Kinetics question

1. May 4, 2015

mooncrater

1. The problem statement, all variables and given/known data
The question says:
The decomposition of a gas on a surface follows the rate law : Rate is proportional to $\frac {kP}{1+kP}$, where $P$ is the pressure and $k$ is a constant. Find the order of the reaction at high pressure.

2. Relevant equations

3. The attempt at a solution
In the solution they have said that since $P$ is high therefore $kP>>>1$ due to which rate is proportional to $\frac {kP}{kP}=1$ which means the rate is a constant so it is a zero order reaction. What I say is that since they don't known the value of $k$ how can they come to the conclusion that $kP>>>1$? If we assume k to be equal to a very very small number such that $1>>>kP$ then their assumption fails, and the rate is not a constant.
So am I right about this thing or making a mistake?

2. May 4, 2015

Staff: Mentor

If doesn't matter what the value of k is. If you take a high enough pressure, the order will approach zero.

chet

3. May 4, 2015

mooncrater

But if we leave that proven part (I. e if pressure is increased order will tend towards zero) and use only this :
Rate is proportional to $$\frac {kP}{1+kP}$$
Then isn't the value of $k$ important?

4. May 4, 2015

Staff: Mentor

Sure. At pressures less than on the order of 1/k.

Chet

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