Answer:
The particles must be in the correct orientation upon impact.
The particles must collide with enough energy to meet the activation energy of the reaction.
Step-by-step explanation:
Particles must collide in order for chemical reactions to take place, but they must collide in the correct orientation.
Heat increases the rate at which particles collide and the force with which they collide, and so will usually increase the rate at which reactions occur. So, although heat can affect reaction rates, it is not the determining factor for their occurrence.
Although pH can influence the reaction rates of many biological reactions, it is generally only relevant to acid-base reactions.
The activation energy is the minimum required kinetic energy that the particles must possess in order for a reaction to take place.