# Kinetics and thermodynamics relationship

It is dangerously easy to confuse thermodynamic quantities like free energy with kinetic ones like activation energy. Probably for this reason, thermodynamics. No. Both I think are equally theoritical or experimental. You must study both seperately. Thermodynamics doesn't speak about rates. Relationships between chemical thermodynamics and kinetics (note that the equivalence of thermodynamic and kinetic equilibrium constants.

So let's go ahead and start with the forward reaction. So I'm just going to go ahead and say that the forward reaction has a negative delta G value. So remember that means it's spontaneous, and visually that means that our reactants start off at a higher free energy than our products. Now, this of course means that the reverse reaction, which will have the same magnitude of delta G, that is the free energy change, will be at the same numerical value.

But of course, since the reaction is going the opposite direction, the sign of delta G will be now positive. And visually, we're saying essentially that our reactant, which in this case is B, starts our at a lower free energy than our product, which is A.

Now thus far in drawing this free energy diagram, we've just been talking about thermodynamics.

### Thermodynamics vs kinetics (video) | Khan Academy

But it turns out that there is also a kinetic energy barrier for the conversion of reactants to products, regardless of whether the reaction is spontaneous or non-spontaneous.

This kinetic barrier of energy is referred to as the free energy of activation, or simply activation energy. So I'm going to go ahead and put in parentheses E sub A, which we'll say stands for activation energy. And remember that delta G of course is talking about thermodynamics. With that said, let's go ahead and add this kinetic energy barrier to our diagrams. And we can do this by understanding that the activation energy is defined as the amount of energy that is required to form a high-energy intermediate during the course of the reactions.

In other words, in our hypothetical reaction of A going to B, it proceeds through an intermediate, that is a high-energy chemical product that won't last very long, but is important in the conversion of A to B or vice versa. So in our diagrams here, we can go ahead and indicate that there is some intermediate, so in between our reactants and our products, that is at much higher energy than everything else. And we can go ahead and then connect the dots.

So go ahead and essentially draw a line from reactants to products that includes our high-energy intermediate. And this ultimately allows us to see the presence of the activation energy as well as the change in free energy. So let's go ahead and actually label these things. So on the left side here, remember our change in free energy, which is looking at our reactants compared to our products-- we're ignoring this little bump.

The change in free energy, which I'm going to indicate with the green line, extends between these two points. And the same on the right side, just again extending between the start and endpoints of our reactions. So I'll go ahead and indicate that this green line in both cases refers to our delta G values. And in a different color, let's say red, I'm going to go ahead and indicate the activation energy, which takes into account the change in energy between the high-energy intermediate and the reactants.

So in the case on the left, that change is indicated here with red. And on the right side, the change between the intermediate and the reactant is a bit longer, so we'll go ahead and indicate that here. Now, activation energy is an important quantity to take into account, because in order for molecules to react, they must have enough energy to overcome this activation energy barrier. Essentially, in the case of a spontaneous reaction for example, I think of it like the energy one needs to get a ball to start rolling down a hill.

We all know that gravity will make a ball roll down a hill, which is like a negative delta G value, it's telling us that the reaction is very thermodynamically favorable. But we need to sometimes give the ball push in order for the reaction to occur. And so that's kind of this little help that it needs to go over before it can actually proceed. For a non-spontaneous reaction, the idea is essentially the same.

We still need to have some activation energy. But in addition, because it requires an input of energy, we can think about it as rolling ball up a hill instead of down a hill. Now in general, the idea is that the lower this free energy change, the faster a reaction will occur. The activation energy is the energy required to reach the transition state. Once this threshold is reached, the reaction proceed in the favorable "downhill" direction. It is important to remember that each reaction has a different transition state threshold, with different activation energies, and determined by the reactants and the conditions in which the reaction is taking place.

Fuel The gas in a fuel tank is not "wasted" or burnt away while the car is sitting in the parking lot. For a video that shows why two elements do not spontaneously combust as fuel would, had it not needed activation energygo to 'outside link' number 5.

This indicates that the reactions' most stable state is that of the products. Thus, going back to Diagram 1, thermodynamics is what describes the free energy between the reactants and the products. Systems The best way to understand thermodynamics is by realizing that anything that transfers, receives, or contains heat can be described as a system.

## Kinetics vs Thermodynamics

Heat can enter or leave a system, which affects the amount of thermal energy it contains. Consider a kettle of water sitting on a stove. As it is heated, thermal energy is added to the system the kettle with the water.

This is an example of the system losing thermal energy. To view an animated diagram of a thermodynamic system, click on 'Outside Link' number 2. Kinetics As mentioned above, the most stable states of a kinetic reaction are those of the reactants, in which an input of energy is required to move the reaction from a state of stability, to that of reacting and converting itself to products.

Kinetics is related to reactivity.