Question

Consider the following reaction: NO(g) + NO3(g) --> 2NO2(g) which has an observed rate law of rate = k[NO][NO3].

a. Describe what would happen to the rate if the concentration of NO was cut in half with NO3 held constant.

b. What does this mean on the particle level in terms of distance between NO and NO3 particles?

c. Describe what would happen to the rate if the concentration of NO was doubled with NO3 held constant.

d. What does this mean on the particle level in terms of distance between NO and NO3 particles?

e. What if the concentration of NO was very high and NO3 was very low -- what would happen to the rate of the reaction (and what would affect the rate the most, a slighter increase to [NO] or [NO3])?

** Would love to fully understand this, not just get an answer! Thank you.

Answer 1

The observed rate law of rate = k[NO][NO3] shows the reaction order with respect to NO and NO3 is one each. Thus, the rate of the reaction is directly proportional to the concentration of NO and also directly proportional to the concentration of NO3.

a.
If the concentration of NO was cut in half with NO3 held constant, the rate of the reaction would also be reduced to half as the rate of the reaction is directly proportional to [NO].
rate = k[NO][NO3]
Subsitute in the above equation

b.
On the particle level, the distance between NO and NO3 particles will increase as the [NO] is reduced. THe molecules of NO and NO3 will be far apart. So the collision frequency will decrease. THis will decrease the number of effective collisions. So the rate of the reaction will decrease.

c.
If the concentration of NO was doubled with NO3 held constant, the rate of the reaction would also be doubled as the rate of the reaction is directly proportional to [NO].
rate = k[NO][NO3]
Subsitute [NO] = 2 [NO] in the above equation

d.
On the particle level, the distance between NO and NO3 particles will decrease as the [NO] is increased. The molecules of NO and NO3 will be closer. So the collision frequency will increase. This will increase the number of effective collisions. So the rate of the reaction will increase.

e.
If the concentration of NO was very high and NO3 was very low the rate of the reaction will be given by the rate law rate = k [NO3]. The reaction will become psuedo first order reaction. The [NO] can be assumed to be constant. A slighter increase to [NO] will not affect the rate. But a slighter increase to [NO3] will affect the rate the most.