Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: kmhard1 on July 15, 2008, 01:24:59 PM
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Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0. He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be. Is he correct?
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In general, forcing anything through zero is a bad idea. There are zillions of possible reasons that can shift your otherwise perfectly straight line in some direction, so that it doesn't go through 0.
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Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0. He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be. Is he correct?
When you plot Ln([A]) = f(t) , you'll get a straight line with an intercept equal to Ln[A]0 , which is a constant ofcourse and not a variable.
The relation ln-([A] - [A]o) =f(t) doesnt work for a 1st-order reaction.
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ln[A]=ln[A0]-kt should though. k is the rate constant. So plotting ln[A] vs. t gives you the rate constant (-k=slope) and ln[A0] (the y-intercept).