Introduction to Hydrogen Peroxide/Physical and Chemical Properties

H₂O₂

Thermodynamic Properties

General note: Determination of thermodynamic properties for H₂O₂ are complicated by:

1) Uncertainties due to H₂O₂ decomposition

2) Irreversibility of H₂O₂ formation and decomposition.

1. Molecular data

Molecular Picture

NOTE: The H₂O₂ molecule has no center of symmetry.

Bond angles:	q (H-O-O angle): 95^o ± 2^o f (Dihedral angle): 120^o ± 3^o Ref: Gmelin "Handbuch der Anorganischen Chemie", Suerstoff – syst. 3, Lief. 7-8 Auflage – Weinheim – VERLAG Chemie, p.429 (1966)
Bond length:	O-H: 0.097 ± 0.001 nm - Ref: P.A. Giguere and O. Bain, J.Phys.Chem. 56:340-42 (1952) O-O: 0.149 ± 0.001 nm - Ref: S.C. Abrahams, et.al., Acta Cryst. 4:15-20 (1951)
Bond strength:	HO-OH: 51 ± 1 kcal/mole - Ref: J.A. Kerr, Chem.Rev. 66:465 (1966) H-OOH: 90 ± 2 kcal/mole - Ref: J.A. Kerr, Chem.Rev. 66:465 (1966)
Dipole moment:	m = 2.2 debyes

Vibration:

Wave Number, cm^-1

O-H stretching
Symmetric bending
O-O stretching
Torsional oscillation
O-H stretching
Unsymmetrical bending

3610
1295
890
520
3610
1266

Moments of inertia (g.cm²):

I_A = 2.78 x 10^-40
I_B =34.0x10^-40
I_C = 33.8 x 10^-40
I_red = I_A/4 = 0.696 x 10^-40

Barrier restricting internal rotation:
Absolute entropy:

V_o = 3.5 kcal/mole
S^o_298.16 = 55.66 cal/mole ^oK

P.A. Giguere, I.D. Liu, J.S. Dugdale, J.A. Morrison. Can. J. Chem., 74:3715 (1952)

NOTES:

1. In rotation as a whole, the molecule remains rigid.
2. Vibration may be considered to be harmonic oscillations.

2. Thermodynamic functions

(for vapor at 1 atm.)

Functions

P.A. Giguere, I.D. Liu, J.S. Dugdale, J.A. Morrison. Can. J. Chem., 74:3715 (1952)

NOTES:

C_p^o = constant pressure heat capacity
F^o = free energy
H_o^o = enthalpy at absolute zero
T = absolute temperature
H^o = enthalpy of H₂O₂ as ideal gas at 1 atmosphere

3. Heat Capacity

Heat Capacity

P.A. Giguere and B.G. Morissette. Can. J. Chem. 33:804 (1955)

NOTES:

1. The value for liquid anhydrous H₂O₂ over the temperature range of 0-27 ^oC is 0.628 cal/gm.^oC
2. The deviation from ideal (mole fraction average heat capacity) is negative (i.e., H₂O₂ solutions have heat capacities lower than the average of two unmixed components).

4. Heat of Dilution

Heaf of Dilution

P.A. Giguere and B.G. Morissette. Can. J. Chem. 33:804 (1955)
G. Scatchard, G.M. Kavanagh, and L.B. Ticknor, J. Am. Chem. Soc. 74:3715 (1952)

NOTE:

The effect of dilution is mildly exothermic (negative D H₁) for all concentrations at > 21 ^oC. Some dilution processes below this temperature are endothermic

5. Heat of decomposition

Heat of Decomposition

P.A. Giguere, Complements au Nouveau Traite de Chemie Minerale – No. 4 Peroxyde d’Hydrogene et Polyoxydes d’Hydrogene, Paris, Mason, p.181 (1975)

NOTES:

1. The standard free energy change (D F^o) is -27.92 kcal/mole at 25 ^oC
2. Rapid decomposition of concentrated H₂O₂ solutions may not be complete, with concentrations up to 10% remaining.

