Question 16.3: CALCULATING THE STANDARD ENTROPY OF REACTION Calculate the s...
CALCULATING THE STANDARD ENTROPY OF REACTION
Calculate the standard entropy of reaction at 25 °C for the Haber synthesis of ammonia:
N_{2}(g) + 3 H_{2}(g) → 2 NH_{3}(g)STRATEGY
To calculate ΔS° for the reaction, subtract the standard molar entropies of all the reactants from the standard molar entropies of all the products. Look up the S° values in Table 16.1 or Appendix B, and remember to multiply the value for each substance by its coefficient in the balanced chemical equation.
TABLE 16.1 Standard Molar Entropies for Some Common Substances at 25 °C
| Substance Formula S°[J/K · mol)] | Substance Formula S°[J/K · mol)] |
| Gases
Acetylene C_{2}H_{2} 200.8 Ammonia NH_{3} 192.3 Carbon dioxide CO_{2} 213.6 Carbon monoxide CO 197.6 Ethylene C_{2}H_{4} 219.5 Hydrogen H_{2} 130.6 Methane CH_{4} 186.2 Nitrogen N_{2} 191.5 Nitrogen dioxide NO_{2} 240.0 Dinitrogen tetroxide N_{2}O_{4} 304.3 Oxygen O_{2} 205.0 |
Liquids
Acetic acid CH_{3}CO_{2}H 160 Ethanol CH_{3}CH_{2}OH 161 Methanol CH_{3}OH 127 Water H_{2}O 69.9 Solids Calcium carbonate CaCO_{3} 91.7 Calcium oxide CaO 38.1 Diamond C 2.4 Graphite C 5.7 Iron Fe 27.3 Iron(III) oxide Fe_{2}O_{3} 87.4 |
ΔS° = 2 S°(NH_{3}) – [S°(N_{2}) + 3 S°(H_{2})]
= (2 mol)\left(192.3 \frac{J}{K · mol}\right) – \left[(1 mol)\left(191.5 \frac{J}{K · mol}\right) + (3 mol)\left(130.6 \frac{J}{K · mol}\right)\right]
= -198.7 J/K
BALLPARK CHECK
As predicted in Worked Example 16.1c, ΔS° should be negative because the reaction decreases the number of gaseous molecules from 4 mol to 2 mol.