For reaction in solution: Increasing concentration of either of the reagents: Greater concentration=more reagent particles in solution, so there are more collisions.Increasing Temperature: reagent particles move faster on average (have more kinetic energy) so will collide more often and will also be more likely to collide with an energy greater than the "activation energy" needed for the reaction.Adding a catalyst: Lowers activation energy for the reaction, increasing the fraction of collisions that will lead to reaction.