Class 11th

9.4. The production of dihydrogen in coal gasification can be increased by reacting CO present in syngas mixtures with steam in the presence of iron chromate as the catalyst. This is called the water-gas shift reaction. Synthesis gas or 'syngas' is produced from sewage, sawdust, scrap wood, newspapers etc. The process of producing 'syngas' from coal is called 'coal gasification'.

                                                CO(g) + H₂?O(g) → CO₂?(g) + H₂?(g)
With the removal of C0₂ by scrubbing with sodium arsenite solution the reaction shifts in the forward direction and thus, the production of dihydrogen will be increased.

Whenever equilibrium is disturbed by change in the concentration, pressure or volume, the composition of the equilibrium mixture changes because the reaction quotient, Q_c no longer equals the equilibrium constant, Kc. However, when a change in temperature occurs, the value of equilibrium constant, K_c is changed.

In general, the temperature dependence of the equilibrium constant depends on the signoff ΔH for the reaction.

• The equilibrium constant for an exothermic reaction (negative ΔH) decreases as the temperature increases.
• The equilibrium constant for an endothermic reaction (positive ΔH)increases as the temperature increases. Temperature changes affect the equilibrium constant and rates of reactions.

The intensity of the red colour becomes constant on attaining equilibrium. This equilibrium can be shifted in either forward or reverse directions depending on adding a reactant or a product. The equilibrium can be shifted in the opposite direction by adding reagents that remove Fe³⁺or SCN^– ions. For example, oxalic acidH₂C₂O₄), reacts with Fe³⁺ ions to form testable complex ion [Fe (C₂O₄)₃]^3–, thus decreasing the concentration of free Fe³⁺ (aq).In accordance with the Le Chatelier's principle, the concentration stress of removed Fe³⁺ is relieved by dissociation of [Fe (SCN)]²⁺ to replenish the Fe³⁺ions. Because the concentration of [Fe (SCN)]²⁺ decreases, the intensity of red colour decreases. Addition of aq. HgCl₂ also decreases red colour because Hg²⁺ reacts with SCN^– ions to form stable complex ion [Hg (SCN)₄]^2–.