Reversible Reactions

Reversible reactions are common in everyday life, an example is ammonia being made through the Haber process. This can be shown like so:

The arrow above is a sign that a reaction is reversible. Often the reverse reaction is the opposite to the forward equation. If the forward reaction needs heating to 100 Degrees Celsius then the reverse will most likely happen when the temperature drops back below this. The rest of the presentation will go deeper into reversible reactions.

A state of balanced change (Equilibrium)

Equilibrium is when the reaction is in a state of balance. So the forward reaction is happening as fast as the reverse reaction. This is also called dynamic equilibrium as the reaction is still going on but it is overall in balance. An example is when sodium chloride is dissolved in water:

You can't see very well in the diagram but as many of the Na+ and Cl- ions are joining together to form the solid as there are ones splitting from the NaCl to become aqueous. In industrial processes they will siphon off the product that they want so the percentage of that substance stay much lower during the reaction. This means that the equilibrium is never reached and more of the product is made in total than, this often makes some reactions much more efficient. So if originally only 10% of the reaction at equilibrium is the desired product then siphoning this off can change this to something around 90% which is much more efficient and cost effective.

Equilibrium and Chemical Change

Calcium Carbonate is decomposed to form calcium oxide and carbon dioxide and it reversible so calcium oxide and carbon dioxide can form calcium carbonate. If these substances are put in a sealed container at a high temperature and are left to react an equilibrium will be produced. Both of the reactions occur until a balance is reached. At this point the rate of formation equals the rate of decomposition.

Reagents are constantly being converted to products and vice versa. At equilibrium the rate of forward process is the same as the backwards one. Not all reactions can reach equilibrium such as the burning of magnesium in air. In many cases the conversion of reactants to products is so large that by the end of the reaction no reactants can be detected by normal analytical means. In other reactions the reagents e.g nitrogen and oxygen in the air, do not seem to react at all. These reactions are called irreversible reactions in such conditions.

If we mix hydrogen and bromine and provide energy for the reaction to occur the orange colour of the bromine disappears and the reverse reaction is so minimal that the reaction seems to go to completion and we are left with hydrogen bromide. If we did this with hydrogen and iodine though the violet colour of the iodine persists. An equilibrium has been set up between the three components (iodine, hydrogen and hydrogen iodide) in which all three components are present in significant amounts.

When you cook on a camping gas stove, butane gas is released and burned. The liquid butane will evaporate to maintain the gas supply. Equilibrium will not be restored in the cylinder unless the gas is turned off at the tap as we must have a closed system for equilibrium to occur. A closed system is a system where substances cannot leave or enter.   

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