Sciencemadness Discussion Board

Basic Kinetics Question(Gen. Chem)

smaerd - 18-2-2011 at 18:26

So here's the question.

CHCl3(g) + Cl2(g) -> CCl4(g) + HCl(g)

Rate-law: Rate = k*[CHCl3]*[Cl2]^(1/2)

What is the overall order and the units of the rate constant for the reaction?

The overall order is 3/2 no problem there :P.

The problem I'm having is this. The CHCl3 is a first order reaction(Unit = 1/S) while the Cl2 is a 1/2 order reaction. In my book it has what a Zero order, First Order, and Second order reaction are but mentions nothing of non-integer order reactions. What am I supposed to do?

Magpie - 18-2-2011 at 20:38

Quote: Originally posted by smaerd  

CHCl3(g) + Cl2(g) -> CCl4(g) + HCl(g)

Rate-law: Rate = k*[CHCl3]*[Cl2]^(1/2)

What is ... the units of the rate constant for the reaction?

What am I supposed to do?


The units of k are whatever are needed to provide equal units on both sides of the equation.

So, presuming the units on the left are moles/(liter-s) the units for k are [(moles/liter)^(-1/2)]/s.


smaerd - 19-2-2011 at 08:50

Well the answer he provided was 1/(S * M^(1/2)) and I'm just not seeing it.

His work shows this: K = M * S^(-1) / M ^(3/2)
I have no idea where the 3/2 came from, one is first order and one is 1/2 order? Unless its from the overall order?

Magpie - 19-2-2011 at 09:38

Quote: Originally posted by smaerd  
Well the answer he provided was 1/(S * M^(1/2)) and I'm just not seeing it.

His work shows this: K = M * S^(-1) / M ^(3/2)
I have no idea where the 3/2 came from, one is first order and one is 1/2 order? Unless its from the overall order?


The answer he gave is the same as the one I gave where M=moles/liter, and S = s.

K=M*S^(-1)/M^(3/2) = M^(-1/2)/S = 1/(S*M^(1/2))

K is just a constant that provides equality based on the units (dimensions) that were chosen. It could just as well have been expressed in (molecules/gallon)/hr.


smaerd - 19-2-2011 at 21:22

Oh wow I feel retarded.

This is what happens when you study calculus 2 for 5 hours then try to hop straight into chemistry. You confuse yourself lol. Didn't help that my professor covered the chapter in 3 one hour class periods. Thanks for bearing with me. Much appreciated. Kinda let down that I couldn't see that...

[Edited on 20-2-2011 by smaerd]