Study of the Shift of Equilibrium Between Ferric Ions and Thiocyanate Ions by Increasing the Concentration of either of Them

Concentration highly affects the shifting of equilibrium. Chemical equilibrium will shift or move towards the reactant when the concentration of the product increases, less product is formed and the concentration of the reactant increases as the concentration of the product decreases.

Some daily life examples of equilibrium are: A book kept on a desk at rest, a car moving on the road at a constant velocity, and also a chemical reaction where the rate of the forward reaction is exactly equal to the rate of the backward reaction. And in ionic equilibrium, the ionic substance dissociates into its constituent ions in the polar solvents.

Table of Content

• Aim

• Apparatus Required

• Theory

• Procedure

• Observation

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To study the shift of equilibrium between ferric ions and thiocyanate ions by increasing the concentration of either of them.

Materials Required

1. Test tubes

2. Test tube stand

3. Glass rod

4. Droppers 

5. Beakers

6. Weight box

7. Measuring cylinders

8. Measuring flask

9. Ferric chloride (0.1 M)

10. Potassium thiocyanate (0.1 M)

11. Potassium chloride (0.1 M)

Theory

1. Effect of Increasing Concentration of Ferric Ions

When ferric chloride solution is added to the red solution containing ferric ions, thiocyanate ions and ferric-thiocyanate complex, the concentration of ferric ions increases, and therefore, the concentration of thiocyanate ions should decrease or that of [Fe(SCN)]2+ should increase to keep concentration quotient equal to the equilibrium constant at a given temperature. Therefore, an increase in the concentration of ferric ions results in more thiocyanate ions combining with ferric ions to give more of [Fe(SCN)]2+ complex, and therefore, the colour intensity of red-solution increases. Thus, an increase in the concentration of Fe3+ ions shifts the above equilibrium in the forward direction.

2. Effect of Increasing Concentration of Thiocyanate Ions

Since the thiocyanate ion is in the denominator in the equilibrium law equation, the addition of more and more of thiocyanate results in more ferric ions reacting with thiocyanate ions to give more of [Fe(SCN)]2+ complex. Hence, the colour intensity of red-solution increases. Thus, an increase in the concentration of SCN- ions shifts the above equilibrium in the forward direction.

3. Effect of Increasing the Concentration of Potassium Ions

An increase in the concentration of K+ ions shifts the equilibrium in the backward direction. This results in a decrease in the concentration of SCN- ions, which in turn shifts the equilibrium in the backward direction. In other words, some [Fe(SCN)]2+ complex dissociates to give Fe3+ ions and SCN- ions. Due to a decrease in the concentration of [Fe(SCN)]2+, the intensity of the red colour decreases. Thus, an increase in the concentration of K+ ions shifts the above equilibrium in the backward direction.

Procedure

1. Take a 250 ml (measure) beaker thoroughly or properly washed and cleaned.

2. Put 10 ml of 0.1 M FeCl3 solution (s) in it by using a measuring cylinder.

3. Add 10 ml of 0.1 M KSCN solution by using or with the help of a measuring cylinder.

4. A deep red colour is obtained or produced due to complex formation [Fe(SCN)]2+ (aq).

5. Dilute the above deep red solution by mixing or adding 50 ml of distilled water.

6. Take four test tubes and name or label them as A, B, C, and D. Add or mix 10 ml of the deep red solution to each of the four test tubes.

7. Arrange or place the test tubes in a test tube stand.

8. Add or mix 5 ml of distilled water to test tube labelled A; 5 ml of 0.1M FeCl3 solution to test tube labelled B; 5 ml of 0.1 M KSCN solution to test tube labelled C and 5 ml of 0.1 M KCl solution to test tube D.

9. Shake or mix all the tubes well.

10. Now compare the intensity (darkness) of the colours in test tubes, B, C, and D with the red colour in test tube A taken as a reference tube.

11. The intensity (darkness) of the red colour corresponds to the concentration (c) of the complex [Fe(SCN)]2+ and if the concentration (amount) of this ion increases, the colour intensity (darkness) will also increase.

The Process of the Shift in Equilibrium between Ferric Ions and Thiocyanate Ions

Observations

The Process of the shift in Equilibrium between Ferric Ions and Thiocyanate Ions

Result

• An increase in the concentration of either of the reactants (Fe3+ ions or SCN- ions) shifts the equilibrium in the right or forward direction.

