reactions in equilibrium

Every chemical reaction starts with reactants (the reacting substances)in order to make products.
This seems as simple as that, but it is not always like that. In practice, most products have the tendency to react back, to reproduce the old reactants.
In such cases we have a 'forth-and-back' reaction.

the 'forth'reaction:     I2+ H2 2HI

the 'back'reaction:     2HI I2+ H2       or:       I2 + H2 2HI

The habits in chemistry are to combine those two arrows, the two reactions in one reaction equarion with a double arrow:

I2+ H2 2HI (ΔH < 0)
Another habit in chemistry is to add the energy effect to the right side of the equation, as it belongs to the forth-reaction:
ΔH > 0 or ΔH < 0.
In the above example, ΔH < 0 means that the forth reaction is exothermic.

The same equilibrium can also be written as follows:

2HI I2+ H 2     (ΔH > 0)

Module 8 is completely about this type of reactions.