  We consider ultracold, chemically reactive scattering collisions of the
diatomic molecules KRb. When two such molecules collide in an ultracold gas, we
find that they are energetically forbidden from reacting to form the trimer
species K$_2$Rb or Rb$_2$K, hence can only react via the bond-swapping reaction
2KRb $\to$ K$_2$ + Rb$_2$. Moreover, the tiny energy released in this reaction
can in principle be set to zero by applying electric or microwave fields,
implying a means of controlling the available reaction channels in a chemical
reaction.
