GCSE OCR Gateway Chemistry C6.1g–j Fertilizer production

GCSE OCR Gateway Chemistry C6.1g–j Fertiliser production

C6.1g To be able to explain the importance of the Haber process in agricultural production

C6.1h To be able to compare the industrial production of fertilizers with laboratory syntheses of the same products

C6.1i To recall the importance of nitrogen, phosphorus and potassium compounds in agricultural production

C6.1j To describe the industrial production of fertilisers as several integrated processes using a variety of raw materials

Compound Fertilizers

What is a compound fertilizer?

Most compound fertilizers contain three essential elements to enhance and supplement soil nutrients and boost plant growth.

The three essential elements are nitrogen (N), phosphorus (P) and potassium (K)

Nitrogen is necessary for the production of plant protein, lack of nitrogen results in poor growth and yellow leaves.

Phosphorus is necessary for the formation of plant DNA, lack of phosphorus leads to poor root growth and discoloured leaves

Potassium assists in plant reproduction but lack of it means poor fruit growth and discoloured leaves

What fertilisers do is replace the nutrients that plants remove from the soil.  They can also provide essential nutrients that might be missing from the soil in a particular area.

Another things is that for plants to absorb these essential elements from the soil they have to be in a water soluble form so that they can be taken up in the plant root systems.

Nitrogen occurs as Nitrate ions NO₃^—  or ammonium ions NH₄⁺

Phosphorus as phosphate ions PO₄^3—

Potassium as potassium ions K⁺

Compounds containing all three essential elements are known as compound fertilisers or NPK fertilisers.

Examples include ammonium dipotassium phosphate NH₄K₂PO₄

Industrial production of fertilisers

The industrial routes to different fertilisers are shown in the flow diagram above.  You can see what kinds of raw materials are required on the left of the chart and trace the kinds of fertilisers produced from them.

Some equations for the formation of fertilisers

Ammonium nitrate

NH₃   +   HNO₃    ⟶     NH₄NO₃

Urea

2NH₃     +     CO₂       ⟶       (NH₂)₂CO    +   H₂O

Ammonium phosphate

3NH₃    +       H₃PO₄       ⟶        (NH₄)₃PO₄

Ammonium sulphate 

2NH₃     +   H₂SO₄    ⟶     (NH₄)₂SO₄

How to make a fertiliser in the laboratory.

You can produce an ammonium fertilizer such as ammonium sulphate in the lab using the titration technique.

Here’s an outline procedure but you will need to define the concentrations of the solutions you use.

1.    The dilute ammonia solution needs to be in a 250ml conical flask.  Use a pipette to measure an accurate volume say 25ml.

2.    Add a few drops of an acid base indicator such as methyl red to the ammonia.

3.    Use a burette to add the dilute sulphuric acid to the ammonia solution until the end point has been reached and the methyl red is at its neutral orange colour.  Methyl red turns yellow beyond the end point of the titration.

4.    Record the volume of the ammonia used and the sulphuric acid used.

5.    Repeat the titration adding the two volumes to each other again but do not use the indicator.

6.    At the end point add a couple of extra drops of acid to ensure all the ammonia is neutralised.

7.    The reaction should now be complete and you can take the final solution of ammonium sulphate and evaporate it slowly to obtain the colourless crystals of the ammonium salt.

This lab method is a batch process.  Industrial processes are more often continuous processes.

Continuous processes are used when vast quantities of product are needed.

Comparing batch and continuous processes.

Characteristic	Batch process	Continuous process
Quantities produced	Small	Large
Cost of equipment	Low	High
Number of workers	Large, labour intensive	Small
Time shut down	Frequently: very stop start	Rarely: every couple of years
Automation	Difficult	Easier
Rate of production	Low	High