Question

How would changing the pH affect the formation of a complex ion between iron (III) and thiocyanate ions? Provide a balanced equation that shows the formation of the complex ion and explain how the equilibrium constant would be impacted by changes in pH.

Answer 1

The formation of a complex ion between iron  III  and thiocyanate ions can be represented by the following balanced equation:Fe aq  + 6 SCN aq   Fe SCN  aq In this reaction, the iron  III  ion  Fe  reacts with six thiocyanate ions  SCN  to form the hexathiocyanatoferrate  III  complex ion  Fe SCN  .The equilibrium constant  K  for this reaction can be expressed as:K = [Fe SCN ] /  [Fe][SCN] Now, let's consider the effect of pH on this reaction. The pH of a solution is a measure of the concentration of hydrogen ions  H  in the solution. A change in pH can affect the concentrations of the species involved in the reaction, which in turn can affect the position of the equilibrium and the value of the equilibrium constant.In the case of the complex ion formation between iron  III  and thiocyanate ions, the reaction does not involve any hydrogen ions  H  or hydroxide ions  OH  directly. Therefore, changes in pH will not have a direct impact on the equilibrium constant  K  for this reaction.However, changes in pH can indirectly affect the reaction by altering the speciation of the iron  III  and thiocyanate ions. For example, at very low pH  high H concentration , the thiocyanate ion  SCN  can react with H to form the weak acid, thiocyanic acid  HSCN :SCN + H  HSCNThis reaction would decrease the concentration of thiocyanate ions  SCN  available to form the complex ion with iron  III , which could shift the equilibrium of the complex ion formation reaction to the left, decreasing the concentration of the complex ion  Fe SCN  .Similarly, at very high pH  low H concentration , iron  III  can form hydroxide complexes, such as Fe OH  and Fe OH , which can decrease the concentration of free iron  III  ions  Fe  available for the complex ion formation with thiocyanate ions.In summary, although changes in pH do not directly affect the equilibrium constant  K  for the complex ion formation between iron  III  and thiocyanate ions, they can indirectly influence the reaction by altering the speciation of the reactants, which can affect the position of the equilibrium and the concentrations of the species involved.