6. Heat, Free Energy, and Equilibrium Constant

H₂O₂ (g) è H₂O (g) + ½ O₂ (g) D H_dec = 23.44 kcal/mole

W.C. Schumb, C.N. Satterfield, R.L. Wentworth. Hydrogen Peroxide, ACS Monograph, Reinhold Publishing, pg. 251 (1955).

7. Decomposition products

Decomposition Products

Decomposition Products 2

8. Decomposition volumes

Oxigen Liberation Capacity

Volume Expansion Ratios

NOTES:

Isothermal refers to the slow, controlled decomposition where ambient temperature and pressure are maintained – the gas volume is comprised of essentially pure oxygen. Adiabatic refers to the rapid, accelerated decomposition where the temperature is allowed to increase but the pressure remains ambient – the gas volume is comprised of both oxygen and steam (water vapor).

9. Self-accelerated decomposition

Self-Accelerated Decomposition

NOTES:

1. H₂O₂ decomposition is highly exothermic (23.44 kcal/mole). Even 10% H₂O₂ can boil if it becomes grossly contaminated.
2. The effect of temperature is such that an increase of 10 ^oC increases the rate of decomposition by a factor of 2.3 (i.e., a first order rate equation). Therefore, decomposition can accelerate if the solution becomes grossly contaminated.
3. As the concentration of H₂O₂ in solution increases, there is less water to absorb the heat of decomposition. A crossover occurs at 63-64% H₂O₂ where rapid, accelerated decomposition becomes self-sustaining and the concentration of H₂O₂ in the decomposing solution can actually increase.

10. Free energy of formation

H₂(g) + O₂ (g) è H₂O₂ (aq) D F^o = -31.95 kcal/mole (25 ^oC)

Free energy of formation

11. Standard electrode potentials

H₂O₂ contains oxygen in a state of oxidation midway between molecular oxygen and water.

	Reduction è
Oxygen moiety ……….	O₂	ó	H₂O₂	ó	H₂O
Oxygen valence ………	0		-1		-2
	ç Oxidation

H₂O₂ è O₂ + 2H⁺ + 2e^-	E^o = -0.682 V
H₂O₂ + 2H⁺ + 2e^- è 2H₂O	E^o = 1.776 V

For perhydroxyl ion (HO₂^-):

OH^- + HO₂^- è O₂ + H₂O + 2e^-	E_B^o = 0.084 V
3OH^- è HO₂^- + H₂O + 2e^-	E_B^o = -0.87 V

12. pH and Ionization Constant

pH and Ionization

13. Dissociation: Heat, Free Energy, and Equilibrium Constant

	D H^o	D F^o (kcal/mole)
Nonionic
H₂O₂ (g) = H₂O (g) + O(g)	+ 33.90	+ 25.60
H₂O₂ (g) = 2H (g) + O₂(g)	+ 136.72	+ 122.41
H₂O₂ (g) = H₂(g) + 2O (g)	+ 150.86	+ 135.23
H₂O₂ (g) = 2H (g) + 2O (g)	+ 255.04	+ 232.39
H₂O₂ (g) = H (g) + O₂H (g)	+ 90.
O₂H (g) = H (g) + O₂ (g)	+ 46.

Ionic
H₂O₂ (aq) = H⁺ + O₂H^-	+ 8.2	+ 15.89

At 25 ^oC, K = [(H⁺) (O₂H^-)] / (H₂O₂) = 2.24 x 10^-12

The free energy of formation for O₂H^- is –15.23 kcal/mole.

14. Related electrochemical values

	Potential, volts
HO₂ + H⁺ + e^- ó H₂O₂	1.5
O₂ + H₂O + 2e^- ó HO₂^- + OH^-	-0.076
O₂ + 2H₂O + 2e^- ó H₂O₂ + 2OH^-	-0.146
HO₂^- + H₂O + 2e^- ó 3OH^-	0.87

Heat for formation (D H_f^o) for:

HO^.	9.2 ± 1 kcal/mole	Ref: J.A. Kerr, Chem.Rev. 66:465 (1966)
HO₂^-	5.3 ± 2 kcal/mole	Ref: J.A. Kerr, Chem.Rev. 66:465 (1966)

[Forward to Electrical Properties]

H2O2

Thermodynamic Properties

H₂O₂