difference between a system and its surroundings in a chemical reaction
the system is usually defined as the chemical reaction and the boundary is the container in which the reaction is run in the course of the reaction heat is either given off or absorbed by the system furthermore the system either does work on it surroundings or has work done on it by its surroundings
the sign convention for the relationship between the internal energy of a system and the heat gained or lost by the system can be understood by thinking about a concrete example such as a beaker of water on a hot plate when the hot plate is turned on the system gains heat from its surroundings