We focus on a coordination model for pervasive computing applications, tightly coupled to Semantic Web technologies to support openness and semantic reasoning. Our approach is based on the ideas of reifying the existence of services, data, and events in terms of RDF-oriented annotations maintained by local agents, and enacting global system behaviour by manipulation rules for such annotations, which resemble chemical reactions and can be seen as sequences of SPARQL queries and SPARUL updates over RDF stores. We show a minimal set of ingredients to equip this framework with space-time computing mechanisms, including reification of space-time information in terms of annotations, and a relocation service for annotations. Some examples of spatial computations in pervasive computing are given to illustrate the approach.