What must happen before a chemical reaction can begin? the activation energy must be exceeded. the activation energy must be reached. the concentrations of products and reactants must be equal. the concentration of reactant molecules must be reduced.

We have that the avtivation energy of a reaction is the energy that is needed for the molecules to start reacting. Hence, the activation energy must be reached; it is sufficient to reach it and there is no need to exceed it. Exceeding it might make the reaction faster. The last two choices are plainly wrong. We can have a reaction like the dissolution of salt into Na and Cl ions where the initial consentration of the products is 0, different fron the reactants. Also, the last sentence is a result of the reaction starting, it does not lead to the start of the reaction.