5.3 Dynamic equilibria Flashcards
Reaching equilibrium
Equilibrium occurs when during the course of a reversible reaction, the rate of the forward reaction equals the rate of the reverse reaction.
This means that products are being formed in the forward reaction as fast as reactants are being formed in the reverse reaction.
It is reached at a faster rate when:
A higher pressure is used as there are more successful collisions.
A higher temperature is used as the particles have greater kinetic energy.
A higher concentration is used as there are more particles per given volume, hence there are more collisions.
A catalyst is used as it speeds up the rate of reaction, allowing it to reach equilibrium faster.
Economic consideration
Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site.
The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken.
In the Haber Process the raw materials are readily available and inexpensive to purify:
Nitrogen - from the air
Hydrogen- from natural gas
If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable.
Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain.
Fertilisers
Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields.
NPK fertilisers are formulations containing appropriate ratios of all three elements.
From these three essential elements:
Nitrogen promotes healthy leaves.
Potassium promotes growth, healthy fruit and flowers.
Phosphorus promotes healthy roots.
Artificial fertilisers
A distinct advantage of artificial fertilisers is that they can be designed for specific needs whereas in natural fertilisers, such seaweed or manure, the proportions of elements cannot be controlled.
Fertiliser compounds contain the following water soluble ions:
Ammonium ions, NH4+ and nitrate ions, NO3–, which are sources of soluble nitrogen.
Phosphate ions, PO43-, which are a source of soluble phosphorus.
Ammonium nitrate
Ammonia is an alkaline substance and neutralises acids producing a salt and water.
The salt it produces contains the ammonium ion, NH4+, which is a component of several fertilisers.
Ammonia also undergoes oxidation to produce nitric acid, HNO3.
Nitric acid is used as the source of the nitrate ion, NO3–, which is another important ion found in fertilisers.
Ammonium nitrate, a fertiliser and one of the most important ammonium salts, is made by reacting ammonia with nitric acid:
NH3 (aq) + HNO3 (aq) → NH4NO3 (aq)
Preparation of ammonium sulphate in a lab
Aim: To prepare ammonium sulphate by titration.
Procedure - Add an exact volume of ammonia to the conical flask and place on the white tile.
Add a few drops of indicator and swirl, it should turn yellow.
Add the acid to the flask solution drop by drop, swirling the flask in between
Continue until the colour turns red sharply and record the titre.
Repeat by adding exactly the same amount of acid but this time without the indicator which is an impurity.
Pour the reaction mixture in an evaporating dish and gently heat in a water bath to remove some of the water.
Stop heating when the volume has been reduced to roughly one third of its volume.
Leave in a dry place so the remaining water evaporates, allowing crystallisation to occur.
This may take a few days depending on ambient conditions.
After a few days ammonium sulphate crystals should appear.
Filter to remove any remaining water.
Industrial preparation of ammonium sulphate
The industrial preparation of ammonium sulphate is a large scale operation consisting of several stages.
Ammonia is prepared by the Haber process and sulphuric acid by the Contact process.
Both processes require their own supplies of raw materials, energy and equipment.
The most common industrial process of manufacturing ammonium sulphate involves filling a large reactor chamber with ammonia gas.
Sulphuric acid is sprayed into the chamber from above and ammonium sulphate powder is produced.
Comparing preparation of ammonium sulphate
Lab -
Equipment: Simple equipment needed, prepared using a titration apparatus.
Reactant concentration: low concentration, less heat given off.
Separation of product: Crystallisation is used which is a slow process.
Industry -
Equipment: Hugely expensive and complex .
Reactant concentration: High concentrations, very exothermic reaction.
Separation of product: The heat produced is used to evaporate water from the reaction mixture to make very concentrated ammonium nitrate product.
