Question

What type of chemical reactions only occur if there is an input of energy?

Answer 1

In fact most chemical reactions require energy input (i.e. `"energy of activation"`) to make them go..........

Explanation:

But given the context of the question, clearly they are looking for the answer of `"endothermic reactions"`, in which the enthalpy of the PRODUCTS is greater than the enthalpy of the reactants:

`"Reactants"+Deltararr"Products"`

That is energy must be put into the reaction to enable the transformation.