[fertilisers] NPK fertilisers

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Ammonia

An important industrial product used to make fertilisers, explosives and dyes.

Haber Process

An industrial process used to manufacture ammonia, which can be used to produce NPK fertilisers. This process involves a reversible reaction between nitrogen and hydrogen.

Haber Process Equations

• N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• nitrogen + hydrogen ⇌ ammonia

Producing Nitrogen:

• fractional distillation of air

• air is then cooled to -200°C (expensive process)

Producing Hydrogen:

• produced by reacting methane gas with steam at very high temperatures (steam reforming)

• methane + steam → hydrogen + carbon monoxide

• expensive process

• methane is a greenhouse gas and carbon monoxide is toxic

Conditions of Haber Process:

• high pressure (200 atmospheres)

• moderate temperature (450°C)

• iron is used as a catalyst

High Pressure’s Effect on Yield:

• high pressure is wanted to be reduced by the closed system, so position of equilibrium is shifted to the right side as it has fewer moles

• this increases yield of ammonia as equilibrium lies to the right (products)

High Pressure’s Effect on Rate:

• high pressure increases the rate of reaction, as there are more particles in a given space → more frequent collisions per unit of time

• high pressures can be dangerous and expensive

Moderate Temperature’s Effect on Yield:

• when the temperature is increased, the position of equilibrium moves in the endothermic direction (reactants) to reduce the temperature

• increased temperatures = decreased yield

Moderate Temperature’s Effect on Rate:

• higher temperatures = faster rate of reaction as particles have more kinetic energy

• a moderate temperature is used as a compromise between rate and yield

Iron Catalyst’s Effect on Yield:

• using a catalyst means that a lower temperature can be used whilst keeping the rate of reaction high → lower temperature helps to keep the yield high

• no affect on the position of equilibrium or the yield of ammonia

Iron Catalyst’s Effect on Rate:

• speeds up the rate of the forward and reverse reactions equally

• this reduces the time taken for the system to reach equilibrium

Reducing Cost of Haber Process:

• reactants recycled

• Haber process is exothermic → this releases energy; this heat is often used to heat up other parts of the process and to generate steam which is passed through a turbine connected to a generator in order to make electricity

Stage 1 of Haber Process

Nitrogen and hydrogen gas are pumped into the compressor through pipes.

Stage 2 of Haber Process

The mixture of gases are compressed to 200 atmospheres inside the compressor.

Stage 3 of Haber Process

The pressurised gases are now pumped into a tank containing layers of catalytic iron beds at a temperature of 450°C. Some of the nitrogen and hydrogen react to form ammonia.

Stage 4 of Haber Process

Unreacted hydrogen and nitrogen and the product, ammonia, pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels

Stage 5 of Haber Process

The unreacted hydrogen and nitrogen gases are recycled. Recycling the unused reactants saves money and increases the effective (overall) yield.

Fertiliser

A nutrient added to the soil to increase the soil fertility. This is an example of a formulation and its compounds must be soluble in water so they can be absorbed by the root hair cells

Formulation

A mixture that is always made with the same proportions of the same substances.

Soluble Fertiliser Compounds:

• ammonium ions, NH₄⁺, and nitrate ions, NO₃^-, are sources of soluble nitrogen

• phosphate ions, PO₄^3-, are a source of soluble phosphorus

• all common potassium compounds dissolve in water to produce potassium ions, K⁺

Nitrogen and Ammonia:

• ammonia is an alkali and when it is involved in neutralisation reactions, it produces the ammonium ions (NH₄⁺)

• ammonia can also be oxidised to make nitric acid (HNO₃), which is the source of the nitrate ion (NO₃^-)

• ammonia can be neutralised by nitric acid to make the salt, ammonium nitrate (NH₄NO₃) → this can also be done in aqueous solution (ammonium hydroxide instead)

• ammonia + sulfuric acid

Sources of Potassium:

• potassium chloride → obtained from mining the ground

• potassium sulfate → obtained from mining the ground

• phosphate rock once its been treated with an acid to form soluble salts

Calcium Nitrate + Phosphoric Acid

Formed when phosphate rock is reacted with nitric acid. The acid is neutralised with ammonia to make ammonium phosphate.

Calcium Sulfate + Calcium Phosphate

Formed when phosphate rock is reacted with sulfuric acid. The products are an example of a single superphosphate (mixture of products).

Calcium Phosphate

Formed when phosphate rock is reacted with phosphoric acid. The product is an example of a triple superphosphate.

Stage 1 of Making Sulfuric Acid

sulfur + oxygen → sulfur dioxide

Stage 2 of Making Sulfuric Acid

sulfur dioxide + oxygen ⇌ sulfur trioxide (at 450°C)

Stage 3 of Making Sulfuric Acid

sulfur trioxide + water → sulfuric acid

Stage 4 of Making Sulfuric Acid

The reaction between ammonia gas and sulfuric acid takes place in a continuous process at 60°C, to form ammonium sulfate on a very large scale.