Chemistry, Third edition

13 · ENERGY CHANGES IN CHEMICAL REACTIONS

Standard enthalpy of formation

The standard enthalpy of formation of a compound H—f^ is the standard enthalpy change

at a particular temperature when one mole of compound is made from its elements.

The standard enthalpies of formation used in this book are those which have been

measured (or calculated) at 25 °C (298 K), and we shall take Hf^ —as meaning H—f^ (298

K). The units of H—f^ are kJ per mole of compoundmade.

Key points about standard enthalpies of formation

●H—f^ is a standardenthalpy change, and therefore applies to a reaction in which

both the elements and compound involved are present in their pure forms at an

atmospheric pressure of 1 atm.

●H—f^ at a particular temperature is a constant for a compound. Extensive com-

pilations of H—f^ values are available. Selected values are shown in Table 13.2.

13.5

226

Aluminium Al(s) 0
Aluminium oxide Al 2 O 3 (s) 1675.7

Bromine Br 2 (l) 0
Bromine Br 2 (g) 30.91
Hydrogen bromide HBr(g) 36.40

Calcium Ca(s) 0
Calcium oxide CaO(s) 635.09
Calcium chloride CaCl 2 (s) 795.8

Carbon (graphite) C(s) (graphite) 0
Carbon (diamond) C(s) (diamond) 1.895
Carbon monoxide CO(g) 110.53
Carbon dioxide CO 2 (g) 393.51
Tetrachloromethane CCl 4 (l) 135.44
Hydrogen cyanide HCN(g) 135.1
Methane CH 4 (g) 74.81
Ethyne (acetylene) C 2 H 2 (g) 226.73
Butane C 4 H 10 (g) 126.15
Cyclohexane C 6 H 12 (l) 156.4
Benzene C 6 H 6 (l) 49.0
Ethanol C 2 H 5 OH(l) 277.69
Sucrose C 12 H 22 O 11 (s)  2222
Urea NH 2 CONH 2 (s) 333.51

Chlorine Cl 2 (g) 0
Chlorine atom Cl(g) 121.68
Hydrochloric acid HCl(aq) 167.16
Hydrogen chloride HCl(g) 92.31

Copper Cu(s) 0
Copper(II) oxide CuO(s) 157.3
Copper(II) sulfate CuSO 4 (s) 771.36
Copper(II) sulfate CuSO 4 ·5H 2 O(s) 2279.7

Fluorine F 2 (g) 0
Fluorine ion F(aq) 332.63
Hydrogen fluoride HF(g) 271.1

Hydrogen H 2 (g) 0
Hydrogen atom H(g) 217.97

Deuterium D 2 (g) 0

Water H 2 O(l) 285.83

Water H 2 O(g) 241.82

Deuterium oxide D 2 O(l) 294.60

Hydrogen ion H(aq) 0

Iodine I 2 (s) 0

Iodide ion I(aq) 55.19

Hydrogen iodide HI(g) 26.48

Iron Fe(s) 0

Iron(III) oxide Fe 2 O 3 (haematite) 824.2

Nitrogen N 2 (g) 0

Nitrogen(II) oxide NO(g) 90.25

Nitrogen(IV) dioxide NO 2 (g) 33.18

Dinitrogen tetroxide N 2 O 4 (g) 9.16

Ammonia NH 3 (g) 46.11

Ammonium chloride NH 4 Cl(s) 314.43

Ammonium chlorate(VII) NH 4 ClO 4 (s) 295.31

Oxygen O 2 (g) 0

Ozone O 3 (g) 142.7

Hydroxide ion OH(aq) 229.99

Phosphorus (white) P (white) 0

Phosphorus (red) P (red)  18

Phosphorus(III) chloride PCl 3 (l) 319.7

Potassium K(s) 0

Potassium hydroxide KOH(s) 424.76

Potassium chloride KCl(s) 436.75

Sodium Na(s) 0

Sodium chloride NaCl(s) 411.15

Sulfur (rhombic) S(s) (rhombic) 0

Sulfur (monoclinic) S(s) (monoclinic) 0.33

Sulfuric acid H 2 SO 4 (l) 813.99

Sulfur dioxide SO 2 (g) 296.83

Sulfur trioxide SO 3 (g) 395.72

Hydrogen sulfide H 2 S(g) 20.63

Table 13.2Standard enthalpies of formation of selected elements and their compounds at 298 K

Substance Formula and state H^ —f/kJ mol^1 Substance Formula and state H^ —f/kJ mol^1