Understanding Equilibrium Shifts: Ferric vs Thiocyanate Ions

Let us study the equilibrium reaction between the ferric chloride and potassium thiocyanate via the change in the solution’s colour concentration. The equilibrium constant and the reaction is given as follows.

Fe3+(aq) + SCN-(aq) ⇌ [Fe(SCN)]2+ (aq)

The equilibrium constant can be given by the formula as follows:

Kc = \[\frac{[C][D]}{[A][B]}\]

Finding the equilibrium constant for the reaction between ferric chloride and potassium thiocyanate can be obtained as follows:

K \[\frac{[[Fe(SCN)]^{2+}(aq)]}{[Fe^{3+}(aq)[SCN^{-}(aq)]]}\]

Considering at a constant temperature, the K value also remains constant.

Increasing the concentration of either the thiocyanate ion or Fe3+ ion would result in an increase in the [Fe(SCN)]2+ ion’s concentration.

Let us discuss the process of shift in equilibrium between ferric ions and thiocyanate ions.

Aim

To understand the process of equilibrium shift between thiocyanate ions and ferric ions either by increasing or decreasing the ion concentration.

Required Materials

• 0.100g Potassium thiocyanate,

• 2 Beakers of 100 mL capacity,

• 0.100g Ferric chloride,

• 6 Boiling tubes,

• 4 Burettes,

• 250 mL Beaker,

• 2 Glass droppers,

• 1 Test tube stand,

• 1 Glass rod.

Experimental Setup

(Image to be added soon)

Procedure

• Dissolve 0.100g of ferric chloride salt in a glass beaker in 100 mL of water and dissolve 0.100 g potassium thiocyanate in the other beaker in 100 mL of water.

• A solution will be obtained in a bright blood red colour by mixing 20 ml of ferric chloride solution with 20 mL of potassium thiocyanate solution.

• Now, fill the same bright blood red colour solution in a glass burette.

• Then, take 5 boiling tubes measuring of equal size and after that label them with the names a,b,c, d, and e.

• Add 2.5 ml of the blood-red solution to every boiling tube from the burette.

• Now, add 17.5 mL of water to the boiling tube ‘a,’ so that the overall volume of solution present in the boiling tube ‘a’ become 20 mL. (for our reference)

• Now take 3 burettes and label them with the names: A, B, and C.

• Fill the ferric chloride solution in A burette.

• And, fill the potassium thiocyanate solution B burette

• Fill the C burette with water.

• After that, add 1 mL, 2 mL, 3 mL, and 4 mL of ferric chloride solution to boiling tubes b, c, d, and e, respectively, from the A burette.

• The C burette adds 16.5 mL, 15.5 mL, 14.5 mL, and 13.5 mL of water to the boiling tubes b, c, d, and e, respectively.

• Now, compare the intensity of the produced colour from the solution in every boiling tube with the reference solution's colour present in the boiling tube ‘a'.

• After that, take another set of 4 clean boiling tubes and then fill them with 2.5 mL of the blood-red solution to every boiling tube from the burette.

• Repeat the same experiment by adding 1 mL, 2 mL, 3 mL, and 4 mL of potassium thiocyanate solution from B burette to the boiling tubes b′, c′, d′, e′ respectively, followed by the addition of 16.5 mL, 15.5 mL, 14.5 mL, 13.5 mL of water.

• Once again compare the intensity of the colour present in the solution of these test tubes using the reference equilibrium solution in the ‘a’ boiling tube,

• Record all the results in the table which is given below,

• We can also repeat the observations with various amounts of ferric chloride solution and potassium thiocyanate and compare them with the reference solution.

Precautions to Be Followed During the Experiment

• Use the mild diluted solutions of potassium thiocyanate and ferric chloride.

• Make a look at the solution’s colour in the boiling tube and reference the test tube

• Also, use boiling tubes of similar size.

Observation

The study of an Equilibrium shift when the concentration of ferric ions becomes increases.

The study of an Equilibrium shift when the concentration of thiocyanate ions becomes increases.

Some Important Viva Questions

Q1. Does the constancy in the intensity of colour indicate the dynamic nature of the equilibrium? Explain the answer with suitable reasons.

Answer: No, since the intensity of the colour becomes constant even after the reaction stops at an equilibrium.

Q2. What is the equilibrium constant, and how does it differs from the rate constant?

Answer: The equilibrium constant can be given as follows:

Kc = \[\frac{[C][D]}{[A][B]}\]

The equilibrium constant is given as independent of the reactants' initial concentration, and it is a function of temperature. However, it remains constant at a constant temperature.