The system is the part of the universe under study. It is the substance or reaction mixture which is the focus of our attention. The surroundings are everything else - the container and the rest of the universe. There is usually a boundary , where the system is on one side and the surroundings are on the other. An exothermic process is a chemical reaction or a physical change that releases heat - energy flows out as a result of a temperature difference between the system and the surroundings.
Many chemical reactions have products with less chemical energy than the reactants.
When we transfer heat to a system at constant pressure , its enthalpy increases. The type of energy change which occurs at constant pressure is what usually happens in the laboratory when we carry out reactions in flasks, beakers, etc. These occur at atmospheric pressure. When energy leaves a system as heat at constant pressure, the enthalpy decreases. The symbol for the change in enthalpy is H .