units of k

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Kyle Rex 1K
Posts: 50
Joined: Sat Aug 17, 2019 12:16 am

units of k

Postby Kyle Rex 1K » Wed Mar 04, 2020 11:38 pm

I know the units of k is dependent on the order of the rate equation, but how do we use that knowledge to determine the units of k?

Victoria Zheng--2F
Posts: 103
Joined: Fri Aug 09, 2019 12:17 am

Re: units of k

Postby Victoria Zheng--2F » Wed Mar 04, 2020 11:43 pm

Since the unit of rate is M/s, if the reaction is zero-order, the unit of k is the same as the unit of rate. If the reaction is first order, then the reactant will have the unit M and the unit for k would be 1/s since the unit of rate is always M/s. If the reaction is second order, the reactant will have the unit M^2, which means that the unit of k has to be 1/M*s since the unit of rate is always M/s.

gabbymaraziti
Posts: 111
Joined: Wed Sep 18, 2019 12:19 am

Re: units of k

Postby gabbymaraziti » Thu Mar 05, 2020 9:54 pm

If the reaction is zero-order, K has units of M/s
If the reaction is first-order, K has units of 1/s
If the reaction is second-order, K has units of 1/M.s

EvanWang
Posts: 101
Joined: Sat Sep 07, 2019 12:16 am

Re: units of k

Postby EvanWang » Thu Mar 05, 2020 11:20 pm

For first order, the units are just s^-1

For second order, the units are L x mol^-1 x s^-1

For third order, the units are L^2 x mol^-2 x s^-2

The pattern continues in this way.

WYacob_2C
Posts: 102
Joined: Sat Jul 20, 2019 12:16 am

Re: units of k

Postby WYacob_2C » Fri Mar 06, 2020 6:25 pm

For the units of k, the zeroth order is M/s, the first order is 1/s, and the second order is 1/M*s. A good way to remember this is to divide by M when going from zeroth to first to second order.
zeroth order: M/s --> divide by M --> first order: 1/s --> divide by M --> second order:1/M*s

Diana Chavez-Carrillo 2L
Posts: 122
Joined: Fri Sep 28, 2018 12:18 am

Re: units of k

Postby Diana Chavez-Carrillo 2L » Fri Mar 06, 2020 9:01 pm

When you are given a problem sometimes they may give you a rate constant to solve for something else like concentration. For example, a given rate constant can be written with the following units: L/mol*s. This is the same as 1/M*s because M=mol/L so if we have L/mol then this is the inverse of mol/L so it will equal 1/M. Then we multiplied 1/M by s to get 1/M*s. This indicates that we will be using rate laws or half-life equations for the second order.

Emily Chirila 2E
Posts: 112
Joined: Sat Jul 20, 2019 12:16 am

Re: units of k

Postby Emily Chirila 2E » Fri Mar 06, 2020 9:44 pm

I found this video to be very helpful in visualizing why the units are different for zeroth, first, and second-order reactions.

https://www.khanacademy.org/science/che ... constant-k

205405339
Posts: 77
Joined: Thu Jul 11, 2019 12:16 am

Re: units of k

Postby 205405339 » Fri Mar 06, 2020 11:41 pm

units of k can be determined by (M/s) / (M^n) where n is the total reaction order

Jasmine W 1K
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Joined: Sat Sep 07, 2019 12:18 am

Re: units of k

Postby Jasmine W 1K » Sun Mar 08, 2020 5:04 pm

for zero-order reactions, K has units of M/s
for first-order reactions, K has units of 1/s
for second-order reactions, K has units of 1/M.s

Megan Kirschner
Posts: 46
Joined: Wed Feb 20, 2019 12:17 am

Re: units of k

Postby Megan Kirschner » Sun Mar 08, 2020 6:39 pm

Always go based on the order of the reaction:

If the reaction is zero-order, K has units of M/s
If the reaction is first-order, K has units of 1/s
If the reaction is second-order, K has units of 1/M.s


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