The Haber process is one of the most important industrial chemical reactions and has allowed mankind to greatly increase their food supply through the use of ammonia (NH3) based fertilizers.
The process involves the reaction of nitrogen (N2) with hydrogen (H2) to produce ammonia (NH3).
The balanced chemical equation is: N2 + 3 H2 --> 2 NH3 and is a reversible reaction as made industrially.
Three variables are generally involved in the production of ammonia. They are temperature, pressure, and a catalyst.
Because both hydrogen and nitrogen are relatively unreactive, it is generally necessary to use a catalyst to bring about a chemical reaction between the two reactants. This lowers the energy necessary to cause the reaction to take place.
Because both reactants are gases, the process generally takes place at relatively high pressure to increase the likelihood that the two reactants will come close enough together to combine. The pressure is usually in the 200 atm range.
To further increase the reaction, the process is generally run at elevated temperatures (400 - 450 C). However, the increase in temperature also contributes to the breakdown of the desired product back into the two reactants. So there is a balancing act between having the reaction take place fast enough and producing enough of the desired product, but not at the same time breaking down the product as fast as it is made.
If you look at LeChatelier's Principle, this will help you understand how changes in a process condition can affect the equilibrium between the reactants and the products. See the link for additional information also.