Hey guys, in this new video, we're going to take a look at Gibbs Free Energy.
We've been talking about the system and the surroundings and we've been talking about
entropy and to a lesser extent delta H, enthalpy.
Now we're going to take a look at the last major variable, our delta G value.
We're going to say we know that according to the second law of thermodynamics, a spontaneous
reaction will be when delta S total or delta S universe is greater than zero.
But now we're going to bring into the mix delta G. Besides looking at delta S total,
it's also great to look at delta G because knowing the sign for delta G, will be a great
way to determine if your reaction is spontaneous or not overall.
To do this we use two different equations.
Here we'll take a look at the one on the left side.
It says delta G zero equals delta H zero minus T delta S zero.
Remember, these zero means that we're under standard conditions.
When it comes to thermal dynamics standards, conditions means one atmosphere and 25 degrees
Celsius.
Then on the other side, we have delta G equals delta G zero plus RTlnQ.
Here, this would be delta G under non-standard condition.
We're not at one atmosphere or our temperature is not 25 degrees Celsius.
Here it equals delta G zero again.
R here is our gas constant, which is 8.314 joules over moles times K. T is our temperature
in Kelvin.
Q is known as our reaction quotient.
We're going to say our reaction quotient is just products over reactants.
We're going to say when it comes to Q, it ignores two states of matter.
It ignores solids and liquids.
If it's a solid or a liquid, it ignores it.
If it's aqueous or gas, it will look at them.
Ignores solids and liquids, but look at aqueous and gases.
We're going say here, if delta G under non-standard conditions is less than zero, then our reaction
is spontaneous.
If it's greater than zero, then it's non-spontaneous.
If it's equal to zero, then our reaction is at equilibrium.
Let's take a look at example one.
When it comes to this particular type of example, this is a standard type of question for any
exams that you take that deal with delta G, delta H, and delta S. Remember our techniques
that we use here in order to solve this particular type of question.
Since we're going to need room to see everything, I'm going to remove myself from the image,
guys, so we have more room to work with.
The question says which of the following statements is true for the following reaction.
We have N2O4 gas breaks down to give us 2NO2 gas.
We're going to say here the delta H value is negative.
The delta S value is positive.
Then we'll see that our options are the reactions could be spontaneous or non-spontaneous depending
on the type of temperatures we're talking about.
In your book, there's going to be a long, large chart, where you have to memorize different
things.
I say you can look at the chart, but there's a much easier way to handle a question like
this.
That has to do with the little square thing that we have to the right side.
Here we're going to use this image to help us figure out how to answer this question
easily without having to learn a huge chart.
Here we're going to say we have our delta S values on top.
They're positive and negative.
On the sides, we have our delta H values positive and negative.
Here's the thing, we're going to say when both delta H and delta S are positive, then
our reaction is spontaneous at high temperatures.
If delta H is positive and delta S is negative, then it's non-spontaneous at all temperatures,
so non at all temperatures.
If delta H is negative and delta S is positive, then we're going to say they're spontaneous
at all temperatures, so spon at all temperatures.
Then finally, if delta H and delta S are both negative, then it's only spontaneous at low
temperatures, so it's going to be high non-spon low.
Easy way to remember.
If we go back to the question, we have a delta H that's negative, a delta S that's positive.
When delta H is negative and delta S is positive, they said the reaction is spontaneous at all
temperatures.
A question like this is very basic on your exam, so it's important you guys remember
this quick, easy technique.
Instead of having to sit down and memorize an entire graph, an entire chart in your book,
just simply use this diagram, you can easily answer the question.
The hard part really is just remembering who goes on the top, who goes on the side.
Just remember, delta S values are on top, delta H's are on the side, then it's high
non-spon low.
That's all it is.
An easy way to break down a very complicated issue.
Just remember the steps, remember how to draw this diagram and you'll be fine when it comes
to a question like this.