Standard 7

Energy is the ability to do work

That is any chemical reaction or physical process energy can be onverted from one form to another but its neither created nor destroyed.

IS the flow of energy from one object to another due to difference in temperature when warmer object loses heat; it's temperature decreases and when the cooler heat, object absorbs energy heat; it's temperature rises.

Is the specific part of the universe that contains the reaction or process you wish to study.

Everything in the universe other than the system is considered the surroundings

Universe is definined as the system plus the surroudnings

the energy is released which means that the heat produced by this reaction flows from the system to the surroundings. Heat maybe considered a product of the reaction.

The energy is absorbed which means the flow of heat is now reversed. Heat flows from the surroundings to the system. Heat maybe considered as one of the reactant of the reaction.

IS heat content of the system at constant pressure 

or

enthalpy is the property that is useful for keeping track of heat transfers in chemical and physical processes at constant pressure

Change in enthalpy is the heat absorbed and released in a chemical reaction

or change in enthalpy is equal to the energy  that flows as heat