Reacting molecules or ions in a solution at room temperature are in constant motion and collide. some collisions lead to a formation of product and some do not. what determines whether a reaction will take place?

A chemical reaction occurs only when the collision between the reacting molecules takes place. However, not all the collisions are efficient in making the reaction. The products are produced only when the molecules colliding exhibits a certain kind of minimum energy known as threshold energy.  

The threshold energy refers to the least amount of energy needed for the reactants to collide effectively, and to produce the activated complex. It is kinetic energy in supplementation with the energy of activation. Hence, threshold energy is more in comparison to the activation energy.  

Explanation:

When three or more particles collide with each other then there are less chances that all of them collide with correct energy and orientation.

So, it is very important for the occurrence of a chemical reaction that its particles or molecules which are taking place in the reaction should have a sufficient amount of energy.

Also, these particles must collide with proper orientation so that they combine at the correct location to result in the formation of product(s).

Thus, we can conclude that energy and proper orientation determines whether a reaction will take place.

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they it depends on if one believes about radioactivity, they make a geiger counter beep but maybe it was going to beep anyway electronics do wierd things all the time or it was programmed to beep near certain metals.