If a reaction is first order, its half-life varies as the reaction proceeds. 

Measurements of a rate constant at several temperatures can be used to calculate the activation energy of a reaction.

In a reaction mechanism the rate-determining step has the largest rate constant. 

If a reaction is first order with respect to a particular reactant, and the concentration of that reactant is made three times larger, the rate for the reaction will also be three times larger. 

The activation energy for a reaction changes when the temperature is changed.