Energy is a physical quantity which characterises the state of a system and which, overall, is conserved during transformations.
Energy is the source of all transformations, both physical and chemical. Whether it's the transformation of light energy into chemical energy during photosynthesis in plants, or the release of chemical energy in the form of light and heat when a log is burnt, energy makes life possible. In fact, it is involved in most of the chemical reactions that surround us.
Studying the energy aspect of transformations provides an opportunity to acquire scientific knowledge about phenomena and applications involving reactions that imply a variation in energy. A reaction can release or absorb energy. Reactions are then described as exothermic or endothermic.
In addition to a qualitative description, the energy aspect of a transformation can be represented graphically. An energy diagram can be used for this purpose. This type of diagram highlights, among other things, the activation complex and the activation energy.
Finally, the energy variations of a reaction can be predicted quantitatively without the need to carry it out in the laboratory. The energy balance of a reaction can be established in various ways, for example using calorimetry or Hess's law.
Burning ethanol releases energy in the form of light and heat - this is an exothermic reaction. When the steak is cooked, it absorbs a large amount of energy; this is an endothermic reaction.
