For a large class of 3D steady laminar fluid flows, dynamicists have shown that there is at least one local direction within a flow where stretching and contraction do not occur. This constraint influences the rate of creation of intermaterial surface area within a chaotic flow. This fact has significant implications in a large class of reactive mixing applications where the rate of reaction (and the resulting product distribution) are transport-controlled. To study this type of behavior, a simple model with one stretching direction, one contracting direction, and one neutral direction has been created. Within this model, the Lyapunov exponent governing stretching and the angle between the stretching and neutral directions can be controlled. A simple consecutive-competitive reaction scheme is created to test the yield and selectivity of desired and undesired products. This model shows that the change in the stretching/neutral angle has a direct effect on the yield and selectivity of reaction products especially in the case of equal reaction rates for the formation of both desired and undesired products. Furthermore, the model also suggests that the stretching/neutral angle may actually have a stronger influence on the yield and selectivity than the rate of stretching. This unexpected result could influence future reactor design for steady 3D unit operations such as laminar static mixers and concentric stirred tanks.