What happens to a pair of molecules that colide with less than the acitivation energy?

When a pair of molecules collide with less energy than the activation energy required for a reaction, they will not undergo a chemical reaction. Instead, they may simply rebound off each other or undergo elastic scattering without any transformation. The insufficient energy prevents the necessary molecular rearrangements and bond breaking that facilitate a reaction. As a result, the reactants remain unchanged and can potentially collide again in the future.