Fertilisers LI 1 Topic 12 National 5 Chemistry Summary Notes Chemistry is extremely important to the future of food production. As the population of the world increases, more and more efficient ways of producing food are required. LI 2 Healthy Plants Plants require nutrients from the soil in order to have healthy growth. The 3 main elements found in essential nutrient compounds are: nitrogen (N) phosphorous (P) potassium (K) Or N, P, K for short. As plants grow, their roots take nutrients from the soil and this means that as time goes on the level of nutrients present in the soil decreases. The next year, plants growing in the same place would not have enough nutrients for healthy growth. Different plants require different proportions of N, P and K. Replacing Nutrients in the Soil for Healthy Plant Growth Nutrient compounds must be soluble if the roots are to take them into the plant. There are three ways farmers can replace nutrients into over-used soils: Grow beans or clover for 1 year (this bacterial method of replacing nitrogen is cheaper than chemical methods) Add synthetic fertilisers (man-made) or natural fertilisers (manure) fertilisers Crop rotation means using the same field to grow different crops from year-to-year. This allows the field to recover levels of over-used nutrients. 1
LI 3 Fertilisers A fertiliser is a soluble compound containing at least one of the following elements - nitrogen, phosphorous and potassium (NPK). Use your data book (page 8) if you wish to find out if a compound is soluble. Compound Solubility Could be used as a fertiliser? Ammonium phosphate Calcium phosphate Potassium nitrate Sodium chloride LI 4 Both natural and synthetic fertilisers can be used on the soil. Examples are: Natural Fertilisers Synthetic Fertilisers Decay of dead plants or animals Ammonia, Ammonium nitrate Decay of plant or animal waste Potassium phosphate LI 5 LI 6 Unfortunately, soluble fertilisers can be washed away by rainwater into rivers and lakes. This causes pollution resulting in fish dying and algal blooms (a green coating) covering water. There are many useful synthetic fertilisers, examples of these are: Synthetic Fertiliser Formula Ammonium salts Potassium salts NH 4 + K + Nitrate salts NO 3 - Phosphate salts PO 4 3- Using both tables on page 8 of the data book, it can be seen that all salts containing the above ions: are very soluble (except some phosphates) contain the elements N,P and K 2
LI 7 Different crops need fertilisers with different proportions of nitrogen, phosphorous and potassium. If you go to buy a fertiliser from a shop, you often find that the fertiliser is classified according to which crop it can improve the best. Nitrogen Fixation Converting nitrogen from the atmosphere into soluble nitrogen compounds is called nitrogen fixation For farmers, it is much cheaper to grow peas or clover than it is to use synthetic fertilisers to replace nitrogen as peas and clover are examples of plants which have bacteria in their root nodules which can fix nitrogen. LI 8 The Haber Process The Haber process is a process used in industry to manufacture ammonia (NH 3 ) which is an essential starting point for the making of fertilisers. In order for ammonia to be made nitrogen and hydrogen need to react together. Diatomic nitrogen has a triple covalent bond holding the atoms together. This triple bond requires a large amount of energy to break it, so the direct reaction of nitrogen to hydrogen is not usually possible. The Haber Process, shown in the flow chart below, manages to overcome these difficulties. LI 9 nitrogen + hydrogen ammonia N 2 (g) + 3H 2 (g) 2NH 3 (l) The double arrow ' ' tells us that the reaction is reversible. This is a problem as the ammonia made easily breaks down into nitrogen and hydrogen. 3
LI 10 To make the Haber Process economic and efficient the following conditions apply: Condition Moderately high temperature High pressure Iron Catalyst Condenser Recycled reactants Reason Low temperature reaction too slow. High temperature lower percentage conversion to ammonia This increases the yield of ammonia (as it squeezes the 4 volumes of reactants towards 2 volumes of product.) This iron catalyst lowers the temperature required and speeds up the production of ammonia, saving energy costs. This cools the reaction down, turning ammonia into a liquid which is easier to remove. Unreacted nitrogen and hydrogen are returned to the reaction, saving money. You can remember some of the main point of the Haber Process by remembering the word HAIR. H- Haber A Ammonia I Iron catalyst R Reversible reaction 4
