Calculate ΔSf^∘ for these compounds in J mol^-1 K^-1. (a) Al2 O3(s) (b) CaSO4 ·(1)/(2) H2 O(s)

step 1 To calculate the delta S of formation, we simply need a chemical reaction that takes the element

Calculate ΔSf^∘ for these compounds in J mol^-1 K^-1. (a)...

Calculate $\Delta S_{\mathrm{f}}^{\circ}$ for these compounds in J $\mathrm{mol}^{-1} \mathrm{~K}^{-1}$.
(a) $\mathrm{Al}_{2} \mathrm{O}_{3}(s)$
(b) $\mathrm{CaSO}_{4} \cdot \frac{1}{2} \mathrm{H}_{2} \mathrm{O}(s)$
(c) $\mathrm{NH}_{4} \mathrm{Cl}(s)$

Key Concepts

Standard Molar Entropy

This concept refers to the absolute entropy of one mole of a substance measured under standard conditions, typically given in units of J·mol?¹·K?¹. It is a fundamental state function in thermodynamics that quantifies the degree of randomness or disorder in a system. Standard molar entropy values are essential for calculating the overall entropy change in chemical reactions.

Standard State

The standard state of a substance is the reference state used when reporting thermodynamic properties, such as entropy. For pure substances, this typically means the form in which the substance exists most commonly at 1 bar pressure and a specified temperature (usually 298.15 K). The concept of the standard state provides a common basis for comparing thermodynamic data and calculating changes in entropy during chemical reactions.

Entropy of Formation

Entropy of formation refers to the change in entropy when one mole of a compound is formed from its constituent elements in their standard states. Unlike formation enthalpies, the standard entropy of formation is not defined as zero for elements in their standard states, so the calculation involves summing the standard entropies of the final compound and subtracting the entropies of the elemental reactants.

Reaction Entropy Calculation

This concept involves using Hess's law for entropy to determine the overall change in entropy for a formation reaction. The reaction entropy change is calculated by taking the difference between the sum of the standard molar entropies of the products and the sum of the standard molar entropies of the reactants. This calculation is crucial in predicting the spontaneity of a reaction and understanding the disorder evolution in chemical processes.

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