Predict the sign of ΔS^∘ for each of the following reactions. (a) 2 Na(s)+Cl2(g) ⟶2 NaCl(s) (b)

Predict the sign of ΔS^∘ for each of the following...

Predict the sign of $\Delta S^{\circ}$ for each of the following reactions. (a) $2 \mathrm{Na}(s)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NaCl}(s)$ (b) $2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)$ (c) $\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(l)$ (d) $\mathrm{NH}_{4} \mathrm{NO}_{3}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NH}_{3}(g)+\mathrm{O}_{2}(g)$

Predict the sign of $\Delta S^{\circ}$ for each of the following reactions.
(a) $2 \mathrm{Na}(s)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NaCl}(s)$
(b) $2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)$
(c) $\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(l)$
(d) $\mathrm{NH}_{4} \mathrm{NO}_{3}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NH}_{3}(g)+\mathrm{O}_{2}(g)$

Key Concepts

Entropy

Entropy is a measure of the disorder or randomness of a system. In thermodynamics, a positive change in entropy indicates an increase in disorder, while a negative change indicates a decrease in disorder. This concept is fundamental to understanding how energy is dispersed in physical and chemical processes.

Standard Molar Entropy (?S°)

Standard molar entropy refers to the entropy content of a substance under standard conditions (commonly 1 atm pressure and a specified temperature, usually 298 K). It provides a baseline for comparing the disorder of reactants and products in a chemical reaction, helping predict the overall change in entropy.

Effect of Phase on Entropy

The phase of a substance greatly affects its entropy. Generally, gases have the highest entropy due to the increased freedom of movement of particles, liquids have intermediate entropy, and solids have the lowest entropy because their particles are arranged in an ordered lattice. Recognizing the phases involved in a reaction is crucial when predicting the change in entropy.

Effect of Number of Gas Molecules

The number of gas molecules is directly related to the entropy of a system. An increase in the number of gas molecules usually leads to higher disorder and a positive change in entropy, while a decrease results in lower disorder and a negative change in entropy. This concept is key when evaluating chemical reactions that involve gaseous reactants or products.

Transcript

00:04 So thermodynamics is the branch of chemistry and physics that deals with the relationships between heat and other forms of energy.

00:11 So in particular, it does describe how thermal energy is converted to and from other forms of energy and how it can affect matter.

00:19 So here we're looking at entropy.

00:21 So this is just a measure of the disorder of the system.

00:24 And so remember, within the universe, we always want entropy to be increasing.

00:28 That's associated with our spontaneous reaction alongside a negative delta.

00:34 G0 values, so gibbs free energy.

00:37 So delta s naught is calculated by taking the sum of the moles multiplied by the entropy values of the products, subtract the sum of the moles multiplied by the entropy values of the reactants.

00:50 So that's the equation that we need to be using and that we can plug our values in order to get some change in entropy values.

00:58 So with this first example, we've got the formation of sodium chloride.

01:02 And so what we should get, is negative 181 .2 joules per kelvin.

01:09 So here we have a negative value.

01:11 And so what this tells you is that we have the disorder that is decreasing because we're going from a solid and a gas and we're forming one gas.

01:21 A one solid, sorry.

01:23 And remember that the solid has less entropy than a gas does.

01:27 So in this next example, we're taking two gases and we're forming one gas...

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