The free energy change for a reaction, ΔG, is an extensive property. What is an extensive proper

The free energy change for a reaction, ΔG, is an extensive...

The free energy change for a reaction, $\Delta G$, is an extensive property. What is an extensive property? Surprisingly, one can calculate $\Delta G$ from the cell potential, $\mathscr{b}$, for the reaction. This is surprising because $\mathscr{B}$ is an intensive property. How can the extensive property $\Delta G$ be calculated from the intensive property $\mathscr{E}$ ?

Key Concepts

Extensive Property

An extensive property is one that depends on the size or extent of the system—its value scales with the amount of matter or number of particles present. Examples include mass, volume, and free energy change. In a chemical reaction, an extensive property such as the free energy change (?G) increases with the quantity of reactants undergoing the reaction.

Intensive Property

An intensive property is independent of the amount of material present in the system. It characterizes the intrinsic nature of the system regardless of scale. Examples include temperature, pressure, and cell potential (E). The cell potential represents the energy per unit charge and remains the same even if the amount of reactants is scaled up or down.

Conversion from Intensive to Extensive Property in Electrochemistry

In electrochemistry, the relationship ?G = -nFE demonstrates how an intensive property (cell potential, E) can be used to calculate an extensive property (free energy change, ?G). Here, n (the number of moles of electrons transferred) and F (Faraday's constant) act as scaling factors. They convert the per-unit-charge energy given by the intensive property E into the total energy change that depends on the system size, thus linking an intensive property to an extensive one.

Transcript

00:01 Hello, my name is margaret, and today i'm going to be helping you with zoomdahl chapter 18 electrochemistry problem number 27.

00:09 And the problem states, the free energy change for a reaction, delta g, is an extensive property.

00:16 What is an extensive property? surprisingly, one can calculate delta g from the cell potential e for the reaction.

00:23 This is surprising because e is an intensive property.

00:27 How can the extensive property, delta g be calculated? from the intensive property e.

00:34 So let's just start out by talking about intensive and extensive properties, which are covered way back towards the beginning of gen.

00:43 Where we're talking about properties of different parts of matter, or different types of matter.

00:50 And so extensive properties are properties that depend on the amount of matter being measured.

01:16 And examples of this are some of them, you know, there are some of the more obvious ones like, you know, mass, length, volume.

01:31 But obviously, there are some of the more tricky ones when you start to get into some of the really more intense types of properties.

01:47 As you get further in.

01:48 So there are things like, for example, some things that we've covered in this chapter, work, and charge are, and then obviously delta g, are examples of properties that are extensive as well.

02:09 Intensive means that it doesn't depend, or it doesn't matter on the amount, of the mountain of matter present for this particular property.

02:47 And examples of this are obviously cell potential, density, things like this.

03:06 So, now here's something very interesting.

03:14 And this is kind of the key to understanding why this particular problem works.

03:22 And it's not something that's mentioned in the answer in the textbook.

03:27 I don't know about the solutions guide, but when you have a ratio of two extensive properties, you get an intensive.

04:15 And density is a great example of this.

04:19 You know, you ratio mass divided by volume, and all of a sudden you get out density, which is something that doesn't change depending on the amount of things present.

04:38 Okay...

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