the purple-red molecule.
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And maybe this brown molecule is separated and left alone.
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We had two molecules, and now we have come to a state with three molecules.
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Disorderedness is greater, more entropy is present.
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There can obviously be more states.
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I tell you the delta H is less than 0.
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And in doing so, these friends, their electrons are lower
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potential, or in a more stable configuration.
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And when electrons move out of their configurations
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with more potential here and become more stable,
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they release energy.
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So we have a plus ... then I just know that as I said
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from the outset that the change in enthalpy
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is less than 0.
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And we add some energy.
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It seems very clear to me that this reaction
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will be spontaneous in this direction to the right.
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Because there is no reason ...
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first it is easier for two particles to collide into each other,
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to go in this direction than if they were three particles -
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if we look at it in terms of probability
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collect the three particles appropriately
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and go in that direction.
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What's more - these girlfriends are more resilient.
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Their electrons are in a state of lower potential.
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And there is not even a reason in terms of enthalpy
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to move in that direction, or for some energy reason
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to move there.
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And that, in my opinion,
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something tells me that no matter what the temperature, it will benefit
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the course of the right reaction.
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In my opinion this
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probably happens spontaneously.
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And what's going on - let's do something we can
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less logical.
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What happens if the delta H is less than 0?
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But let's just say I'm losing entropy.
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And it seems that according to the second law of thermodynamics,
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if the entropy of the universe goes up ...
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I'm only talking about my system.
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That would be a situation where I'm moving, let's say,
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from a place with two other particles.
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Let's say I have that particle, and then that particle.
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And then, if they collide with each other the right way,
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their electrons will be more stable, and that's probably what happens.
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And when they do, electrons can go lower
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potential states, and when they do, they release energy,
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ie we have plus energy here.
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And we know this because the change in enthalpy is less than zero.
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In this state we have lower energy than in
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and the difference is released here.
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Will this reaction take place?
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Well it looks like ... let's enter the temperature here.
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What will happen at lower temperatures?
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At low temperatures, these elements have average very low kinetic energy.
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They just wander around in slow motion.
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And as they wander very slowly ...
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Remember when I talked about spontaneity ... I spelled "spontaneous".
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This is something spontaneous.
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"Spontaneous" is another word for thermodynamic.
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A really fun word.
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When we have a low temperature - what I was talking about before finding the mistake,
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When I talk about spontaneity, I mean whether
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the reaction will take place on its own.
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I'm not talking about how fast it will go or its speed.
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This is really a key point.
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Do we know if this will happen?
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I don't care if it takes a million years for that to happen.
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I just want to know if it will happen by itself?
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If the temperature is low, these elements may be true
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they wander around and rarely encounter one another.
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But in the end they will collide with each other.
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And when they do, they will just go side by side.
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And as they pass by, they will take shape
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in a certain way - things want to go lower
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potential status.
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I'm just trying to tell you,
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to awaken your intuition.
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But because it will release energy, it will pass
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in a lower potential state, the electrons are somehow configured,
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when they approach each other and go into this state.
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And they release energy.
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And once that energy is exhausted, maybe in the form of heat
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or something of the variety, it becomes somehow difficult for her to return
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and we go in the other direction.
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And it seems as if there will be spontaneity, if
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the temperature is low.
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Let me write it.
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The process is spontaneous if the temperature is low.
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And what if the temperature is high?
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Let's not forget that these are not the only particles here.
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We have more of them.
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I'll choose another such friend, and another friend like that.
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And then on this side I will have, apparently, more particles.
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Obviously there is not only one particle.
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Then all these macro variables really don't make sense,
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if we talk only about specific molecules.
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These are whole systems.
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But what if our system temperature is high?
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Let's think about a situation where the temperature is high.
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So, suddenly ... on this side, the particles will collide into each other very quickly.
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If this friend crashes into this "over-speed", this collision can almost be accepted
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like a car accident.
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Even better, it could be a disaster.
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If all these individual cars, if the atoms were part of the cars,
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if they bump into each other,
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even though they want to be attached to each other,
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they have screws and whatever it takes to hold them together,
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if two cars are moving fast enough against each other,
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all those screws, adhesives and solders won't matter.
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They are just going to mess up.
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So, high kinetic energy - let me show it.
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If they have high kinetic energy, my intuition tells me that
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on this side of the reaction these elements are simple
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they will disintegrate and the resulting disorder will be directed here.
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And since these friends also have high kinetic energy,
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they will move so fast past each other,
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and they will jump out of each other in such a fast way,
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that the opposing force, or the opposite
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tendency for their electrons to be more configured
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it won't matter.
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It's like, imagine trying to attach a tire,
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as you run past the car.
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You have to do it somehow ... even though it's a little ...
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perhaps this comparison is inappropriate.
