Question

Write the equations involved in the manufacture of nitric acid by Ostwald’s process by maintaining reaction conditions.

Answer 1

Hint: Ostwald’s process is closely associated with Haber’s process. It is a two-step process which involves firstly the oxidation of ammonia followed by the catalytic oxidation of the product formed in the first step.

Complete step by step answer:
-Ostwald process was developed in the year 1902 by a German chemist Wilhelm Ostwald.

-Ostwald process for the synthesis of nitric acid involves the following two steps-
(i) Catalytic Oxidation Reaction - The oxidation in this step is two-step oxidation, i.e. first is the primary oxidation and second is the secondary oxidation.
-Primary oxidation (Formation of Nitric Acid) - The main aim of this process is to convert ammonia into nitric acid. The process begins by taking one part of ammonia and eight parts of oxygen introduced by volume in a catalyst chamber which uses platinum gauze or copper and nickel type catalysts. The chamber has a temperature of about $600{}^\circ C$. The oxidation of ammonia takes place in a reversible manner which releases heat. The change in temperature favours a forward reaction.

The reaction for this step is given as -
 $4N{{H}_{3}}+5{{O}_{2}}\leftrightarrow 4NO+6{{H}_{2}}O(\Delta H = -905.2\text{ kJ/mol)}$

-Secondary oxidation (Formation of Nitrogen Dioxide) – The nitric oxide gas produced in the primary oxidation is in a very hot state. The nitric oxide is passed through a heat exchanger in which the temperature of the nitric oxide is lowered to$150{}^\circ C$. After the cooling of nitric oxide, it is then transferred to another oxidizing tower where nitrogen dioxide is oxidized to about $50{}^\circ C$/
The reaction for this step is given as –
$2NO(g)+{{O}_{2}}(g)\leftrightarrow 2N{{O}_{2}}(g)(\Delta H=-114\text{ kJ/mol)}$

(ii) Absorption of $N{{O}_{2}}$ (Formation of $HN{{O}_{3}}$) – Nitrogen dioxide produced in the secondary oxidation is then introduced in a special absorption tower containing water where it absorbs water. This final step produces nitric acid at the end.
The reaction for this step is given as-
$3N{{O}_{2}}(g)+{{H}_{2}}O(l)\to 2HN{{O}_{3}}(aq)+NO(g)(\Delta H=-117\text{ kJ/mol)}$

-The nitric acid formed by this method is very dilute. NO produced in the last step is recycled to increase the concentration level which is generally kept around $68%$. The water from the acid can also be dehydrated by distillation with sulfuric acid where the pressure is kept between 4-10 standard atmospheres and the temperature is at 870 - 1,073 K.
-The Ostwald process is an exothermic process. However, since the water and oxygen are constantly added to this cycle as reactants, therefore the increasing concentration will create optimum equilibrium conditions.
-The overall reaction for the manufacturing of nitric acid by the Ostwald process can be summarized as
$2N{{H}_{3}}(g)+4{{O}_{2}}(g)+{{H}_{2}}O(l)\to 3{{H}_{2}}O(g)+2HN{{O}_{3}}(aq)(\Delta H= -370.3\text{ kJ/mol)}$

Note: A special care needs to be taken in the first step that reverts the nitric oxide to nitrogen.
$4N{{H}_{3}} + 6NO\to 5{{N}_{2}} + 6{{H}_{2}}O$

The conversion back to nitrogen can be minimized by reducing the time the gas mixture is in contact with the catalyst.