As the search continues for useful applications of noisy intermediate scale quantum devices, variational simulations of fermionic systems remain one of the most promising directions. Here, we demonstrate an experimental quantum simulation of chemistry with more than ten times times the number of entangling gates as the largest prior implementation. We variationally model the isomerization of diazene and correctly distinguish between competing reaction mechanisms to within the model chemistry. The parameterized circuits explored in our work realize the Givens rotation approach to free fermion evolution. This ubiquitous algorithmic primitive performs an arbitrary change of orbital basis and is required by many proposals for correlated simulations of molecules and Hubbard models. Because free fermion evolutions are classically tractable to simulate yet still generate highly entangled states over the computational basis, we are able to use these experiments to benchmark the performance of our quantum processor while establishing a foundation for scaling up intermediate scale correlated electron simulations.