Question

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.

Explanation:

For a chemical reaction to start, the reactant molecules need to gain enough energy to overcome the activation - energy barrier. Activation energy is the minimum energy required for a reaction to occur. Reaching or exceeding this energy level allows the reaction to proceed. Equal concentrations of products and reactants relate to chemical equilibrium, not the start of a reaction. Reducing reactant molecule concentration would slow or prevent a reaction rather than initiate it.

Answer:

The activation energy must be reached.