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.

The activation energy must be REACHED before a chemical reaction can begin.

A chemical reaction can be defined as a process where one or more substances called reactants, are converted to one or more products of that reaction.

The activation energy is the minimum amount of energy required to carry out a chemical reaction.

The higher the activation energy of a given chemical reaction, the slower this reaction will be.

An enzyme is a biological catalyst that helps to perform a chemical reaction by lowering its activation energy.

In conclusion, the activation energy must be REACHED before a chemical reaction can begin.

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Explanation:

For any enthalpy change in a chemical reaction, (exothermic/endothermic), the activation energy must be reached so as to break the strong chemical bonds between the reactant molecules.