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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: tdiv on February 16, 2015, 01:55:47 PM

Title: Using Rate Laws
Post by: tdiv on February 16, 2015, 01:55:47 PM

Hello,

I have a question about a rate law problem. The problem is:

The reaction 2NO₂ → 2NO + O₂ follows the rate law
Δ[O₂]/Δt=1.50*10^-2[NO₂]²
at 250. K.
If the initial concentration of NO₂ is 1.00 M, how long will it take for [NO2] to reach 31.8% of its initial value?

I do not know what to do with the NO₂ squared on the right because that makes it seem like an instantaneous concentration, and I need the average concentration over time. Could someone help?
Thanks!

Title: Re: Using Rate Laws
Post by: OmniReader on February 16, 2015, 05:57:40 PM

What do you mean "average concentration over time". You need instantaneous concentration.

Note first that d[NO2]/dt = -2*d[O2]/dt from the stoichiometry of the reaction. Now, you are given the equation for d[O2]/dt. Substitute that into the stoichiometry and you will have an equation for d[NO2]/dt in terms of [NO2]. Solve that by separation of variables (look it up if you don't know how) and you will get [NO2] in terms of [NO2]_0 and t. Substitute [NO2] = [NO2]_0 * 0.318 and [NO2]_0 = 1 M and find t and you're done.

Title: Re: Using Rate Laws
Post by: mjc123 on February 16, 2015, 06:04:19 PM

You also need the units of time. What is k? - 1.5*10^-2 M^-1s^-1? M^-1min^-1?...