Rate-determining Step

[hamzah]

I understand that the rate-determining step (rds) is the slowest step in a reaction. However I don't really understand how we establish which "step" it is. Here's the problem:

 

Bromine can be formed by the oxidation of hydrogen bromide with oxygen.

 

The following mechanism has been suggested for this multi-step reaction.

 

1. HBr + O2 --> HBrO2

2. HBrO2 + HBr --> 2HBrO

3. HBrO + HBr --> Br2 + H2O

4. HBrO + HBr --> Br2 + H2O (a repeat of step 3)

 

rate = k[HBr][O2]

 

Explain which of the four steps is the rate-determining step for this reaction.

 

Also I would like to know in general how you establish the RDS for any given reaction provided you're given the rate equation for the reaction.

 

Thanks

[Hades]

are steps 3 and 4 in equilibrium with one another? If so, then that is a fast step; all equilibrium steps are fast.

[Skye]

Whichever species show up in the rate equation are the ones that the rate depends on, because they are the slowest, and so they'll be part of the rate determining step. In the case HBr and O2 are the only species in the rate equation and they're both part of step 1, so I'd assume the rate determining step is that one.

[hamzah]

The notes in my study guide say:

 

In the rate law the order of reaction with respect to a reactant indicates how many molecules of that reactant participate in the rate-determining step.

If a reactant is first order, one molecule reacts in the rds.

If a reactant is second order, two molecules react in the rds.

 

example:

 

H2 (g) + 2ICl (g) --> I2 (g) + 2HCl (g)

rate = [H2][iCl]

 

The stoichiometric equation tells us that overall, 1 molecule of H2, reacts with 2 molescules of ICl to give the products. But it does not tell us about the individual steps which make up the overall reaction. There is no link between the stoichiometric equation and the reaction mechanism or the rate equation.

 

The rate equation tells us that the rate-determining step involves 1 molecule of H2 and and 1 molecule of ICl, because it is a first order with respect to both.

[chemistry]

It's the first one. Look at the rate law to determine which equation stands for the rate determining step.

[hamzah]

thanks everyone