Units of k

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Nancy Le - 1F
Posts: 52
Joined: Fri Sep 29, 2017 7:07 am

Units of k

Postby Nancy Le - 1F » Mon Mar 05, 2018 12:35 am

How do you calculate the units of k for the order of each reactant?

Elizabeth Ignacio 1C
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Joined: Fri Sep 29, 2017 7:07 am

Re: Units of k

Postby Elizabeth Ignacio 1C » Mon Mar 05, 2018 12:54 am

You find the units of k by cancelling out all other known units in the rate law, like all of the different mol/L and the units for time. You can also just memorize them, because they're the same for each rxn order.

Zero order: mol/L/s or basically, Molarity/Time
First Order: 1/s, or basically 1/Time
Second Order: L/mol/s, which is (molarity flipped)/Time

Ya Gao
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Re: Units of k

Postby Ya Gao » Mon Mar 05, 2018 1:56 am

You can calculate the unit of k based on the rule that the rate of reaction has an unchanging unit which is mol*L-1*s-1

Alyssa Pelak 1J
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Re: Units of k

Postby Alyssa Pelak 1J » Mon Mar 05, 2018 9:10 am

The simplest way is to write out the rate law and divide.

1st order: rate=k[A] and divide rate by [A]. rate is written in mol.L-1.s-1. and [A] is mol.L-1. If you divide the two you are left with s-1.

2nd order: rate=k[A]^2. as with first order divide rate (mol.L-1.s-1.) by [A]^2 to get L.mol-1.s-1.

0 order: rate=k. that leaves you with mol. L-1.s-1

Cali Rauk1D
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Re: Units of k

Postby Cali Rauk1D » Mon Mar 05, 2018 10:00 am

Each unit of K will be different for each order

Yutian Zhao -1J
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Joined: Fri Sep 29, 2017 7:05 am

Re: Units of k

Postby Yutian Zhao -1J » Mon Mar 05, 2018 2:33 pm

you can write the rate law (with units) first and then cancel units on both sides, then whatever units that are not cancelled should be the units of k. the units of k depends on the order of reaction. And pay more attention when writing the rate law if any concentration is to be squared (or higher), since your units for that concentration should also be squared. Hope it helps.

Akash_Kapoor_1L
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Re: Units of k

Postby Akash_Kapoor_1L » Mon Mar 05, 2018 4:58 pm

What is the best way to convert between different units of k? Would we ever have to convert k from L/ mol*s, into 1/ kPa*s?

Michelle Steinberg2J
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Joined: Fri Sep 29, 2017 7:04 am

Re: Units of k

Postby Michelle Steinberg2J » Mon Mar 05, 2018 5:00 pm

Last year in chemistry I learned that the units of k = 1/M^(overall order-1)*unit of time.
:)

Sarah 2F
Posts: 33
Joined: Fri Sep 29, 2017 7:06 am

Re: Units of k

Postby Sarah 2F » Wed Mar 07, 2018 2:42 pm

It is different for each reaction order


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