Reversible reactions are ones that can go forward and backwards depending on the conditions. A very simple example is melting ice, this is not a reaction, yet it has the same idea. If you heat a block of ice it melts into water, consider this the forward direction of the reaction. If you freeze the water, it turns back to ice, this is the backward reaction. This change depends on temperature.
Dehydration and Hydration:
In reversible reactions the reaction will be going forward and backwards at the same time and at the same rate. This is called the Dynamic equilibrium. In the state of equilibrium, the rate of forward reaction is equal to the rate of backward reaction and the amount of products and reactants remain constant.
In the state of equilibrium, conditions could be changed and factors could be altered to increase the rate of the forward reaction and thus increasing the yield of the products (Shifting the equilibrium to the right) or to increase the rate of the backward reaction and thus obtaining the reactants (Shifting the equilibrium to the left).
For example, if in a reversible reaction the forward reaction is exothermic, that means that making the product increases the temperature. If we want more products, we will decrease the temperature, the equilibrium will be shifted forward and the rate of the forward reaction will increase to try and raise the temperature back to normal. This will increase the yield of the product. The rate of the backward reaction is endothermic. So if we want the reactants and not the products, we will increase the temperature. This will shift the equilibrium to the left the rate of the backward reaction will increase and the rate of the forward reaction will decrease, thus absorbing the temperature which drops back to normal and the amount of reactants increase.
If the reactants are solids or liquids and one or more of the products are a gas, adjusting the pressure will have an effect on the equilibrium. This is because the gas of the products exerts pressure. If want more of the product, we will decrease the pressure. This shifts the equilibrium to the right and increases the rate of the forward reaction making more gas, thus raising the pressure back to normal. This increases the yield of the products. If we want more of the reactants instead, we could increase the pressure, this will shift the equilibrium to the left and increase the rate of backward reaction. This is to convert some of the gas which exerts pressure to reactants to drop the pressure down to the normal. This gives us more of the reactants. If the case is opposite (the gas is in the reactants rather than the products) the opposite is done.
Haber process is the manufacture of ammonia by reacting nitrogen and hydrogen together. Nitrogen needed for this process is obtained from fractional distillation of liquid air. Hydrogen needed could be obtained by three ways, either reacting methane with steam, electrolysis of brine or cracking of alkanes.
In this reaction, very little of the reactants react forming ammonia. Some factors must be changed to shift the equilibrium to the right and obtain more ammonia. We want to shift the equilibrium to the right, so we should do the opposite of what the forward reaction does:
To speed the reaction even more, a catalyst could be used. The best catalyst for this reaction is powdered iron. A catalyst increases the rate of both the forward and the backward reaction, but since we’ve changed other factors, the equilibrium will still be shifted to the right. A catalyst does not have an effect on the position of the equilibrium.
To make sure that all the reactants react and form ammonia, a trick could be used. The boiling point of ammonia is -33ºC, nitrogen is -196ºC and hydrogen is 299ºC. When equilibrium is reached, there will be a mixture of gases (ammonia, nitrogen & hydrogen). This mixture is cooled to about -40ºC so that ammonia condenses but nitrogen and hydrogen don’t. Ammonia is collected and stored. The unreacted remainder of the reactants is then recycled and the ideal conditions are brought back in for it to react. This is repeated until no more of the reactants remain.