C6.1 Improving processes and products Flashcards
Describe the importance of nitrogen, phosphorus and potassium compounds in agricultural production (and the typical symptoms of deficiency of these elements)
- these 3 are essential elements needed by plants
- plants don’t grow well if these are in limited supply in the soil, and may also show symptoms of mineral deficiency. Quality and yield of food will also be reduced.
Element - typical symptoms of deficiency
Nitrogen - poor growth, yellow leaves
Phosphorus - poor root growth, discoloured leaves
Potassium - poor fruit growth, discoloured leaves
- fertilisers are substances that replace the elements used by plants as they grow
- plant roots can only absorb these elements if they’re in a water-soluble form:
- nitrogen in nitrate ions or ammonium ions
- phosphorus in phosphate ions
- potassium as potassium ions
- Fertilisers that provide N, P and K in water-soluble compounds are called ‘NPK fertilisers’
Explain the importance of the Haber process
- manufactures ammonia from nitrogen + hydrogen
- reversible reaction
- over 150 million tonnes of ammonia is manufactured in the world each year, and more than 80% of this is used to produce fertilisers
- the raw materials needed is air, natural gas and steam
- nitrogen is manufactured by fractional distillation of liquefied air (78% nitrogen)
- hydrogen is manufactured by reacting natural gas (mostly methane) with steam
Describe the industrial production of fertilisers
- several processes happen in a fertiliser factory
- lots of raw materials needed, e.g. sulfur for sulfuric acid and phosphate rock for phosphoric acid
- the different processes in a fertiliser factory are integrated so that a range of compounds for fertilisers can be made
ammonium sulfate NH4NO3 - ammonia + nitric acid
ammonium sulfate (NH4)2SO4 - ammonia + sulfuric acid
ammonium phosphate (NH4)3PO4 - ammonia + phosphoric acid
potassium nitrate KNO3 - potassium chloride + ammonium nitrate
Describe how to make potassium sulfate (compound found in fertiliser) in the lab
- made from potassium hydroxide + sulfuric acid
1 - put dilute KOH into a conical flask and add a few drops of phenolphthalein indicator. Phenolphthalein changes colour at the end-point of the titration, so adding the indicator enables you to determine when the alkali has been neutralised.
2 - Add dilute H2SO4 from a burette or dropping pipette, stopping when indicator changes from pink to colourless.
3 - Add ‘activated charcoal’. This attracts the phenolphthalein, and you can filter the mixture to remove activated charcoal with the phenolphthalein attached to it.
4 - Warm the filtrate to evaporate the water, leaving potassium sulfate behind. You must not heat thus to dryness.
- hazard - potassium hydroxide is alkaline
Describe how to make ammonium sulfate (compound found in fertiliser) in the lab
- similar to method used to make potassium sulfate
1 - Place dilute NH3 solution in a conical flask with methyl orange indicator. Ammonia solution releases small amounts of ammonia in the gas state, which has an irritating sharp smell, so you need to take care to avoid breathing it in
2 - Add dilute H2s04 from a burette/ dropping pipette, stopping when indicator changes from yellow to red
3 - When you reach the end-point, you can add a little extra ammonia solution to ensure that the reaction is complete. Any remaining ammonia will be lost during evaporation.
- hazard - ammonia solution is alkaline
- excess ammonia is given off in the gas state when solution is warmed
Compare the industrial and lab production of fertilisers
Lab:
- small amount - batch process
- start with pure substance bought from chemical manufacturer
Industrial:
- large amounts frequently - continuous process
- start with raw materials which must purified before use, or product is purified at the end
feature - batch process - continuous process
rate of production - low - high relative cost of equipment - low - high no. of workers needed - large - small shut down periods - frequent - rare ease of automating the process - low - high
Describe what conditions are chosen for the Haber process
- manufactures ammonia from nitrogen + hydrogen:
N2(g) + 3H2(g) 2NH3(g) (tH + -93kJ/mol) - reversible reaction
- forward reaction is exothermic (tH is the triangle shape H)
Conditions usually chosen: - pressure - 200 atm (20 MPa)
- temperature - 450 degrees C
- an iron catalyst
Under these conditions, the equilibrium yield of ammonia is about 30%
Explain what factors determine the pressure chosen in the Haber process
- in the balanced equation, there are (1 + 3)= 4 mol of gas on the left, but only 2 mol on the right
- if the pressure is increased, the equilibrium position moves to the right and the equilibrium yield of ammonia increases
- but, it would be hazardous + expensive to choose a very high pressure
- the higher equilibrium yield wouldn’t justify the additional costs, so the pressure chosen is a compromise
Explain what factors determine the temperature chosen in the Haber process
- forward reaction of Hager process is Exothermic , so backward reaction is endothermic
- if temp is increased, the equilibrium position moves to the left and the equilibrium yield of ammonia decreases
- a high equilibrium yield is favoured by a low temp
- the temp chosen is a compromise: low enough to achieve a reasonable equilibrium yield, but high enough to achieve a reasonable rate of reaction
- the iron catalyst works more efficiently above 400°C
Explain what other conditions are chosen in the Haber process
- mixture of gases leaving the reaction vessel is cooled so that the ammonia is liquefied
- this allows ammonia to be removed, and unreactive nitrogen + hydrogen to be recycled
- this improved the overall yield to around 97%
Describe what conditions are chosen for the Contact process
- 3 raw materials needed to make sulfuric acid:
- sulfur
- air (oxygen)
- water
- these are used in 3 stages
The conditions for the 2nd stage are usually:
- pressure of 2 atm (200 kPa)
- temp of 450°C
- a vanadium (V) oxide catalyst V2O5
Under these conditions, the equilibrium yield of sulfur trioxide is about 96%
Describe and include equations for for the 3 stages of the Contact process
Stage 1: sulfur burns in air to produce sulfur dioxide
S(s) + O2(g) —> SO2(g)
(tH = - 297kJ/mol)
Stage 2: sulfur dioxide and oxygen react to get the to produce sulfur trioxide
2SO2(g) + O2(g) 2SO3(g)
(tH = - 144kJ/mol)
Stage 3: sulfur trioxide is converted to sulfuric acid
H2O(l) + SO3(g) —> H2SO4(aq)
Explain what factors determine the pressure chosen in the Contact process
- in the balanced equation in stage 2, there are (2+1)= 3 mol of gas on the left, but only 2 mol on the right
- if the pressure is increased, the equilibrium position moves to the right and the equilibrium yield of sulfur trioxide increases
- but in this reaction the equilibrium position is already far to the right, and so there isn’t any need for high pressures, so 2 atm is just enough to push the gases through the converter
Explain what factors determine the temperature chosen in the Contact process
- forward reaction in stage 2 is exothermic, so backward reaction is endothermic
- if temp is increased, the equilibrium position moves to the left and equilibrium yield of sulfur trioxide decreases
- a high equilibrium yield is favoured by a low temp
- the temp is chosen at a compromise: low enough to achieve a reasonable equilibrium yield, but high enough to achieve a reasonable rate of reaction
- vanadium (V) oxide catalyst only works above 380°C
Describe how the hazards in the Contact process are controlled
- reaction between sulfur trioxide and water in stage 3 is very exothermic
- it would produce a hazardous acidic mist, so stage 3 is carried out in 2 steps
1- Sulfur trioxide is passed through concentrated sulfuric acid (made previously) to make a compound called oleum - H2S2O7
H2SO4(l) + SO3(g) —> H2S2O7(l)
2- The oleum is then added to water, and the reaction makes a larger volume of concentrated sulfuric acid
H2S2O7(l) + H2O(l) —> 2H2SO4(aq)
