When chemical reactions occur energy is transferred to or from the surroundings
An exothermic reaction is one that transfers energy, usually heat, to the surroundings
Examples of exothermic reactions are combustion, many oxidation reactions and neutraisation
During an exothermic reaction, you say that the material giving out the heat is getting hotter too
If the temperature is raised, the yield decreases
If the temperature is lowered, the yield increases
An endothermic reaction is one that takes in energy, often heat, from the surroundings
Endothermic reactions include thermal decompositions and some dissolving reactions
During an endothermic reaction, you say the material taking in heat is getting colder
If the temperature is rised, the yield increases
If the temperature is lowered, the yield decreases
In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules
In some chemical reactions, the products of the reaction can react to produce the original reactants. This is called a reversible reaction.
A + B = C + D
This is the way a reversible reaction is presented, however instead of the = sign a two headed arrow is used which points in both directions.
In a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction
If a reversible reaction is exothermic in one direction, it is endothermic in the other direction
Reversible Reactions Continued...
A common example of a reversible reaction:
hydrated copper sulfate = anhydrous copper sulfate + water
The reverse of this reaction can be used to test for water
The relative amounts of all the reacting subbstances at equilibrium depend on the conditions of reaction
It is important for sustainable development as well as economic reasons to minimise energy requirements and energy wasted in industrial processes.
Non-vigorous conditions mean less energy is used and less is released in to the environment.
Varying Conditions on reversible reactions
Increasing the temperature increases the rate of reaction equally in both directions. So the product is formed faster but also breaks down faster.
Increasing the pressure favours the smaller volume. High pressure favours a reaction where less molecules are beig made from more molecules.
An increase in pressure would favour the production of ammonia as four molecules are being changed in to two molecules, so here the pressure would increase the yield.
The Haber Process
Although reversible reactions may not go to completion, they can still be used in industrial processes e.g. the Haber Process
The raw materials for the Haber Process are nitrogen and hydrogen. Nitrogen is obtained from the air and hydrogen is obtained from natural gas or other sources
The purified gases are passed over a catalyst of iron at a high temperature (450C) and a high pressure (about 200-250 atmospheres)
Some of the nitrogen and hydrogen reacts to form ammonia. The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen
The Haber Process Continued...
On cooling, the ammonia liquefies and is removed.
The remaining hydrogen and nitrogen is recycled.
nitrogen + hydrogen = ammonia
Dont forget that instead of the equals sign, there will be a two headed arrow pointing in both directions
The reaction conditions are chosen to produce a reasonable yield of ammonia quickly
A Compromise Solution - Haber Process
The formation of ammonia is exothermic so a low temperature increases the yield but the reaction is very slow. A high temperature makes the reaciton faster but produces lower yield.
The volume of ammonia produced is less that the total volume of the reactants so a high pressure favours the reaction but is expensive. A lower pressure is more affordable but produces a low yield.
So a compromise is reached:
200atmos. pressure is used
450C is used