The factor that initiates chemical reactions by breaking bonds in the reactants is activation energy.

Here's why:

* Bonds are stable: Chemical bonds hold atoms together within molecules. To break these bonds, energy is required.

* Activation energy is the "kickstart": Activation energy is the minimum amount of energy that reactant molecules must possess in order to overcome the energy barrier and initiate a reaction.

* Collision and energy transfer: When molecules collide with sufficient energy, they can overcome the activation energy barrier. This energy is used to stretch and weaken existing bonds, making them susceptible to breaking.

Think of it like this: Imagine a ball at the top of a hill. It has potential energy but won't roll down unless given a push (activation energy). Once pushed, the ball will roll down the hill (the reaction proceeds).

How activation energy is supplied:

* Heat: Heating the reactants provides more kinetic energy to the molecules, increasing the likelihood of successful collisions with enough energy to break bonds.

* Light: Some reactions are initiated by light, which can provide the energy needed to break bonds.

* Catalysts: Catalysts lower the activation energy of a reaction, making it easier to initiate. They do this by providing an alternative pathway for the reaction with a lower energy barrier.

In summary, activation energy is the key factor that triggers chemical reactions by providing the energy necessary to break bonds in the reactants.