Ammonia

Harber Process :

Raw Materials are :

Nitrogen
Made from fractional distillation or by burning something, e.g. Methane (CH₄) in the air to remove Oxygen

Hydrogen
Obtained by a reaction between Methane and steam

Nickel (Used as catalyst)

Conditions : Finely divided Iron catalyst, 450^oC, 200 atmospheres

N_2(g) + 3H_2(g) ó2NH₃(g)

D H is negative

N.B. The symbol ó means that the reaction is Reversible: it doesn't go to completion. After a while, Ammonia molecules are decomposing (breaking up) to N2 and H2 at the same rate as they are being made. We say that EQUILIBRIUM has been set up : the concentrations of NH3, N2, and H2 reach a constant value in the reaction mixture. We say that the equilibrium is DYNAMIC - the reaction does not stop.

Ammonia is removed from the reaction mixture by cooling the mixture so the Ammonia condenses.

Boiling temperature /°C

NH₃ -33
N₂ -196
H₂ -253

The Ammonia (liquid) is tapped off, and remaining H₂ and N₂ are recirculated after being topped up with new Nitrogen and Hydrogen. Reactions in which unused reactants are recirculated in industry are called Continuous Process.

The quicker the Ammonia is produced, the lower the costs of making it - catalyst speeds up reaction. Reactions are speeded up by increased temperature because the speed of the particles increases, and so distances between them are more frequent, AND more of the collisions are sufficiently energetic to produce a reaction.

Increased pressure increases rate of reaction because there are more particles per unit of volume, so collision frequency increases. Increased pressure does not increase the energy of collisions.

The equilibrium position, i.e., yield of Ammonia, can be altered by a change in conditions. The position moves in the direction which opposes or minimises the change in conditions.

N_2(g) + 3H_2(g) ó 2NH_3(g)

(forward reaction is exothermic)

High temp, position moves left, as backward reaction ABSORBS heat

High pressure, position moves right, as 2 molecules occupy less space than 4, and so reduce pressure. So, for high yield, chose low temp, and high pressure.

HOWEVER, this takes too long to be cost effective. Therefore, a compromise is made of 450^oC, and 200 atmospheres. Higher pressure causes problems of :

Designing suitable equipment to withstand high pressure.

The cost in energy use of generating high pressure.