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But I think you get the idea that if the temperature
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is really high, it seems less likely
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these particles move smoothly side by side in the right way,
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so they can connect with each other and make their electrons more stable,
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and perform this whole exothermic act.
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So my feeling is that if the temperature is high enough,
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I mean, you know, maybe, let's just say she's not high enough.
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And what about ultra-high temperatures?
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If we have very high temperatures, then probably
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even this friend will run into that one.
00:12:06.880 --> 00:12:10.000
Instead of getting this, he will kick this other blue friend,
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and will later be found here.
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I have to color this friend in blue.
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And maybe he'll kick this one out for his other components,
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if there is enough kinetic energy.
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And here comes the idea that this does not happen spontaneously.
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Even more so, in the reverse reaction, if the temperature
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is high enough, there will probably be spontaneity.
00:12:29.930 --> 00:12:31.860
If the temperature is high enough, these friends will
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react, they will collide with each other
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and the reaction will proceed in this way.
00:12:36.400 --> 00:12:38.390
So the temperature is high - we take this path,
00:12:38.390 --> 00:12:41.000
the temperature is low - we take on the one.
00:12:41.000 --> 00:12:43.000
Now let's see if we can arrange everything,
00:12:43.000 --> 00:12:47.920
which we saw so far and see what it would look like
00:12:47.920 --> 00:12:50.000
a formula for spontaneity.
00:12:50.000 --> 00:12:54.000
We can start with enthalpy.
00:12:54.000 --> 00:12:58.000
We already know that if this is less than 0, probably
00:12:58.000 --> 00:13:00.710
we are dealing with something spontaneous.
00:13:00.710 --> 00:13:03.920
Now let's say I want a whole expression where if the whole expression
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is less than 0, this tells me
00:13:07.000 --> 00:13:10.740
that there will be spontaneity.
00:13:11.000 --> 00:13:19.400
And we know that positive entropy is a good thing for spontaneity.
00:13:19.400 --> 00:13:21.760
We have seen it in each of our situations.
00:13:21.760 --> 00:13:24.820
And to have more states is always welcome.
00:13:24.820 --> 00:13:29.000
Something spontaneous is more likely to happen.
00:13:29.000 --> 00:13:32.000
So, we want our whole expression to be negative,
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if there is spontaneity, right?
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And positive entropy should make my whole expression even more
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more negative, so maybe we have to subtract the entropy.
00:13:45.570 --> 00:13:48.000
If this is positive, then my whole expression
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will be more negative.
00:13:49.000 --> 00:13:51.930
Which just says, "Hey, there's spontaneity here."
00:13:51.930 --> 00:13:54.230
So if it is negative, we release energy.
00:13:54.230 --> 00:13:56.770
And if it is positive, we become more messy,
00:13:56.770 --> 00:13:58.620
so this whole thing will be negative.
00:13:58.620 --> 00:14:00.350
Things look more decent.
00:14:00.350 --> 00:14:04.000
What if the entropy is negative?
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If the entropy is negative, then this is also a reminder of
00:14:07.000 --> 00:14:10.000
the idea that if entropy is negative,
00:14:10.000 --> 00:14:13.000
then one kind of reaction is less spontaneous.
00:14:13.000 --> 00:14:16.000
In this situation, entropy was negative.
00:14:16.000 --> 00:14:18.000
We have gone from more clutter to less
00:14:18.000 --> 00:14:20.330
disorder, or fewer particles.
00:14:20.330 --> 00:14:21.720
And what did we say?
00:14:21.720 --> 00:14:29.260
When the temperature is high, spontaneity and entropy are important.
00:14:29.260 --> 00:14:32.000
When the temperature is high, this is a less entropic state,
00:14:32.000 --> 00:14:34.640
they will collide and will become more entropic.
00:14:34.640 --> 00:14:38.860
When the temperature is low, they are likely to move slowly toward each other,
00:14:38.860 --> 00:14:43.000
and then the enthalpy part of the equation
00:14:43.000 --> 00:14:44.000
will be more important.
00:14:44.000 --> 00:14:45.790
So let's see if we can find that.
00:14:45.790 --> 00:14:49.000
When the temperature is high, entropy matters.
00:14:49.000 --> 00:14:51.690
When the temperature is low, entropy does not matter.
00:14:51.690 --> 00:14:54.650
And what if we express entropy through temperature?
00:14:54.650 --> 00:14:59.000
What if I take a temperature variable here?
00:14:59.000 --> 00:15:02.970
Now, my statement, or my feeling based on everything,
00:15:02.970 --> 00:15:05.610
what we have been experimenting with so far is that if this expression
00:15:05.610 --> 00:15:12.420
is less than 0, we will deal with spontaneous reaction.
00:15:12.420 --> 00:15:15.700
And let's see if that goes with everything we say here.
