Textbook Question
Calculate the change in entropy that occurs in the system when 1.00 mole of isopropyl alcohol (C3H8O) melts at its melting point (-89.5 °C). See Table 12.9 for heats of fusion.
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19. Chemical Thermodynamics
Entropy Calculations
Multiple Choice
Calculate ∆S_rxn for the reaction 2NO(g) + O2(g) → 2NO2(g) given the following standard molar entropies: S°(NO) = 210 J/mol·K, S°(O2) = 205 J/mol·K, S°(NO2) = 240 J/mol·K.
A
-45 J/mol·K
B
-87 J/mol·K
C
45 J/mol·K
D
87 J/mol·K
Verified step by step guidance1
Understand that the change in entropy for a reaction, ∆S_rxn, is calculated using the standard molar entropies of the reactants and products. The formula is: ∆S_rxn = ΣS°(products) - ΣS°(reactants).
Identify the standard molar entropies given in the problem: S°(NO) = 210 J/mol·K, S°(O2) = 205 J/mol·K, S°(NO2) = 240 J/mol·K.
Calculate the total entropy of the products. Since there are 2 moles of NO2, multiply the standard molar entropy of NO2 by 2: 2 × S°(NO2) = 2 × 240 J/mol·K.
Calculate the total entropy of the reactants. For NO, there are 2 moles, so multiply the standard molar entropy of NO by 2: 2 × S°(NO) = 2 × 210 J/mol·K. For O2, there is 1 mole, so use the given value directly: S°(O2) = 205 J/mol·K.
Subtract the total entropy of the reactants from the total entropy of the products to find ∆S_rxn: ∆S_rxn = [2 × S°(NO2)] - [2 × S°(NO) + S°(O2)].
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