e.g: N2 (g) + 3H2 (g)
2NH3(g)note the arrow that has been used to indicate a reversible reaction.
In this reaction, Nitrogen and hydrogen becoming ammonia is known as the forward reaction
Ammonia becoming nitrogen and hydrogen again is known as the backward reaction. In a reversible reaction both the forward and backward reaction takes place and after sometime they come to an equilibrium where rate of forward reaction and backward reaction become a constant.
This equilibrium can be altered by several factors so that forward or backward reaction is favored. This is important is several industries which make use of products made by reversible reactions such as the one given in the example , ammonia production.
Factors affecting the equilibrium of a reversible reaction
1. Pressure
If the pressure is increased in a reversible reaction of a gaseous system, the system tries to decrease the pressure by favoring the side of the reaction producing less number of moles. According to the example reaction of producing ammonia, left hand side total number of moles (1+3 =4) and right hand side total number of moles=2.
If the pressure was increased, the forward reaction is favored which produces a less number of moles.
If the pressure was decreased, the backward reaction will be favored which produces more number of moles to the reaction.
2. Temperature
For the given example the forward reaction is exothermic. Which means if forward reaction takes place it produces heat for the system. In a reversible reaction if it is given that the forward reaction is exothermic, we can assume that the backward reaction is endothermic, the opposite. which means if the backward reaction takes place it will absorb heat from the system.
When the temperature is increased in a reversible system, the system tries to decrease the the temperature by altering the equilibrium to the side which will absorb the heat, the endothermic reaction. according to the ammonia example it would be the backward reaction.
When the temperature in decreased, the exothermic reaction will be favored.
3. Adding a reactant
If a reactant is added, the system tries to decrease the concentration of the added one by favoring the reaction that uses that reactant. for example in the above example if Nitrogen was added, the forward reaction is favored. If ammonia was added the backward reaction will be favored.
4. Removing a reactant
If a reactant is removed, the system tries to produce more of the removed reactant. for example if nitrogen was removed, the backward reaction will be favored to produce more nitrogen from ammonia. If ammonia was removed the forward reaction is favored .
5. Adding a catalyst
Adding a catalyst does effect a reversible reaction but not by favoring only one reaction as above. Adding a catalyst will faster both the forward reaction and the backward reaction making the time taken to reach the equilibrium faster. Iron fillings are used as the catalyst in the above reaction.
4:39 AM
Adding a catalyst
Adding a reactant
chemistry
Factors affecting reversible reactions
GCSE Chemistry
Pressure
Removing a reactant
Reversible reactions
Temperature
