We present a new platform for the production and manipulation of microfluidic droplets in view of measuring the evolution of a chemical reaction. Contrary to existing approaches, our device uses gradients of confinement to produce a single drop on demand and guide it to a pre-determined location. In this way, two nanoliter drops containing different reagents can be placed in contact and merged together, in order to trigger a chemical reaction. The reaction rate is extracted from an analysis of the observed reaction-diffusion front. We show that the results obtained using this platform are in excellent agreement with stopped-flow measurements, while decreasing the sample consumption 5000 fold. We also show how the device operation can be parallelized in order to react an initial sample with a range of compounds or concentrations, on a single integrated chip. This integrated chip thus further reduces sample consumption while reducing the time required for the experimental runs from hours to minutes.