Reaction energy; exothermic & endothermic

Every substance has a certain amount of (chemical) energy. The most suitable place for this chemical energy is in the bondings between the atoms of the substance. It is there that change can take place.
Bondings can be very rich in energy, for example the activated bondings in ATP or ADP (the energy carriers in living organisms).
But most bondings are normal, representing an amount of energy that we can find in tables.
If bondings have little energy, they probalby will be more stable.

In a chemical reaction, the reactants change (they had their own amount of energy in there bonds) into product (also having their own amount of energy, but different from that of the reactants).

During a chemical reaction change in chemical energy will be realised.

There are three possibilities:
I The products have more energy than the reactants In this case de products have gained energy;
this is only possible when the system gained energy from outside (ΔH > 0).
II The products have less energy than the reactants Here the reactants lost energy;
the system has donated energy to the outside environment. (ΔH < 0).
III The products have the same energy as the reactants In this case there is a chemical equilibrium. (ΔH = 0).

Energy diagrams

If petrol reacts with Oxygen (these are two rather energy rich substances) than the products water and carbondioxyde are formed (two substances with very little internal energy).
So in this example the system lost a lot of energy during this combustion. The lost energy does not disappear (energy never get lost). This energy goes outside, in this case as heat.
The difference in energy of products and reactants is also called: the reaction energy with the symbol: ∆H.
If energy is delivered by any system, ∆H has a negative value. We talk about an 'exothermic' reaction.
If the system gains energy, than ∆H has a positive value. We talk about an 'endothermic' reaction.


Reactants change to products, with or wihtout catalyst.
ΔGo gives the reaction energy for what often is used the symbol ΔH.
Eais the activating energy.

In a way you may say: there are "weak substances" (with little reactivity) and "strong substances" (with a lot of reactivity).