Si anisotropic etching is simulated on the atomic level with a simple algorithm (Monte Carlo method). The comparison of simulated sequences with in situ real-time STM observations of n-Si(111) in NaOH demonstrates the relevance of the model. Analytical expressions for the growth of triangular etch pits are given and a method proposed to determine experimentally the reaction rates on the atomic scale. The bias dependence of reaction rates and the mechanism of nucleation of etch pits are also discussed in the framework of the chemical description of Si etching.