2. Nitric oxide (NO) is a potent vasodilator released from endothelial cells. NO
ID: 866119 • Letter: 2
Question
2. Nitric oxide (NO) is a potent vasodilator released from endothelial cells. NO also plays a role in platelet aggregation and leukocyte adhesion. Nitric oxide is short lived, reacting with free radicals and other oxidants. The kinetics of NO breakdown can affect the biological activity of the molecule. Consider the following stoichiometry for the reaction of NO with oxygen: 4NO + O2 + 2H2O - > 4H+ + 4 NO2- (a) The following kinetic data show the rate of appearance of NO2- for different concentrations Find the reaction order with respect to NO and O2 and state the resulting rate law. Determine the rate coefficient. (b The following mechanism has been proposed to explain the observed rate law: Assemble that the Intermediates N2O3 and NO2 are in a quasi-steady State shown that this mechanism gives the rate expression developed in part (a).Explanation / Answer
a)
Rate = k * [NO]a [O2]b , we need to find "a" and "b"
1) 0.2218 x10-6 M s-1 = ( 10 x 10-6 )a (2.31x10-4 )b
2) 1.9958 x10-6 M s-1 = ( 30 x 10-6 )a (2.31x10-4 )b
3) 0.4752 x10-6 M s-1 = ( 30 x 10-6 )a (0.55x10-4 )b
Now we have to work with the equations:
2) / 1) : 1.9958 x10-6 M s-1= / 0.2218 x10-6 M s-1 = ( 30 x 10-6 )a (2.31x10-4 )b / ( 10 x 10-6 )a (2.31x10-4 )b
3a = 9 ----> a = 2
2) / 3) : 1.9958 x10-6 M s-1 / 0.4752 x10-6 M s-1= ( 30 x 10-6 )a (2.31x10-4 )b / ( 30 x 10-6 )a (0.55x10-4 )b
4.199 = 4.200b -----------> b = 1
b) Steady state for N2O3 :
[N2O3] = k2 [NO] [NO2] /(k3 + k4 [H2O] )
Steady state for NO2 :
[NO2] = (2 k1 [NO]2 [O2 ] + k3 [N2O3] ) / k2 [NO]
Now the rate of reaction is:
RATE = k4 [N2O3] [H2O]
if you replace in the rate of reaction equation the value of [N2O3] and [NO2] you will get a rate equation compatible with the experimental data. (a = 2 , b = 1)
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