Fertilisers supply the elements plants need
Definition
Fertiliser
A substance added to soil to supply plants with the elements they need to grow.
- Growing crops take nitrogen, phosphorus and potassium out of the soil.
- A fertiliser returns those elements as soluble compounds that roots can absorb.
- Nitrogen compounds support leaf growth, so they are the largest part of most fertilisers.
- Compounds of phosphorus and potassium are supplied alongside them.
- A compound only works as a fertiliser if it dissolves, because roots take up solutions.
Key Idea
Ammonium salts are valuable fertilisers because they are soluble and rich in nitrogen.
Ammonia reacts with nitric acid to give a fertiliser salt
Definition
Neutralisation
The reaction in which hydrogen ions from an acid join with hydroxide ions from an alkali to form water.
Definition
Salt
The compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion.
- Ammonia solution is an alkali, so it reacts with an acid to form a salt.
- Ammonia and nitric acid give ammonium nitrate: NH3+HNO3→NH4NO3\text{NH}_3 + \text{HNO}_3 \rightarrow \text{NH}_4\text{NO}_3NH3+HNO3→NH4NO3
- Ammonium nitrate is especially valuable because both of its ions supply nitrogen.
- Ammonia is unusual among bases in giving a salt and no water.
- Being soluble, it dissolves in soil water and reaches the roots of the crop.
Note
A reaction between an acid and a base normally gives a salt and water, and ammonia is the exception.
Preparing ammonium sulfate in the laboratory
Definition
Titration
A method that finds the exact volume of one solution that reacts with a measured volume of another.
Definition
Crystallisation
A method that obtains a dissolved solid from its solution by evaporating some of the solvent and letting crystals form as the solution cools.
- A measured volume of ammonia solution is placed in a flask with a few drops of indicator.
- Dilute sulfuric acid is added from a burette until the indicator just changes colour.
- The reaction taking place is: 2NH3+H2SO4→(NH4)2SO42\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_42NH3+H2SO4→(NH4)2SO4
- Repeating the run with the same volumes and no indicator keeps the product uncontaminated.
- The solution is evaporated to the point of crystallisation and left to form crystals, which are then dried.
Common Mistake
- Ammonium salts decompose on strong heating, so the solution is warmed gently and never boiled dry.
- Ammonia solution gives off a choking vapour, so it is handled in a well ventilated place.
Producing ammonium sulfate industrially
- Industry makes the same salt from ammonia and sulfuric acid, but on a far larger scale.
- The ammonia has first to be manufactured from nitrogen and hydrogen by the Haber process.
- The sulfuric acid has to be manufactured from its own raw materials in several stages.
- The neutralisation is therefore only the last step of a much longer sequence.
- The process runs continuously, with the reactants flowing in and the product flowing out.
Example
- Laboratory: one flask, one titration, a few grams of crystals.
- Industry: several linked plants, running continuously, producing tonnes.
Comparing the two scales
- The chemistry is the same in both cases, and so is the equation.
- The laboratory route uses bought-in ammonia and acid, while industry manufactures both.
- A laboratory preparation is a batch, made once and then worked up.
- Industry runs continuously, because starting and stopping a large plant wastes energy.
- Purification differs too: crystals are filtered and dried by hand, while industry uses large scale separation.
Self review
- Which three elements do fertilisers supply?
- Why must a fertiliser compound be soluble?
- Write the equation for ammonia reacting with nitric acid.
- Why is the titration repeated without indicator?
- Give two ways in which the industrial production differs from the laboratory preparation.
