I myself am confused so anything helps! Thank you! Suppose you are Dr. Chemistry

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I myself am confused so anything helps!

Thank you!

Suppose you are Dr. Chemistry, the question-and-answer person for a popular reddit page. Readers of your reddit page are invited to submit questions to Dr. Chemistry, who answers them in a "Dear Abby" style. One day you receive the following question: Dear Dr. Chemistry: You've got to help me settle this disagreement I am having with my laboratory partner. While performing a very exciting and trilling chemical kinetics experimental exercise, we observed that when we doubled the initial concentration of a reactant, the rate of the reaction doubled. My laboratory partner concluded that this reaction must therefore be second order with respect to that reactant. This conclusion didn't quite sit well with me .. and I... well... I kinda just said WRONG! However, I wasn't able to explain my reasoning .. as my professor just went over the material in lecture minutes before lab and the textbook is a bit overwhelming and unclear. Needless to say, ever since then my laboratory partner hasn't spoken to me. I really hope you can help me figure this out, so that I can smooth things over with my laboratory partner. But, Dr. Chemistry, don't just tell me the answer. Please explain it to me so that I can understand the concept well enough to explain it to my laboratory partner. Sincerely, Chemical Kinetics Blues Write an answer to Chemical Kinetics Blues. Imagine that you are writing to a classmate who doesn't yet understand how to determine the order of a reaction with respect to a reactant. Your task is to explain your reasoning so clearly that your micro theme serves as a little textbook, teaching your classmate the chemical kinetics principles involved. Thus, your micro theme will be judged not simply on whether or not your figure out the correct answer, but also on whether or not you can write clearly enough to teach a fellow classmate. If you think an equation(s) is needed to support your answer include it(them), however make sure to explain or/and define every variable.

Explanation / Answer

The rate law describes, as its name says, the rate at which reactants are either consumed or produced again, in case of equilibrium. The law is defined as follows:

A + B -> C + D

rate = k[A]a[B]b

Where:

k is the reaction constant

[A] and [B] are concentration of reactants

a and b are exponents which define the order of the reaction according to that specific reactant, and which have no relation at all with stoichiometry.

These are the ones we're interested in, and for which we need to define if indeed the reaction was second order or other order. We'll go back to that in a minute.

First, we need to know that usually these exponents are determined through experimental trials where initial concentrations and rates are known. With this, and with several trials, we can make systems of equation which would give us the corresponding order of reaction.

Usually, we classify known reactions as of zero, first, and second order.

Zero order reactions are usually just directly proportional to the rate constant. This means, reactants will be consuming in a constant way:

rate = k

First order reactions are usually directly proportional to a single reactant's concentration:

rate = k[A] or k[B]

Second order reaction: these are proportional to the product of both reactants' concentrations or, if it's the case, to the single reactant concentration elevated to the second power

rate = k[A]2 or k [A] [B]

These are common orders of reaction, but as mentioned earlier, they are determined experimentally and can also be decimal numbers. The overall order of the reaction is the sum of both exponents in both concentrations.

So, going back to the doubt regarding the second order, we can have two cases of second order, as we mentioned:

rate = k[A]2

rate = k [A] [B]

But, as your partner is stating second order with respect to that reactant, we are looking at the first case:

rate = k[A]2

In which we clearly see that if we double concentration, the rate will increase by four times, and not by double.

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