Using data from Appendix 4 calculate ΔH^∘, ΔS^∘, and ΔG^∘ for the reaction N2(g)+O2(g) ⟶2 NO(g)

Using data from Appendix 4 calculate ΔH^∘, ΔS^∘, and ΔG^∘...

Key Concepts

Standard Gibbs Free Energy Change (?G°)

The standard Gibbs free energy change combines the effects of enthalpy and entropy to determine the overall spontaneity of a reaction under standard conditions. Expressed as ?G° = ?H° - T?S°, it indicates whether a reaction is thermodynamically favorable. A negative ?G° signifies a spontaneous process, while a positive ?G° means the reaction is non-spontaneous under standard conditions. This parameter is crucial for predicting the feasibility of chemical reactions and understanding their equilibrium positions.

Thermodynamic vs Kinetic Control

This concept differentiates between the conditions that determine the final equilibrium state of a reaction (thermodynamics) and those that affect the rate at which the reaction proceeds (kinetics). Thermodynamics, as described by ?G°, tells us whether a reaction is energetically favorable, while kinetics focuses on the activation energy barrier that must be overcome for the reaction to occur. In scenarios where reactants form products under high-temperature conditions—as in an automobile engine—a reaction can proceed rapidly; however, at lower temperatures, even thermodynamically favorable reactions may be hindered by high activation barriers, making the reverse process slow or negligible.

Standard Enthalpy Change (?H°)

This concept refers to the heat absorbed or released during a chemical reaction under standard conditions (usually 1 bar, 298 K). It is calculated using the standard enthalpies of formation of the reactants and products. A positive ?H° indicates an endothermic reaction (heat absorbed), while a negative ?H° indicates an exothermic reaction (heat released). Understanding ?H° is essential for predicting whether energy is required or released during the transformation of substances in a reaction.

Standard Entropy Change (?S°)

Standard entropy change quantifies the change in disorder or randomness as a reaction progresses under standard conditions. It is determined by the difference between the total standard entropy of the products and that of the reactants. A positive ?S° generally indicates an increase in randomness (for example, an increase in the number of gas molecules), while a negative ?S° suggests a decrease in disorder. This concept helps in understanding how the distribution of energy and particles changes during a chemical reaction.

Transcript

00:01 So in this video we're going to work through problem 54 from chapter 20, which says using data from appendix 4, calculate delta h, delta s, and delta g for the reaction of n2 plus o2 forming 2no, where all our reagents and products are gases.

00:16 Why does n0 form in an automobile engine, but then does not readily decompose back to n2 in the atmosphere? so before we pick apart that last question, let's go ahead and calculate delta h, delta, s, and delta g.

00:30 By first visiting appendix 4.

00:33 So in pendix 4, we have delta h and delta g in units of kilojoules per mole, and we have the entropy s in units of joules per kelvin mole.

00:43 And then your book lists these thermodynamic data for compounds both in the gaseous phase or in solution or in a liquid or solid phase.

00:56 You need to make sure you're looking at nitrogen, oxygen, and in o, in the gas phase.

01:03 But anyway, this would be the data that you would get from appendix 4.

01:08 So let's plug this into our calculation for the enthalpy, where we just take the products minus the reactants and we multiply each reagent by the stoichiometric coefficient.

01:20 So we have two times no, which is 90, minus zero and zero for n2 and o2.

01:26 And that gives us 180 kilojoules per mole for our change in enthalpy for this reaction.

01:33 What about gibbs free energy? well, we do the same thing.

01:36 We take products minus reactants, and our reaction is still n2, plus o2 yields 2 in o.

01:40 So we have 2 times n0, which is 87, minus n2, which is 0, minus o2, which is zero.

01:47 So 2 times 87 gives us 174 kilojoules per mole for our gibbs free energy.

01:54 And then for entropy, we do the same thing.

01:56 Again, we take products minus reactants and we multiply everything by its stoichiometric coefficient.

02:01 So we have two times no, which is 211, minus n2, which is 205, minus o2, which is 195.

02:11 So that works out to 25 joules per kelvin mole.

02:19 So what does delta g tell us? we know that a reaction is spontaneous if it releases free energy.

02:26 Remember, free energy is energy that's available to do work.

02:29 So we need to release free energy in order for this reaction to be spontaneous, which means that we need delta g to be negative.

02:38 That would correspond to releasing energy.

02:41 So is delta g negative in this case? well, delta g is 174 kilojoules per mole...

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