## Finding out order

$\frac{d[R]}{dt}=-k[R]^{2}; \frac{1}{[R]}=kt + \frac{1}{[R]_{0}}; t_{\frac{1}{2}}=\frac{1}{k[R]_{0}}$

ShravanPatel2B
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### Finding out order

Can the units of the rate constant in a rate law be used to identify the order of the reaction? Also would the units of the rate constant be different in in a rate law of species with different order?

Riya Sood 4G
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Joined: Sat Aug 24, 2019 12:18 am

### Re: Finding out order

Yes the units of the rate constant can be used to find the order of the reaction.
Depending on the order, the units of the rate constant will be different. For example, for a first order reaction, the units of k will be s^-1.

The best way to ensure that you have the correct units is to plug in k and multiply the concentrations and it should also give the rate in terms of concentration/time

Sebastian Lee 1L
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### Re: Finding out order

Just to expand on the previous comment, you can generalize the rate constant's units as $\frac{1}{M^{n-1}s}$ where n is the order of the reaction. So for a zero order reaction, the units would be M/s. For first order, it's just s^-1. For second order, it's $\frac{1}{Ms}$, etc.

The rate constant's units will apply to the overall rate law of the step you are looking at. If you have a first order reactant and a second order reactant colliding, you can expect the units for a third order rate constant (assuming this is an elementary step).

Jessa Maheras 4F
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### Re: Finding out order

Yes, the units of the rate constant are representative of the order of the reaction because the units of the rate constant must cancel out with tge units of the rate law.

Osvaldo SanchezF -1H
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### Re: Finding out order

The units do represent the rate constant but I would like to add that this is different to the total order of the whole reaction as a whole. You would add all of the constants to get the overall order. Do not forget to make that distinction.

William Chan 1D
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### Re: Finding out order

Technically, yes the rate order could be determined if you know the units of k. The units for k do change for different order equations. If you check to see that the units cancel out and are equal on both sides of the equation, you should be okay.

Bryan Chen 1H
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### Re: Finding out order

yes, if you look at the rate constant and its units

Luc Zelissen 1K
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### Re: Finding out order

For order (m + n), the rate constant has units of mol1−(m+n)·L(m+n)−1·s−1
For order zero, the rate constant has units of mol·L−1·s−1 (or M·s−1)
For order one, the rate constant has units of s−1
For order two, the rate constant has units of L·mol−1·s−1 (or M−1·s−1)
And for order three, the rate constant has units of L2·mol−2·s−1 (or M−2·s−1)

Wendy 1E
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### Re: Finding out order

Yes, the units can be used to determine the order of the reaction. The way I think about it is, you start out with the units M/s for a zero-order reaction and then you keep dividing by M for each reaction after. For instance, the units for a first-order reaction are 1/s and the units for a second-order reaction are 1/M*s. Hence, each time you divide by M to get the units of the next order.

Tanmay Singhal 1H
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### Re: Finding out order

smurphy1D
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### Re: Finding out order

Units are useful because each order reaction has different units

Posts: 102
Joined: Tue Oct 02, 2018 12:16 am

### Re: Finding out order

Units correlate to the reaction order to ensure the final units work out. Hence, the units for the rate constant will indicate the rate order.

CalvinTNguyen2D
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Joined: Thu Jul 25, 2019 12:16 am

### Re: Finding out order

For each different order, the rate constant will have its own units. If you can determine the units that the constant has, you can deduce the reaction order.