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Kinetics & Mechanism of the Reaction Between Molecular Hydrogen H2 and Nitrogen Oxide NO
posted August 21, 2004

Long Abstract

by

Professor David L. Allara, Department of Chemistry,
185 MRI Building, 230 Innovation Boulevard,
Penn State Univ., University Park PA 16802

Distinguished professor Sidney W. Benson
Scientific Co-Director Emertitus,
Loker Hydrocarbon Research Institute,
University Park - MC1661
University of Southern California
Los Angeles CA 90089-1661

Dr Michael Cowperthwaite
1536 Bittern Avenue
Sunnyvale CA 94087
  Professor Dan McElroy, Computer Information Systems
San Jose City College, 2100 Moorpark Avenue
San Jose CA 95128-2799

Bob Shaw
191 East El Camino Real Spc 245
Mountain View CA 94040-2708
(accountable for any errors)

Frederick Armstrong Shaul
zap technologies
(article web hosting)


The Hinshelwood-Mitchell-Green experiments of the 1920s and 30s on the reaction between molecular hydrogen, H2, and nitrogen oxide, NO
2H2 + 2NO -> 2H2O + N2 [0]

have been re-interpreted using thermochemical kinetics.

Because many questions remain, especially about HNO & HON, this is a progress report. Please correct us verbally. By inspection, the mechanism is
H2 <-> 2H             [H2 equilibrium]
HNO <-> H + NO        [HNO equilibrium]
HON <-> H + NO        [HON equilibrium]
2HNO -> H2O + N2O      [1]
2HON -> H2O + N2O      [2]
HNO + NO -> HO + N2O   [3]
HON + NO -> HO + N2O   [4]
HO + H2 -> H2O + H     [5]
H + N2O -> HO + N2     [6]

Reactions [1] through [4] are rate determining.

The rate of the overall reaction is
-d[N2]/dt = a[H2] + b[H2]0.5            [7]
where
a = {(k1/KHNO)2) + (k2/KHON)2)}KH2[NO]2   [8]
and
b = {(k3/KHNO) + (k4/KHON)}(KH2)0.5[NO]2  [9]
From a least squares treatment of the data fo [NO] = 300 mm, and [H2] from 15 to 414 mm at 1074 K,
a = 10-3.1s-1    b = 10-3.9M0.5s-1
Using equilibrium constants, x,
x/M    form of log(x/M)         log(x/M) at 1074 K
KH2     3.3 - (103.7)/theta)   -17.9
KHNO     3.6 - (48.1)/theta)    -6.2
KHON    3.6 - (23.2)/theta)    -1.1

where theta is 10-32.303RT kcal/mole, 4.91 kcal/mole at 1074 K.

At 1074 K, 300 mm NO = 10-2.34 M, & 1 mm/(100s) = 10-6.82 M s-1.

If reaction [2] is ignored, then k1 = 107.1M-1s-1, in line with results determined by other workers.

From literature, k3 = 104.2 M-1 s-1 at 1074 K.

This gives k4 = 108.9 M-1 s-1, compared with 109.5 M-1 s-1, estimated at a meeting of the Western States Section of the Combustion Institute at Stanford in 1995.

Because the rate of [1] is almost independent of temperature, if the hydrogen equilibrium is set up, say catalytically, the overallreaction will go at a measurable rate at much lower temperatures.

We thank all those who have helped.