Advances in cold and ultracold techniques for molecules have enabled a variety of opportunities, including the use of molecules for precision measurements, and the harnessing of their rich internal structure for quantum information and simulation. In this talk, I will examine another exciting application: using chemical reactions between quantum-state-controlled molecules to test state-of-the-art quantum chemistry and scattering calculations. In particular, I will discuss our recent work on the exchange reaction, 2KRb → K2 + Rb2, where the KRb reactants are prepared at a temperature of 500 nK in their rovibronic ground state. By combining state-selective photoionization with ion velocity-map imaging, we achieve state-resolved coincident detection of the products. This allows us to measure the full product state distribution, including the scattering probabilities for all rotational state-pairs allowed by the reaction exoergicity. From a comparison of our results to the predictions of a state-counting model, we quantify the degree to which the reaction redistributes energy statistically among the available modes of motion. In addition, our measurements reveal that the system conserves the reactants’ nuclear spin state. Using this, we demonstrate a technique to alter the relative occupation of product states by controlling the KRb nuclear spins with an applied magnetic field. This new form of control could enable future studies of entanglement between reaction products.