Rate depends on concentration?!

That is, the change in concentration with respect to time depends on the concnetration...

For example... the following concentration and time data was collected.

Time
concentration
average rate
NO2
NO
O2
rate = d[NO2]/dt
0
0.01
0
0

50
0.0079
0.0021
0.0011
4.2x10-5
100
0.0065
0.0035
0.0018
2.8x10-5
150
0.0055
0.0045
0.0023
2.0x10-5
200
0.0048
0.0052
0.0026
1.4x10-5
250
0.0043
0.0057
0.0029
1.0x10-5
300
0.0038
0.0052
0.0031
1.0x10-5
350
0.0034
0.0066
0.0033
8.0x10-6
400
0.0031
0.0069
0.0035
6.0x10-5

The rate is the average rate during the period from the first to the second time.

Notice that as the concentration of NO2 decreases the rate at which NO2 is consumed also decreases.

the rate of the reaction depends
on the concentration concentration of the reactants

 

However, the rate is not always linearly dependent on concentration that is why the x is there. If the rate was proportional to the concentration the the rate law would be

If the rate increased with concentration squared the rate law would be

The order of the reaction with respect to NO2 CANNOT be determined by inspection of the balanced equation. The order of a reaction must be determined experimentally. Additionally, the order of a reactant has a physical significance which we will discuss later.

By the way, did I mention

 

The order of a reaction cannot be determined by the stoichiometry of the reaction.

 

Let me repeat that

 

The order of a reaction cannot be determined by the stoichiometry of the reaction.

 

One more time

 

The order of a reaction cannot be determined by the stoichiometry of the reaction.

 

OK then how does one determine the order of a reation?