Units of Reactions. [ENDORSED]

[Jose Avelar]

Can someone please explain to me how to know which unit of k to use for zero order, first order, and second order reactions? Why are they different for each order?

[Richmund Tan 1L]

Units of k for a zero order reaction is just M*s^-1 because the Rate = k. There is no concentration that affects the equation.

Units of k for a first order reaction is s^-1 because Rate = k[A]. Since there is one concentration value, s^-1 must multiply with M to give us a rate of M*s^-1.

Finally, units of k for a second order reaction is M^-1 * s^-1 because Rate = k[A]². It makes sense that M^-1 must cancel out M^2 to give us a rate of M*s^-1.

[Michael Lee 2I]

Basically whatever the exponent of [A] is the order of reaction.

[Esin Gumustekin 2J]

If you forget the units, plug in mol/Ls for the rate and mol/L for the concentration of A and find k according to what power A is raised to (so that the units cancel on both sides of the equation). I usually don't memorize the units for the rate constant but find it like this when I need to.

[Adam Enomoto 1L]

What helps most is just knowing that rate must be defined as mol/L*S so whatever reactants you have and what they multiply out to, just convert K to make sure the final units are in mol/L*S

[Ilan Shavolian 1K]

Just subtract 1 from the total order of the reactants (x), and than put L^x / mol^x * s