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National 4/5 Sub-topic 3C – Fertilisers Summary Fertilisers Increasing world population means that we need more efficient means of food production. Growing plants take nutrients from the soil. y These nutrients include compounds of nitrogen, phosphorous and potassium – these are called essential elements. Fertilisers are substances which are added to the soil to replace the essential elements needed for plant growth. lkkkkkrhggdf Different plants require fertilisers containing different proportions of these nutrient elements. (N, P and K). • Natural fertilisers are made from plant/animal waste i.e. compost and manure. • Synthetic fertilisers are made by chemists and need to be soluble because plants take up nutrients through their roots. • Synthetic Fertilisers also need to contain at least one of the three essential elements. (N, P or K) • Synthetic fertilisers are compounds which can contain: • Nitrate ion : NO3- • Ammonium ion : NH4+ • Potassium ion : K+ • Phosphate ion: PO43- this contains nitrogen this contains nitrogen this contains potassium contains phosphorus 1 Bacterial Methods of Increasing Nitrogen in Soil • Certain plants have nitrifying bacteria present in nodules in their roots. • These bacteria can convert atmospheric nitrogen (called free nitrogen) into nitrogen compounds (fixed nitrogen). • These nitrogen compounds increase the fertility of the soil. • Examples of these plants are peas, beans and clover. • This bacterial method of increasing the nitrogen content of soil is cheaper than chemical methods i.e. using fertilisers. Ammonia Formula is NH3 colourless gas, distinctive sharp smell, highly soluble in water producing an alkaline solution (ammonium hydroxide), turns moist pH paper blue Fountain Experiment This experiment shows how soluble ammonia gas is in water and also that it is an alkali. The water rises up the tube showing that the ammonia dissolves in water. The universal indicator changed to blue in the upper flask showing that ammonia is an alkaline gas. Plastics can be made by either:1. Addition Polymerisation 2. Condensation Polymerisation 2 Ammonia Ammonia can be prepared by the reaction of ammonium compound with alkali: Ammonium + chloride NH4Cl + Sodium Sodium hydroxide chloride NaOH NaCl + Water + Ammonia + H2O This is also used as a test for the ammonium ion NH4+. Ammonia gas will be produced which will turn the pH paper blue. 3 + NH3 Making Fertilisers Ammonia can be converted into an ammonium compound by its reaction with acid: 2NH3 + ammonia H2SO4 (NH4) 2SO4 sulphuric acid ammonium sulphate This new ammonium compound can be used as a fertiliser. These ammonium compounds can be used as fertilisers as they contain nitrogen. This is a neutralisation reaction since the ammonia is an alkali and it is reacting with an acid. ESTER LINK More example include: 2NH3 + ammonia HNO3 NH4NO3 Nitric acid ammonium nitrate 2NH3 + ammonia HCl These are fertilisers NH4Cl Hydrochloric ammonium acid chloride Percentage Composition of Fertilisers The percentage of each of the essential elements in a fertiliser can be varied to suit the crop being grown. Percentage composition allows us to calculate the percentage of any element present in a compound. Example: Calculate the percentage of nitrogen in ammonium sulphate, (NH4) 2SO4. Step 1. Write the formula for the compound. Step 2. Calculate the formula mass.(2 x 14) +(8 X 1)+(1 x 32)+(4 x16) = 132 Step 3. Work out the mass of the required element (nitrogen) Step 4. Calculate the percentage of the element (NH4) 2SO4 = 28 28 x 100 132 4 = 21.2% Problems with Synthetic Fertilisers They are washed into lakes and rivers by rainwater where they encourage bacteria and algae to grow. Bacteria and algae use up dissolved oxygen in the water. This results in the death of fish as they cannot get enough oxygen to live. 5 Industrial Manufacture of Ammonia – The Haber Process Ammonia and nitric acid are nitrogen compounds which are used to make fertilisers. We will look at the industrial production of ammonia first. Ammonia gas is produced industrially by the Haber Process. Nitrogen gas comes from the air. This combines with hydrogen ( which comes from chemicals in the petrochemical industry). • nitrogen + N2 + ammonia 3H2 2NH3 Nitrogen and hydrogen react over an iron catalyst to produce ammonia: N2 • hydrogen + 3H2 2NH3 The formation of ammonia is a reversible reaction and so not all of the nitrogen and hydrogen are converted into ammonia. • At low temperatures a large amount of ammonia is produced slowly. • At high temperatures a smaller amount of ammonia is produced more quickly. • So the Haber Process is carried out at a moderately high temperature to produce ammonia at the most economical rate. • Temperature too low, ammonia production is too slow. • Temperature too high, yield of ammonia is too low. 6 Summary Diagram of The Haber Process air natural gas N2 CH4 + O2 fractional distillation of liquid air hydrogen nitrogen H2 N2 * a mixture of nitrogen and hydrogen, * high pressure, Unreacted nitrogen and * moderately high temperature, * iron catalyst hydrogen are recycled. The ammonia formed is separated from unreacted nitrogen and hydrogen by liquid ammonia NH3 cooling. ammonia gas NH3 (g) Production of Nitric Acid For Fertilisers Nitric acid is also used in the industrial manufacture of fertilisers. This acid is formed when nitrogen dioxide dissolves in water. Nitrogen and oxygen can be obtained from the air. However, nitrogen is not a very reactive gas due to the energy required to break the triple bonds in the molecules and only combines with oxygen in the presence of a spark, e.g. during lightning conditions or at the spark-plugs in car engines. Due to the energy involved in the reaction, it does not provide an economical route to nitric acid. The industrial manufacture of nitric acid is by the catalytic oxidation of ammonia, a route known as the Ostwald Process. 7 The Ostwald Process The first step is the catalytic oxidation of ammonia : ( Ammonia reacting with oxygen) 2NH3 (g) + 31/2O2(g) 2NO(g) + 3H20(g) This requires a platinum catalyst, atmospheric pressure and 600-900oC Catalytic Oxidation of Ammonia in the Lab This process is carried out at a moderately high temperature to allow it to proceed fairly quickly and produce a good yield of nitrogen monoxide. A platinum catalyst is used. Since the reaction is exothermic it is not necessary to continue heating it after the reaction has started since it will supply sufficient energy to continue at a reasonable rate. The second step is : nitrogen monoxide reacts with oxygen to produce nitrogen dioxide. 2NO(g) • + O2(g) 2NO2(g) The third step involves the nitrogen dioxide reacting with more oxygen and water to form nitric acid. 4NO2(g) + O2(g) + 2H2O(l) 4HNO3(aq) 8 Since the reaction is exothermic (heat is given out) there is no need to keep heating once the reaction has started. As with the Haber Process, the higher the temperature the faster the rate of the reaction but the lower the yield … so a moderately high temperature is used. In the manufacture of nitrogen fertilisers, nitric acid is converted to nitrate compounds (solids) by the reaction with alkalis. Summary of The Ostwald Process 9 What Happens to the Nitric acid ? • The nitric acid produced in the Ostwald process is used to make fertilisers. • It can be reacted with ammonia to produce the fertiliser ammonium nitrate. Ammonia + Nitric Acid Ammonium nitrate NH3 + H+NO3- NH4+NO3- Fertiliser • This is a neutralisation reaction. • The nitric acid can react with other bases in neutralisation reactions to from more fertilisers. Nitric acid + H+NO3- + Potassium Potassium hydroxide nitrate K+OH- K+NO3- Fertiliser 10 + + Water H 2O