Units of k  [ENDORSED]


Moderators: Chem_Mod, Chem_Admin

RuchaDeshpande1L
Posts: 60
Joined: Sat Jul 22, 2017 3:00 am
Been upvoted: 1 time

Units of k

Postby RuchaDeshpande1L » Wed Feb 28, 2018 9:15 pm

Can someone go over how we determine the units of k for different orders of reactions? What impacts do the units make on our calculations?

Alyssa Parry Disc 1H
Posts: 53
Joined: Sat Jul 22, 2017 3:01 am

Re: Units of k

Postby Alyssa Parry Disc 1H » Wed Feb 28, 2018 9:18 pm

Basically you take the rate law (rate=k[a] for first order) and the units of rate are always mol.L-1.s-1 and then the units of concentration are always mol.L-1 and then you plug those in to find units of k, which would be s-1.

SantanaRodriguezDis1G
Posts: 56
Joined: Sat Jul 22, 2017 3:00 am

Re: Units of k

Postby SantanaRodriguezDis1G » Wed Feb 28, 2018 9:19 pm

The units of k can change the units for rate when you are using it in a calculation

Abigail Urbina 1K
Posts: 102
Joined: Thu Jul 27, 2017 3:01 am

Re: Units of k

Postby Abigail Urbina 1K » Thu Mar 01, 2018 8:41 am

Rate = k[A]^n, where n is the exponent that represents order of the reaction. Concentration is always going to have units of M/L, which is the equivalent to mol.L-1.s-1. Be sure to apply the exponent to the units of the concentration as appropriate. You want to isolate k to be alone on one side of the equal sign to find its units. Since the units of rate are always mol.L-1.s-1, you can divide the units of the rate by the units of [A]^n.

Sarah_Stay_1D
Posts: 57
Joined: Sat Jul 22, 2017 3:00 am
Been upvoted: 1 time

Re: Units of k

Postby Sarah_Stay_1D » Thu Mar 01, 2018 10:04 am

Also the time units do not always have to be in seconds. Any time unit works, just make sure that you units are consistent throughout the calculation.

Ishita Monga 1B
Posts: 31
Joined: Thu Jul 27, 2017 3:00 am

Re: Units of k

Postby Ishita Monga 1B » Thu Mar 01, 2018 3:41 pm

the units themselves have no impact on the numerical answer, unless they involve a conversion factor (for example mmol to mol)

Yeyang Zu 2J
Posts: 58
Joined: Fri Sep 29, 2017 7:06 am

Re: Units of k

Postby Yeyang Zu 2J » Fri Mar 02, 2018 9:20 am

Since Rate= mol•L^-1•S^-1
AND [A]= mol•L-1

First order rate = K1*[A]

And calculate the unit of K1

Josh Moy 1H
Posts: 58
Joined: Thu Jul 13, 2017 3:00 am

Re: Units of k

Postby Josh Moy 1H » Fri Mar 02, 2018 11:07 am

Unit of K is in a format so that the rate becomes mol/L S

David Minasyan 1C
Posts: 54
Joined: Thu Jul 13, 2017 3:00 am

Re: Units of k

Postby David Minasyan 1C » Fri Mar 02, 2018 11:11 am

Units of k start out at mol/(L*s) at the zero order and every time you go up one order you can multiply by L/mol to get the new units of k.

Grace Han 2K
Posts: 33
Joined: Tue Nov 15, 2016 3:00 am

Re: Units of k

Postby Grace Han 2K » Fri Mar 02, 2018 1:04 pm

I think the units of k is the same units as concentration.

Emily Mei 1B
Posts: 50
Joined: Fri Sep 29, 2017 7:04 am

Re: Units of k

Postby Emily Mei 1B » Fri Mar 02, 2018 1:14 pm

For a 1st order reaction, k units = s^-1
2nd order reaction, k units = L/(mol*s)
and increasing the powers of L and mol as you go up in reaction order so that rate will be in unit of mol/(L*s)

Liz White 1K
Posts: 35
Joined: Thu Jul 13, 2017 3:00 am

Re: Units of k

Postby Liz White 1K » Fri Mar 02, 2018 2:15 pm

I know this question has been answered many times, but the simplest response is just to do dimensional analysis and multiply everything out. The units of k will be whatever makes the equation work out, so they differ according to the order of the reaction.

Yu Chong 2H
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am

Re: Units of k

Postby Yu Chong 2H » Fri Mar 02, 2018 2:45 pm

The unit of the rate of reaction is mol•L^-1•S^-1, so you just have to manipulate the unit of K so that when put into the equation will give you mol•L^-1•S^-1.

Sophie 1I
Posts: 51
Joined: Fri Sep 29, 2017 7:04 am

Re: Units of k  [ENDORSED]

Postby Sophie 1I » Fri Mar 02, 2018 2:56 pm

An easy way of rememberin gthis is to to take (M^1-x)/s where s is the order of the reaction.
For example, for a 1st order reaction 1-1=0 and M^0=1 so the unit for k will be 1/s for 1st order rxn.


Return to “Zero Order Reactions”

Who is online

Users browsing this forum: No registered users and 1 guest