LI 11 Properties and uses of ammonia Ammonia is a colourless gas with a very pungent smelling salts odour. It is very soluble in water and dissolves to produce an alkaline solution. Its solubility can be demonstrated by performing the following experiment. Ammonia + water ammonium hydroxide NH 3 (g) + H 2 O(l) NH + 4 (aq) + OH - (aq) The ammonia in the flask quickly dissolves when water is added from the syringe. This reduces the gas pressure in the flask and the universal indicator solution is drawn up to the flask creating a fountain effect. The ammonia forms ammonia solution when it reacts with water. Ammonia solution is alkaline and therefore the universal indicator solution turns blue when it reaches the flask. Ammonia can be used as a fertiliser and for cleaning. It can be used to make other fertilisers, pharmaceuticals and explosives. Ammonia Properties Summary colourless gas form an alkaline solution i.e. turns damp ph paper blue 5
The Ammonia Molecule LI 12 Making Ammonia in the Lab Ammonia can be made by heating any ammonium compound with a metal hydroxide Ammonium chloride + calcium hydroxide calcium chloride + ammonia + water 2NH 4 Cl(s) + Ca(OH) 2 (s) CaCl 2 (s) + 2NH 3 (g) + 2H 2 O(l) The ammonia gas collected can be tested with damp ph paper to show that it is alkaline. 6
LI 13 Making Nitric Acid using The Catalytic Oxidation of Ammonia (Ostwald Process) Due to the unreactive nature of nitrogen, reacting nitrogen and oxygen is not an economic way of making nitric acid. The German chemist Wilhelm Ostwald discovered an economical way to react oxygen and ammonia to produce nitric acid. The Haber process and the Ostwald Process together provide this route from nitrogen to nitric acid. LI 14 Ammonia (from the Haber process) and oxygen (from the air) are passed over a platinum catalyst at a temperature of 800 C. This creates nitrogen monoxide which reacts with oxygen to form the brown gas nitrogen dioxide. Nitric acid (HNO 3 ) is made if the nitrogen dioxide formed is shaken with water and oxygen. 2H 2 O + 4NO 2 + O 2 4HNO 3 7
Similar to the Haber Process, the higher the temperature, the faster the reaction rate but the lower the % conversion (due to the breakdown of NO and water back to reactants.) A moderately high temperature of 800 C is best. Once the reaction has started however, it does not need continued heating to keep the reaction going as it is an exothermic reaction (i.e. one which gives out heat energy). The catalyst continues to glow once the reaction starts, without further heating. You can remember some of the main point of the Ostwald Process by remembering the word OPEN O Ostwald P Platinum catalyst E Exothermic Reaction N Nitric acid LI 15 The Catalytic Oxidation of Ammonia in the Lab The catalytic oxidation of Ammonia in the lab reaction is an example of oxidation as ammonia reacts with oxygen from the air on the platinum catalyst. When the reaction begins, a colourless gas is formed and collects in the flask. This gas is called nitrogen monoxide, NO. 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(l) Nitrogen monoxide reacts with air (oxygen) to form brown nitrogen dioxide. 2NO(g) + O 2 (g) 2NO 2 (g) This brown acidic gas dissolves in water with oxygen to make nitric acid. 2H 2 O(l) + 4NO 2 (g) + O 2 (g) 4HNO 3 (aq) 8
LI 16 Importance of Nitric Acid One of the most important fertilisers is ammonium nitrate, NH 4 NO 3. It is made by reacting alkaline ammonia gas with nitric acid. Ammonium nitrate is a good fertiliser as it contains a high % of essential nitrogen and is very soluble. NH 3 (g) + HNO 3 (aq) NH 4 NO 3 (aq) Ostwald process Oxygen Nitrogen Hydrogen Haber process Ammonia Nitric acid Ammonium nitrate Making Fertilisers acids Haber Process Ammonia (NH 3 ) Ammonium compounds e.g. NH 3 + H 2 SO 4 (NH 4 ) 2 SO 4 ammonia + sulphuric acid ammonium sulphate fertiliser Ostwald Process Nitric acid (HNO 3 ) alkalis Nitrate compounds e.g. HNO 3 + KOH KNO 3 + H 2 O nitric acid + potassium hydroxide potassium nitrate + water 9
LI 17 Percentage Composition The percentage of an element in a compound can be calculated using the following formula. Example 1 Calculate the percentage, by mass, of nitrogen in ammonium nitrate, NH 4 NO 3. Formula mass of NH 4 NO 3 = 2N, 4H, 3O = (2 x 14) + (4 x 1) + (3 x 16) = 80 Mass due to nitrogen = 28 by using the formula above, percentage composition = x100 = 35% Example 2 Mercury can be extracted from the ore cinnabar, HgS. Calculate the percentage by mass of mercury in cinnabar. Formula mass of HgS = 1Hg, 1S = 200 5 + 32 = 232 5 Mass due to mercury = 200 5 by using the formula above, percentage composition = x100 = 86 2% Example 3 Calculate the percentage, by mass, of phosphorus in ammonium phosphate, (NH 4 ) 3 PO 4. Formula mass of (NH 4 ) 3 PO 4 = 3N, 12H, 1P, 4O = (3 x 14) + (12 x 1) + (1 x 31) + (4 x 16) = 42 + 12+ 31+ 64 = 149 Mass due to phosphorus = 31 by using the formula above, percentage composition = x100 = 20 8% 10
