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For any rate equation, the units have to be equal on either side of the equation. The rate of reaction always has the units mol dm^{-3} s^{-1}, and the concentration species in the equation typically have units of mol dm^{-3}, from this any rate coefficient unit can be determined. For example, a first order reaction; r = k[A]For the units to be equal on either side of the equation, mol dm^{-3} s^{-1} = k [mol dm^{-3}], the rate coefficient, k has to have units of s^{-1}An another example can be a second order reaction; r = k[A]^{2}Here, the units of species A is a little different. As there is two A for each reaction, the units work out to be mol^{2} dm^{-6}. Therefore to make the units equal, k has to have units of mol^{-1} dm^{-3} s^{-1}

In every reaction rate, the units of k can be worked out through simple calculations, you don't even have to memorise the answer for each type of reaction.

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Have a Free Meeting with one of our hand picked tutors from the UK’s top universities

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