00:15:15.700 --> 00:15:19.000
If the temperature is high - ie. this reaction here
00:15:19.000 --> 00:15:21.370
is exothermic, pointing to the right.
00:15:21.370 --> 00:15:23.660
When we go to the right, more than these molecules
00:15:23.660 --> 00:15:26.000
to say the least, I told you it was an exothermic reaction.
00:15:26.000 --> 00:15:29.750
So at low temperatures, logic tells me that, hey, here it is
00:15:29.750 --> 00:15:30.930
to have some spontaneity.
00:15:30.930 --> 00:15:32.570
These friends will move close to each other when they fall
00:15:32.570 --> 00:15:34.000
in this more stable configuration.
00:15:34.000 --> 00:15:35.690
And that makes sense.
00:15:35.690 --> 00:15:39.000
At low temperatures this term would not mean much.
00:15:39.000 --> 00:15:40.000
You can imagine an extreme situation.
00:15:40.000 --> 00:15:42.320
At absolute zero, this article will disappear.
00:15:42.320 --> 00:15:44.000
We can't get quite there, but space
00:15:44.000 --> 00:15:45.510
it will get smaller and smaller.
00:15:45.510 --> 00:15:47.710
And this article dominates.
00:15:47.710 --> 00:15:51.990
At high temperatures, this member will suddenly dominate.
00:15:52.000 --> 00:15:56.800
And if our delta S is less than 0, then this term
00:15:56.800 --> 00:15:59.000
will dominate and become positive.
00:15:59.000 --> 00:16:02.000
And even if it is negative, we subtract.
00:16:02.000 --> 00:16:03.520
So our delta S is negative.
00:16:03.520 --> 00:16:04.680
We put a negative sign here.
00:16:04.680 --> 00:16:06.380
And that will be positive.
00:16:06.380 --> 00:16:08.640
So, with the positive sign here, if the temperature is high enough ...
00:16:08.640 --> 00:16:10.250
and remember working with Kelvin so the temperature
00:16:10.250 --> 00:16:11.500
can only be positive.
00:16:11.500 --> 00:16:14.280
If the temperature is positive enough here, it will dominate
00:16:14.280 --> 00:16:15.950
over any negative enthalpy.
00:16:15.950 --> 00:16:17.790
And so there would be no more spontaneity.
00:16:17.790 --> 00:16:21.580
So if the temperature is high enough, this direction
00:16:21.580 --> 00:16:23.100
will not be spontaneous.
00:16:23.100 --> 00:16:24.860
And this equation tells us.
00:16:24.860 --> 00:16:31.475
And if we go to positive entropy, positive enthalpy, I'm sorry,
00:16:31.475 --> 00:16:33.525
negative enthalpy, positive entropy,
00:16:33.525 --> 00:16:36.000
there's energy release here so it's a negative,
00:16:36.000 --> 00:16:38.000
our entropy is increasing ... our entropy,
00:16:38.000 --> 00:16:41.440
we get more and more messy ... then it also becomes negative.
00:16:41.440 --> 00:16:43.660
So this article will definitely be negative.
00:16:43.660 --> 00:16:46.500
And we've already got the idea that, you see, if it's negative,
00:16:46.500 --> 00:16:48.820
and this positive, we have more entropy
00:16:48.820 --> 00:16:51.570
and energy is released, this must definitely be spontaneous.
00:16:51.570 --> 00:16:53.640
And this equation also shows it to us.
00:16:53.640 --> 00:16:56.500
So I'm sure of that equation so far.
00:16:56.500 --> 00:17:00.660
And as you can imagine ... I don't make it up ...
00:17:00.660 --> 00:17:03.610
This is actually the equation that predicts spontaneity.
00:17:03.610 --> 00:17:06.940
And I'll show you this in a more accurate way in the future,
00:17:06.940 --> 00:17:08.000
maybe we'll go back to some
00:17:08.000 --> 00:17:12.610
from our basic formulas for entropy and the like.
00:17:12.610 --> 00:17:15.550
But here we have the formula for whether something is spontaneous.
00:17:15.550 --> 00:17:17.190
And the goal I had in this clip was to wake up
00:17:17.190 --> 00:17:19.320
your intuition about why this formula makes any sense.
00:17:19.320 --> 00:17:23.500
This quantity is here called the Delta G
00:17:23.500 --> 00:17:26.000
or a change in Gibbs' free energy.
00:17:26.000 --> 00:17:29.320
And that really predicts whether a reaction is spontaneous.
00:17:29.320 --> 00:17:31.980
And in the next clip we will apply this formula several times.
00:17:32.000 --> 00:17:35.050
And a few clips then we'll learn a little more
00:17:35.050 --> 00:17:37.000
about how we can actually get these results
00:17:37.000 --> 00:17:40.000
through some of the basic principles of thermodynamics.

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