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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: calvin coolidge on February 18, 2011, 07:52:20 PM

Title: Arrhenius equation.....what are the units for k?
Post by: calvin coolidge on February 18, 2011, 07:52:20 PM

In the equation, k=Ae^-Ea/RT, what units is the reaction rate constant, "k", expressed in? I did a couple of web searches and only came up with reaction collisions per second....but what is the unit(s) for THAT?!?!?!

Thanks for any help

~CC

Title: Re: Arrhenius equation.....what are the units for k?
Post by: rabolisk on February 18, 2011, 10:07:00 PM

k in Arrhenius equation is the rate constant k. As you know, k has different units based on the molecularity of the reaction. That is, a first-order reaction has different units for k than a second-order reaction, and so on.

Title: Re: Arrhenius equation.....what are the units for k?
Post by: calvin coolidge on February 19, 2011, 09:19:25 AM

I think I might be getting in my own way on this concept because I'm trying to put it in terms of measurement units I'm familiar with...ie g/ml, J/K, etc. So are you saying, for example, I've a 1st order reaction, so my k units would be, ln A/T(mins.)?


Thanks,

~CC

Title: Re: Arrhenius equation.....what are the units for k?
Post by: rabolisk on February 19, 2011, 02:05:22 PM

For a first order reaction, k would be in units of inverse time. Most commonly, it is s^-1.

Rate = k[A]
Rate is always measured as d[A]/dt, or the rate of disappearance of [A], so the unit for rate is mol/(L*s). The unit for [A] is mol/L, so k has to be s^-1.

http://en.wikipedia.org/wiki/Rate_constant

Title: Re: Arrhenius equation.....what are the units for k?
Post by: calvin coolidge on February 19, 2011, 02:23:39 PM

Thanks for the *delete me*  ;D.....I may be back w/more questions concerning chemical kinetics as its quite confusing and my lab manual for the experiment we did seems quite vague....

In the meantime, have a mole snack my friend!

thanks,

~CC