Find ΔS rxn ^∘ for the reaction of nitrogen monoxide with hydrogen to form ammonia and water vap

Find ΔS rxn ^∘ for the reaction of nitrogen monoxide with...

Key Concepts

Reaction Entropy Change Calculation

This concept involves determining the entropy change for the entire reaction by summing the standard molar entropies of the products and subtracting the sum of the standard molar entropies of the reactants. The formula, ?S° = ?S°(products) ? ?S°(reactants), is widely used in thermodynamics to predict the spontaneity and feasibility of reactions.

Impact of Stoichiometry and Phase Changes on Entropy

The overall entropy change in a reaction is heavily influenced by changes in the number of moles of gaseous species. Generally, reactions that result in a decrease in the number of gas-phase molecules lead to a decrease in entropy (negative ?S°). Understanding the stoichiometry and the physical states of the reactants and products helps in anticipating whether the sign of ?S° is positive or negative.

Standard Molar Entropy

This is the measure of entropy for a substance under standard conditions (usually 1 bar or 1 atm pressure), expressed per mole. It provides a baseline for quantifying the disorder or randomness inherent in a substance's molecular arrangement and is essential for calculating overall entropy changes in chemical reactions.

Transcript

00:01 So problem 37 asks us to find the delta s reaction of the following reaction.

00:07 It doesn't say a following reaction.

00:09 It says the reaction between nitrogen monoxide and hydrogen gas, which yields ammonia, gase ammonia, and gaseous water.

00:20 And it asks what to determine to see if the sign calculated is exactly what we expect.

00:28 There is a sign that we expect of the delta s.

00:31 So to start off, we have our reaction here that they give us, well they give in words, but i translated it to this following equation.

00:43 So one thing to note is that this is not balanced.

00:46 So we just got to balance that, which should be no biggie right here.

00:49 We have two moles of nitrogen monoxide, five moles of gaseous hydrogen gas, two moles of ammonium gas, two moles of ammonia, and two moles of gaseous water.

01:01 Okay, so now, at this point, we can determine.

01:03 Is we can find out or we can determine for ourselves what can we expect from the delta s reaction it would be positive would be negative is an increase in increase entry or is it decrease entry so let's start off with this so things they're all gases so that's very nice makes things a lot simpler so all we got to do is just kind of we have to determine where we have more moles of gas so we have seven moles of gas here and we have four moles of gas here so clearly that is a decrease, equals from 7 to 4.

01:35 Thus, we have a decrease in entropy.

01:39 Okay, and unless our delta -s reaction, it should be negative.

01:43 It should be based off our equation, it should be negative.

01:47 Right here.

01:49 That's good up.

01:50 Oops.

01:51 Okay.

01:52 And to calculate it, we simply just kind of take products minus reactants...

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