# Chemical Kinetics

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Hi everyone, this is my first post on this forum! I've been stalking it for a little while and now I really need some help. I am a biology major and I am currently taking a relatively basic Chem 112 class. However, my professor moves through lecture very quickly and I'm having trouble understanding the first part on chemical kinetics.

Also guys, sorry if I don't provide enough information in this post, i can provide it tomorrow, I've had a long day and i'm making this thread as a last resort before i go to bed. my lecture is tomorrow and i'll be talking to my professor then.

Anyways, the part i'm stuck on is first, second, third order reactions. I'm pretty much confused about them in general. Can someone point me in the right direction, to a page that has a general overview or even just explain it to me? I'm also confused about rate law and how it works. I need to be able to learn how they work and then use that information to crunch data from lab reports, or in this case, the problems out of the back of my chem textbook.

yes, i am reading my textbook too but I want to cover all my bases, so thats why I'm making the thread. Thanks guys.

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First order kinetics means that the reaction rate (usually expressed in mol/s or mol/(m3s)) goes up linear with the concentration of a component.

Second order kinetics means that the reaction rate goes up as a quadratic function of the concentration (double the concentration, four times higher rate, 3 times higher concentration gives 9 times higher rate).

Third order kinetics means that the reaction rate goes up as a 3rd power function of the concentration (2 times higher concentration, 8 times higher rate).

A general equation is:

$rate = k\cdot{C_A^n}$

The $C_A$ is the concentration. The n is the order. The k is the reaction constant, which is a function of the temperature.

You can have more than 1 chemical in the formula:

$rate = k\cdot{C_a^n\cdot{C_b^m\cdot{C_c^p}}}$

A good wiki site is this one about the rate equation.

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just as a note, rate is usually expressed in terms of $molL^{-1}s^{-1}$

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Hi everyone, this is my first post on this forum! I've been stalking it for a little while and now I really need some help. I am a biology major and I am currently taking a relatively basic Chem 112 class. However, my professor moves through lecture very quickly and I'm having trouble understanding the first part on chemical kinetics.

Also guys, sorry if I don't provide enough information in this post, i can provide it tomorrow, I've had a long day and i'm making this thread as a last resort before i go to bed. my lecture is tomorrow and i'll be talking to my professor then.

Anyways, the part i'm stuck on is first, second, third order reactions. I'm pretty much confused about them in general. Can someone point me in the right direction, to a page that has a general overview or even just explain it to me? I'm also confused about rate law and how it works. I need to be able to learn how they work and then use that information to crunch data from lab reports, or in this case, the problems out of the back of my chem textbook.

yes, i am reading my textbook too but I want to cover all my bases, so thats why I'm making the thread. Thanks guys.

1st order reaction will have a rate equation like;

r = k[X]

2nd order will have the rate equation like

r=k[X][Y] or r=k[X]^2

Order is the power that is raised in the concentration term in a rate equation. If the concentration term(s) have power 1 then the reaction is first order, 2nd if the concentration term(s) have total power of 2.

The equation of the form r=k[X] is usually called the rate equation or rate law.

The thing is that they are many chemical reactions around and it is difficult to characterise all of them according to speed so kinetist(sorry do know the spelling) have come up with way with which the chemical reaction behave. for example the first order reaction could be in plenty amount but any rection that is first order will have rate decreasing exponentially with time. and simliarly for 2nd order the rate will increase with time.

hope that helps somewhat. Good luck!

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1st order reaction will have a rate equation like;

r = k[X]

2nd order will have the rate equation like

r=k[X][Y] or r=k[X]^2

Order is the power that is raised in the concentration term in a rate equation. If the concentration term(s) have power 1 then the reaction is first order, 2nd if the concentration term(s) have total power of 2.

hmm...

I always learned to say: "it is second order in [enter name chemical] concentration"... that would then mean that you take the square of that [enter same name chemical] concentration in the rate equation.

The equation

r = k[X][Y] is first order in X and in Y.

r = k[X]^2[Y] is second order in X and first order in Y.

Saying that a reaction is second order when there are 2 concentrations involved doesn't say much... So,

r = k[X][Y] may be second order, but if you don't specify anything (and if you don't have that equation), the info is useless.

just as a note, rate is usually expressed in terms of $molL^{-1}s^{-1}$

pffft... Sadly that's true. I still recommend to use SI units! Volume is in $m^3$, not $liter$... although liters are the second best volume unit.

I think it has to do with the fact that rates are researched in laboratories. The glass equipment there is in the order of 1 liter in volume, not 1 m3...

Edited by CaptainPanic
pimping my post
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Hey guys. Thanks a lot for the help. I read all of your posts and it definitely provided me with some additional reading to get other views of that subject. I had met with my chem professor yesterday and wednesday and we went over that and the integrated rate law, and I pretty much get it now.

Look for me posting mainly in the biology section. Thanks guys!

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no problem. I like your avatar, by the way!

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Haha thanks, I want to get it on a shirt.

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