what is the definition of "reaction rate constant"?

Answer 1

This is chemistry, and it's useful, so physics people don't know it.

A reaction rate constant is the term in the mathematical equation that describes the change in concentration of a material vs time, in a chemical reaction.

like this

d[H+]/dt= k [H+]

Here if an acid (H+) were reacting, and this equation described the rate of change, the rate constant would be k, and the reaction would be called "first order" because of the exponent on the [H+]

Expressions for most chemical reactions can be written in this form. Regardless of the order of the reaction, k is the "reaction rate constant". It should be noted that it is only "constant" at a given temperature, and also changes w/ the presence of catalysts.

The units on a rate constant also vary depending on the order of the reaction.

Actually such expressions can be written for nuclear decay as well. They are all first order, so you can solve the equation for the time to get to half the concentration, and generate the "half-life" which is a more useful term in these circumstances.