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16.7: Standard Molar Entropies

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    • Because the entropy of a substance depends on the amount of substance, the pressure, and the temperature, it is convenient to describe the entropy of a substance in terms of Sm°, its standard molar entropy, i.e., as the entropy of 1 mol of substance at the standard pressure of 1 atm (101.3 kPa) and given temperature. Values of the standard molar entropies of various substances at 298 K (25°C) are given in the table. A table like this can be used in much the same way as a table of standard enthalpies of formation in order to find the entropy change ΔSm° for a reaction occurring at standard pressure and at 298 K.

    Table \(\PageIndex{1}\) The Standard Molar Entropies of Selected Substances at 298.15 K (25°C)

    Compound Smo /J K-1mol-1 Compound Smo /J K-1mol-1
    Solids Diatomic Gases
    C (diamond) 2.377
    C (graphite) 5.74 H2 130.7
    Si 18.8 D2 145.0
    Ge 31.1 HCl 186.9
    Sn (gray) 44.1 HBr 198.7
    Pb 64.8 HI 206.6
    Li 29.1 N2 191.6
    Na 51.2 O2 205.1
    K 64.2 F2 202.8
    Rb 69.5 Cl2 223.1
    Cs 85.2 Br2 245.5
    NaF 51.5 I2 260.7
    MgO 26.9 CO 197.7
    AlN 20.2 Triatomic Gases
    NaCl 72.1 H2O 188.8
    KCl 82.6 NO2 240.1
    Mg 32.7 H2S 205.8
    Ag 42.6 CO2 213.7
    I2 116.1 SO2 248.2
    MgH2 31.1 N2O 219.9
    AgN3 99.2 O3 238.9
    Liquids Polyatomic Gases( > 3)
    Hg 76.0 CH4 186.3
    Br2 152.2 C2H6 229.6
    H2O 69.9 C3H8 269.9
    H2O2 109.6 C4H10 310.2
    CH3OH 126.8 C5H12 348.9
    C2H5OH 160.7 C2H4 219.6
    C6H6 172.8 N2O4 304.3
    BCl3 206.3 B2H6 232.0
    Monatomic Gases BF3 254.0
    He 126.0 NH3 192.5
    Ne 146.2
    Ar 154.8
    Kr 164.0
    Xe 169.6

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