Chemistry In Society #2: Fertilisers
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What do plants do when they grow?
They extract nutrients from the soil via their roots
What happens to plant soil over time?
The level of nutrients decreases and they need to be replaced
What do fertilisers, both natural and synthetic, do?
They allow farmers to increase the crop yield and farm intensely
What elements are essential for healthy plant growth?
Nitrogen, phosphorus, potassium
Nitrogen in fertilisers give
Nitrates and ammonium salts
Phosphorus compounds in fertilisers give
Phosphates
Potassium compounds in fertilisers give
Potassium salts
What is the Haber Process?
The industrial production of ammonia from nitrogen and hydrogen using an iron catalyst
Why can’t true equilibrium be achieved in the Haber Process?
It is a continuous process and an open system
What does the catalyst do in the Haber Process?
It provides a fast reaction rate and reasonable yield at lower temperature
Effect of temperature on the haber process
• at lower temperatures, the forward reaction is too slow to be economical • at higher temperatures the rate of reaction increases but favours the reverse reaction
Effect of pressure on Haber Process
• high pressure favours ammonia production but the required equipment is costly to build & maintain • compromises must be reached between the competing factors
How in the Haber Process made effective, and the temperature and pressure problems resolved?
• maximum yield in the Haber Process requires high pressure + low temperature • the process is made effective using 200 atmospheres, 400 °c and an iron catalyst, giving about 15% yield • the ammonia produced is condensed and the unreacted gases are recycled reacting in and overall yield on 98%
What does maximum yield in the Haber process require?
High pressure and low temperature
How is the Haber Process made cost effective?
Using 200 atmospheres, 400 °c and an iron catalyst = 15% yield Ammonia produced is condensed and unreacted gases recycled = 98% overall yield
Haber Process Equation
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92kJ (exothermic therefore -)
What are the starting materials for the commercial manufacturing of nitric acid? (Ostwald Process)
Ammonia (Haber) and Oxygen
What catalyst is used in the Ostwald Process? To give what yield?
Platinum, 98%
Products of the Ostwald Process
Nitrogen monoxide, nitrogen dioxide, nitric acid
Ammonia and nitric acid are important compounds used to produce what?
Soluble, nitrogen containing salts
What happens to the backward reaction in Haber?
It is essentially cancelled as the unreacted gases are recycled
Fertilisers are made by what reactions?
Neutralisation
Ammonia + nitric acid -> ammonia nitrate
NH3 + HNO3 -> NH4NO3
Ammonia + phosphoric acid -> ammonia phosphate
3NH3 + H3PO4 -> (NH4)3PO4