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March 17th, 2017, 06:26 AM   #1
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From: Illinois

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Trying to derive a chem problem

I am trying to find a rate law for a chemical reaction through mechanisms.

I have the mechanisms set out below and I am left with the rate listed

1: 2A + 2B -> 2C (fast)
2: 2C + B -> D (slow)
3: D + E -> F (fast)

Step 2 is the rate determining step.

--------------- = rate

My question is, How do I get the [C]^2 from the bottom to the top. In all of the class examples, the bottom was never squared, but in our examples, the prof took the inverse of the bottom and put it on the top.
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March 17th, 2017, 11:26 AM   #2
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We have no clue what the variables are, what the arrows mean, why you need to calculate a rate if you are given a formula for the rate, why that is a formula for a rate? Are those supposed to be chemical equations involves moles? Are they equations about times? About rates?

Please give the problem exactly and completely.

By the way, if you are talking about multiplicative inverses of exponentiated variables

$\dfrac{A^2 B^2}{C^2} = A^2 B^2 C^{-2}.$

Last edited by JeffM1; March 17th, 2017 at 11:31 AM.
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March 18th, 2017, 04:44 AM   #3
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Hello austin and welcome.

First please let me ask you if you are confusing the reaction rate constant with the equilibrium constant?
This is a common misconception.

The rate of reaction is a differential equation (you should have three simultaneous ones)
not a fraction equal to a constant.

The reaction rate and its constant is determined purely empirically ( by experimental observation)

Are you sure that the reaction rates are second order as you suggest?
Second order reactions depend upon the initial concentrations / partial pressures which appear as the constants in the limits of integeration when the integration is performed to find the rate law.

Hopefully you also realise that just because the proposed rate law 'fits' the experimental data it is necessary but not sufficient for it to be correct.
Additional corroborating observational evidence is required for that.

The reactions themselves you are discussing are interesting.

Is there truly only a single product in each case?
You seem to be constructing quite a complex molecule.

@JeffM1 and other mathematicians

This is the Chemistry section!
The rate of a chemical reaction is defined as the rate of change of activity of some desired reaction component (reactant) with respect to time.
The activity may be measured by various properties depending so concentration or partial pressure are common.
The actual value is conventionally denoted by enclosing in square brackets.

Almost the simplest rate of reaction equation declares

$\displaystyle \frac{{d\left[ X \right]}}{{dt}} = {k_1}\left[ X \right]$

Which says that the rate of change of (say) concentration of X is proportional to the concentration of X.

With some reactions the indices are simple integer powers but some have fractional powers thus

$\displaystyle \frac{{d\left[ X \right]}}{{dt}} = {k_1}{\left[ X \right]^m}$

Last edited by studiot; March 18th, 2017 at 05:00 AM.
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