Increased concentration of a reactant will lead to more collisions because there are more particles present in the same volume of space, increasing the chances of successful collisions and therefore increasing the reaction rate.
Increased concentration of a reactant will lead to more collisions between particles because when the concentration of a reactant is increased, there are more particles present in the given volume. As a result, the chance of collision between these particles increases, creating more opportunities for the reaction to occur.
