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 S_m°, 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 ΔS_m° 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	S_m^o /J K^-1mol^-1	Compound	S_m^o /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