Every collision that occurs isn't always successful, one in every thousand million result in a reaction. It is only the most energetic which are effective. There is some sort of energy barrier that needs to be overcome before a chemical reaction can occur. The energy that they need is called activation energy. If the activation energy needed is high, then the most energetic molecules succeed and reactions will be slow. If the activation energy needed is not very high then more collisions will be successful and the reaction will take less time. Activation energy is the minimum kinetic energy needed. Reactions need energy to occur.
The molecules are moving because they have kinetic energy. Liquid molecules have quite a lot of kinetic energy. They can roll over each other but still have weak bonds with one another. Molecules need kinetic energy because when they collide, they slow down and the energy is turned into potential energy. If the combined kinetic energies of the reactants are equal to or bigger than the activation energy needed they can overcome the barrier and reactions take place.
When molecules collide together this is called the collision theory, they collide together because they have enough kinetic energy to move around and roll over each other. The collision theory is the number of collisions per second, they are lots of ways you can change this. By increasing the concentration you increase the number of molecules present. If they are more molecules present there is a higher chance of particles colliding, because more collisions occur then more successful collisions occur and then they react and increase the reaction rate. They are also less water molecules and they will then not get in the way as much.
I will keep the temperature of the reaction mixture the same because if you increase the temperature you increase the number of collisions per second and the effective collisions.