The kinetics of oxidation of nitrite to nitrate by aqueous bromine have been
investigated using a spectrophotometric stopped flow technique. In the pH range 4.2-5.8
(acetate buffer) the rate law is:
- d[Br21,/dt = [Br21[N02 -I2 (a + b/[Br-1)
(where [Br,], = [Br2]+[Br,-1) with a = (4.61-0-1) x lo4 l2 m01-~ s-l and b =
(3.3 1-0.1) x lo4 1. mol-l s-l at 298.2 K and with the temperature dependences,
- R d(lna)/d(l/T) = (46k 4) kJ mol-l and - R d(ln b)/d(l/T) = (45 k 2) kJ mol-'.
In the pH range 0.8-2.5 (perchloric acid) the rate law is :
- d[Br2],/dt = [HN0212[Br21 (w + v/[Br-l)/(l+ z[H+ItBr,l,)
with w = (5.9+0.2)x lo4 l2 m01-~ s-l, v = (3.41-0.1)~ lo4 1, mol-l s-I, and z =
(1.90i 0.06) x lo7 l2 mol-2 at 298.2 K. In addition:
- R d ln(w/z)/d(l/T) = (31 1 4 ) kJ mol-I
and
- R d ln(v/z)/d(l/T) = (46 f 4) kJ mol-l
In the pH range 2.8-3.3 (chloroacetate buffer) a combination of these two rate
laws adequately describes the kinetic results.
These rate laws have been interpreted in terms of two reversible initial reactions:
6) NO2- +Br2 + N02Br +Br-
(followed by attack on N02Br by NO2-)
(ii) NO2-+NO2- (or HNOJ + N204'- (or HN204-)
(followed by attack by Br2 upon N204'- or HNZO4- or upon N203 formed from
HN204-).
The kinetics of oxidation
of nitrite to nitrate by chlorine in aqueous acid solution (pH 0-1) have been
studied using a spectrophotometric stopped flow technique. The rate law is
���������������� -d[Cl2]a/dt =([Cl2][NO2-]/Ka[Cl-]2)(f+g[HNO2])
where [Cl2]a
= [Cl2]+[Cl3-] and where Ka is the
ionization constant for nitrous acid. At 298.2 K and ionic strength 2.75M, f =
60.8�0.5 mol2 l-2 s-1 and g = (2.35�0.05)x105
mol l-1 s-1: the associated activation energies are 68�3
and 44�2 kJ mol-1 respectively. A mechanism is proposed involving
the reversible initial step:
�������������������������� NO2-+Cl2
↔ NO2Cl+Cl-
with the NO2Cl
undergoing two parallel subsequent reactions, one a unimolecular decomposition
and the other an attack by HNO2 upon NO2Cl. ��� Oxidation of nitrite by the three halogens,
Cl2, Br2, I2, is discussed.
Ihe kinetics of the reversible reaction H s A s O~ + 13-+ HzO Z? H~A s O~ + 2H' + 31have k e n studied spectrophotometrically in acid solution under conditions in which both the forward and reverse reactions go to virtual completion and in which the reaction comes 10 a practical equlibrium. The rates of rhe forward (I?,) and reverse ( R r ) reacrions are givm b y R , = if + gl[H+] + h/[H+]'] [HJAsOS] [I3-]/"I-I2 R, = /u[H-] + u [H+I21 [ H~A s O~] [I-] Lzheref. g, h. u. and u have the values (4 =t 1) X 10-j mole/'l. . S , (4.2 =k 0.2) X 10-j mole2 I.*.s. (5.0 =t 0.3) X lo-' m~l e~/ I .~. s . ( 1 . 1 f 0.1) X I.zjmole2.s, and (3.7 + 0.2) X 10-3 1.3/mole3.s at 298.2"K and at an ionic strength of 2.00M maintained by adding sodium chloride. The stoichiometric equilibrium constant under similar conditions is 0.022 + 0.003. Different values of these parameters were obtained when sodium perchlorate and sodium nitrate were used to control ionic strength.The results are compared with those from previous reports and a mechanism is proposed hased upon an initial rapid equilibrium H~A s O~ + I ? S H J A s O~I + + 1lollowed by a rate-determining attack of water upon HaAsOd+. HyAsO3I. and HAsOaI-.
Spectrophotometric methods have been used to obtain rate laws and rate parameters for the following reactions: with k,, kb, E,, Eb having the values 85 f 5 l./mole.s, 5.7 f 0.2 s-l (both at 298.2'K), and 56 f 4 and 66 f 2 kJ/mole, respectively. (2) BrCN + 31-+ H + + 1 3 -+ Br-+ HCN -d[BrCN]/dt = k,[BrCN][I-] with k, = 0.106 f 0.004 I./mole .s at 298.2'K and E , = 67 f 2 kJ/mole.(3) S(CN)z + 31-+ H + -+ 1 3 -+ SCN-+ HCN d[S( CN)tI/dt = kd[S (CN)zI [I -1 with k d = (3.06 f 0.15) X l./mole.s at 298.2'K and Ed = 66 f 2 kJ/mole. Mechanisms for these reactions are discussed and compared with previous work.205
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