If the reactants have less energy than the products, what kind of a reaction must have taken place?

If the reactants have less energy than the products, then an endothermic reaction must have taken place.

Here's why:

1. In an endothermic reaction, energy is absorbed from the surroundings. This energy is used to break the bonds in the reactants.

2. Once the bonds in the reactants are broken, new bonds are formed to create the products. The energy required to form these new bonds is less than the energy absorbed to break the initial bonds.

3. Therefore, the products end up with more energy than the reactants, which is characteristic of an endothermic reaction.

4. This is in contrast to an exothermic reaction, where energy is released to the surroundings, resulting in products with less energy than the reactants.