National 5 Topic 12 Fertilisers Number Learning Intention Success Criteria 1 I will find out that the increasing world population has led to a need for more efficient food production 2 I will find out that different crops need nutrients containing different proportions of nitrogen, phosphorus and potassium 3 I will find out that fertilisers are substances which restore the essential elements for plant growth to the soil (N,P,K) 4 I will find out that fertilisers can be natural or synthetic(man made) 5 I will find out about the effect of artificial fertilisers on lochs and rivers 6 I will find out about why the solubility and essential elements present determines if the following are useful, synthetic fertilisers: ammonium salts potassium salts nitrates phosphates 7 I will find out that nitrogen-fixing bacteria in root nodules of some plants can convert atmospheric nitrogen into nitrogen compounds and that bacterial methods of fixing nitrogen are cheaper than chemical methods I can state that the increasing world population has led to a need for more efficient food production I can state that different crops need nutrients containing different proportions of nitrogen, phosphorus and potassium I can state that fertilisers are substances which restore the essential elements for plant growth to the soil (N,P,K) I can state that fertilisers can be natural or synthetic(man made) I can give examples of the effect of artificial fertilisers on lochs and rivers I can explain in terms of solubility and essential elements why the following are useful, synthetic fertilisers: ammonium salts potassium salts nitrates phosphates I can state that nitrogen-fixing bacteria in root nodules of some plants can convert atmospheric nitrogen into nitrogen compounds and that bacterial methods of fixing nitrogen are cheaper than chemical methods 8 I will find out that nitrogen gas is not very reactive I can state that nitrogen gas is not very reactive 9 I will find out that the Haber process is a reversible reaction and that not all of the nitrogen and hydrogen are converted to ammonia 10 I will find out that the industrial manufacture of ammonia from nitrogen and hydrogen (Haber Process) and explain why I can state that that the Haber process is a reversible reaction and that not all of the nitrogen and hydrogen are converted to ammonia I can state that the industrial manufacture of ammonia from nitrogen and hydrogen (Haber Process) and explain why the reaction is carried out at 11
the reaction is carried out at moderately high temperature 11 I will find out that ammonia has the following properties: is a colourless gas has a pungent smell is very soluble in water turns damp ph paper/universal indicator an alkaline colour 12 I will find out that ammonia can be prepared in the laboratory by the reaction of ammonium compound with alkali 13 I will find out that the catalytic oxidation of ammonia to form nitrogen dioxide is a step in the industrial manufacture of nitric acid (Ostwald Process) why the reaction is carried out at a moderately high temperature. that the combination of nitrogen and oxygen does not provide an economic industrial route to nitric acid 14 I will find out that nitric acid is formed when nitrogen dioxide, in the presence of air, dissolves in water 15 I will find out that the catalytic oxidation of ammonia can be carried out in the laboratory 16 I will find out that ammonia and nitric acid are nitrogen compounds which are used to make fertilisers 17 I will find out how to work out percentage mass of elements in compounds from formulae moderately high temperature I can state that ammonia has the following properties: is a colourless gas has a pungent smell is very soluble in water turns damp ph paper/universal indicator an alkaline colour I can state that ammonia can be prepared in the laboratory by the reaction of ammonium compound with alkali I can state that the catalytic oxidation of ammonia to form nitrogen dioxide is a step in the industrial manufacture of nitric acid (Ostwald Process) why the reaction is carried out at a moderately high temperature. that the combination of nitrogen and oxygen does not provide an economic industrial route to nitric acid I can state that nitric acid is formed when nitrogen dioxide, in the presence of air, dissolves in water I can state that the catalytic oxidation of ammonia can be carried out in the laboratory I will find out that ammonia and nitric acid are nitrogen compounds which are used to make fertilisers I can work out percentage mass of elements in compounds from formulae 12