How to interpret the Maxwell-Boltzmann distribution to find the activation energy?
The marked location corresponds to a level of kinetic energy in the reactants sufficient to result in a successful collision (energy wise, it says nothing about orientation).  The energy required for a successful collision is the gap in potential energy between the reactants and transition state.
A heterogeneous system is a system where the reactants are in different phases.  A common example is a gaseous reactant that collides with a solid catalyst.  In that case the Maxwell Boltzmann distribution can be applied to the gaseous reactant.
Can you clarify this?  What reactions have negative activation energies?  The reactions you'll commonly encounter have positive activation energies.