## 15.51

$K = \frac{k_{forward}}{k_{reverse}}$

Merzia Subhan 1L
Posts: 33
Joined: Thu Jul 13, 2017 3:00 am

### 15.51

I was wondering why the rate law is the way that it is:

Step 1: NO + Br2 --> NOBr2 (slow)
Step 2: NOBr2 + NO --> NOBr + NOBr (fast)

Write the rate law for the formation of NOBr implied by this mechanism.

The answer in the back of the book is rate=k[NO][Br2]

I understand that NOBr2 cancels and the overall reaction is 2NO + Br2 --> 2NOBr
Why is the [NO] not squared in the rate law so it would be rate=k[NO]^2[Br2]?
Also what does the slow and the fast next to the equation mean?

Silvino Jimenez 1A
Posts: 52
Joined: Fri Sep 29, 2017 7:04 am

### Re: 15.51

The slow reaction determines the overall reaction rate.

Merzia Subhan 1L
Posts: 33
Joined: Thu Jul 13, 2017 3:00 am

### Re: 15.51

So would I completely disregard the fast one?

Grace Ramey 2K
Posts: 57
Joined: Thu Jul 27, 2017 3:01 am

### Re: 15.51

Yes, the fast step doesn't factor into the overall reaction rate, only the slow step does. So, since the rate for the slow step is k[NO][Br2], that should be the overall reaction rate.

Justin Chang 2K
Posts: 53
Joined: Fri Sep 29, 2017 7:04 am

### Re: 15.51

It is important to not think that the rate is based on the stoichiometric coefficients. Usually, the rate will NOT agree with the stoichiometric coefficients of the balanced equation. If they do, that's merely coincidence.

zanekoch1A
Posts: 25
Joined: Thu Jul 13, 2017 3:00 am

### Re: 15.51

Since the exponents do not come from the coefficients, how are they determined?

KayleeMcCord1F
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Re: 15.51

To determine the exponents we would need some experimental data.