• A decrease in the concentration of the reactants shifts the equilibrium towards the left or in the backward direction.

Precautions

1. Use tubes of almost identical diameter.

2. Dilute solutions of thiocyanate should be used.

3. The intensity of the colour of a solution should be compared by keeping it and reference side by side and then observing from the top.

Lab Manual Questions

1. What is a thiocyanate ion?

Ans: Thiocyanate is a pseudohalide anion obtained by deprotonation of the thiol group of thiocyanic acid.

2. What is potassium sulphocyanide?

Ans: Potassium sulphocyanide or potassium thiocyanate is one of the known chemical compounds with the molecular formula KSCN. It is a potassium salt, which is the monopotassium salt of thiocyanic acid.

3. What is the colour of ferrous ions?

Ans: Ferrous ions have a light green colour.

4. What is the colour of ferric thiocyanate?

Ans: The colour of ferric thiocyanate is red.

Viva Questions

1. Define a reversible reaction.

Ans: The reaction in which the products formed react back to give the reactant molecules is called a reversible reaction.

2. State the law of chemical equilibrium.

Ans: For a reversible reaction in equilibrium, the product of the molar concentration of products, divided by the product of the molar concentrations of the reactants, each concentration raised to the power equal to its coefficients is constant at a particular temperature. This constant is called the equilibrium constant.

3. What is chemical equilibrium?

Ans: In a chemical reaction when the rate of the forward reaction becomes equal to the rate of the backward reaction, that state is known as a chemical equilibrium.

4. How does the concentration of reactants affect the equilibrium?

Ans: If the concentration of the reactants is increased, the equilibrium shifts in the forward direction.

5. Does the constancy of colour intensity indicate the dynamic nature of equilibrium? Explain your answer with appropriate reasons.

Ans: No, because the colour would become constant even if the reaction stops altogether at equilibrium.

6. Does temperature affect equilibrium?

Ans: Yes.

7. What will be the effect of increasing the temperature of the reaction mixture at equilibrium?

Ans: On increasing the temperature, the equilibrium shifts in favour of the endothermic direction.

8. What is the colour of [Co (H2O)6]2+  ions?

Ans: Pink.

9. What is the formula of the complex ion formed when a solution containing [Co (H2O)6]2+ ions is treated with hydrochloric acid?

Ans: [CoCl4]2-

Practical Questions

1. In a reversible chemical reaction at equilibrium, if the concentration of any one of the reactants is doubled, then the equilibrium constant will

1. Be doubled

2. Become one-fourth

3. Be halved

4. Remain the same

Ans: The equilibrium constant will remain the same.

2. When a neutral atom undergoes oxidation, the atom's oxidation state

1. Decreases as it loses electrons

2. Increases as it gains electrons

3. Decreases as it gains electrons

4. Increases as it loses electrons

Ans: Increases as it loses electrons.

3. If a system is at equilibrium, the rate of forwarding to the reverse reaction is

1. Less

2. High

3. Equal

4. none

Ans: If a system is at equilibrium, the rate of forward-to-reverse reaction is equal.

4. On doubling P and V with constant temperature, the equilibrium constant will

1. Become double

2. Remain constant

3. Become one-fourth

4. none

Ans: The equilibrium constant will remain constant.

5. What happens when an inert gas is added to an equilibrium, keeping volume unchanged?

1. Less product will form

2. More products will form

3. More reactants will form

4. Equilibrium will remain unchanged

Ans: When an inert gas is added to an equilibrium keeping volume unchanged, more product will form.

6. What is the formula of potassium sulphocyanide?

1. K2SO4

2. KSCN

3. KCN

4. none

Ans: KSCN is the formula of potassium sulphocyanide.

7. What is the colour of ferric thiocyanate?

1. Red

2. Blue

3. Green

4. yellow

Ans: The colour of ferric thiocyanate is red.

8. What is the colour of ferrous ions?

1. Dark red

2. Light green

3. Black

4. None

Ans: The colour of ferrous ions is light green.

9. An aqueous solution of sodium cyanide is

1. Acidic

2. Basic

3. Neutral

4. Amphoteric

Ans: An aqueous solution of sodium cyanide is basic.

Conclusion

From the above experiment, we can conclude that the increase in the concentration of either of the reactants (Fe3+ ions or SCN- ions) shifts the equilibrium in the forward direction (towards the right); on the other hand, a decrease in the concentration of the reactants shifts the equilibrium in the backward direction (towards left).