Method of Initial Rates
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Method of Initial Rates
Can someone explain the steps for the method of initial rates to determine k & n ? I couldn't find an example of it in my notes
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Sophia Diaz - Dis 1B
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Re: Method of Initial Rates
In order to find the value of n, you need to have experimental data which will be given in a table the problem.
First, you need to find 2 experiments where only one of the compounds is changing. For example, if [A] is 0.5 in experiment 1 and 1.0 in experiment 2 while [B] is 0.5 in both experiments 1 and 2, these would be good experiments to determine the order (n) for [A].
When you want to find the order of [B], then just make sure to choose two experiments where the concentration of [B] changes, but [A] stays the same.
In order to find n for a particular compound, divide the experimental concentrations and initial rates you have chosen for each compound.
For the [A] example, Exp 2 / Exp 1 = 1.0 / 0.5 (concentrations) = 4.0 x10 ^ -2 / 2.0 x 10 ^-2 (initial rates). When you divide the concentrations and rates, you want to be aware that: concentration^order = rate, meaning that in this example its 2^x = 2. *keep in mind in just making these numbers up*
You need to find what the value of x is, which in this case is really simple. x = 1 so the order of [A] is 1. You would use the same process for [B], just make sure the concentrations of [A] don't change.
In order to find K, you will need to have found the rate law by finding the order of all the compounds using the above method. Once you find the rate law, do algebra to isolate the k.
k = rate / [A][B] (for my example)
From here, use ANY experiment and just plug in the values that are given in the table at the beginning of the problem. Be very careful with your units and they are completely dependent on the rate law!
k = 2.0X10^-2/[0.5][0.5] (for my example)
First, you need to find 2 experiments where only one of the compounds is changing. For example, if [A] is 0.5 in experiment 1 and 1.0 in experiment 2 while [B] is 0.5 in both experiments 1 and 2, these would be good experiments to determine the order (n) for [A].
When you want to find the order of [B], then just make sure to choose two experiments where the concentration of [B] changes, but [A] stays the same.
In order to find n for a particular compound, divide the experimental concentrations and initial rates you have chosen for each compound.
For the [A] example, Exp 2 / Exp 1 = 1.0 / 0.5 (concentrations) = 4.0 x10 ^ -2 / 2.0 x 10 ^-2 (initial rates). When you divide the concentrations and rates, you want to be aware that: concentration^order = rate, meaning that in this example its 2^x = 2. *keep in mind in just making these numbers up*
You need to find what the value of x is, which in this case is really simple. x = 1 so the order of [A] is 1. You would use the same process for [B], just make sure the concentrations of [A] don't change.
In order to find K, you will need to have found the rate law by finding the order of all the compounds using the above method. Once you find the rate law, do algebra to isolate the k.
k = rate / [A][B] (for my example)
From here, use ANY experiment and just plug in the values that are given in the table at the beginning of the problem. Be very careful with your units and they are completely dependent on the rate law!
k = 2.0X10^-2/[0.5][0.5] (for my example)
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