All the oxides of nitrogen have positive values of ΔGf^9 at 298 K , but only one common oxide of

All the oxides of nitrogen have positive values of ΔGf^9 at...

All the oxides of nitrogen have positive values of $\Delta G_{f}^{9}$ at 298 $\mathrm{K}$ , but only
one common oxide of nitrogen has a positive $\Delta S_{\mathrm{f}}^{\mathrm{a}}$ Identify that oxide of nitrogen without reference to thermodynamic data and explain.

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Key Concepts

Standard Entropy of Formation

This term describes the change in entropy when one mole of a substance is formed from its constituent elements in their standard states. It is a foundational concept in chemical thermodynamics that helps predict whether a reaction will involve a gain or loss of disorder, independent of enthalpy or energy considerations.

Gas?Phase Reactions and Mole Count

In reactions involving gases, the number of gaseous molecules has a significant effect on the overall entropy change. When the number of gas molecules does not decrease, the disorder (entropy) of the system can be maintained or even increased. This concept is often utilized to qualitatively assess whether a reaction’s standard entropy change might be positive or negative.

Molecular Structure and Radical Character in Nitrogen Oxides

Certain molecules, such as nitric oxide (NO), incorporate features like unpaired electrons (radical character) and unique bonding that contribute additional intrinsic disorder. These electronic features and the associated increased number of accessible microstates can lead to a positive change in entropy upon formation from the elemental gases. In this context, NO stands out among the common nitrogen oxides.

Transcript

00:01 In this problem, we are beginning to understand the relationship between gibbs free energy, spontaneity, and entropy.

00:09 Now, all of these concepts are very foundational and important topics in chemistry.

00:16 All of these things are really what we use to understand any chemical reaction.

00:21 So it's really interesting that we are now applying this to something called nitrogen oxides.

00:28 So let's first review the equation for gibbs free energy.

00:31 Energy.

00:32 We denote gibbs free energy as delta h, or pardon me, delta g, and delta g is equal to delta h minus t delta s.

00:42 Now, what do all of these things mean? delta h is enthalpy.

00:47 That is our change in heat.

00:49 T is our temperature, which is a constant, and delta s is our entropy.

00:55 Entropy, you can think of as the freedom of motion, the movement of particles, and how they, they can essentially arrange themselves.

01:07 So the thing that we need to determine here is which conditions of these variables make a reaction spontaneous.

01:16 Well, that's when delta g is negative.

01:19 And if delta g is negative, that means that delta s is negative and delta h is positive.

01:27 And we want these things in order for our reaction to be spontaneous...

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