Effect of Change in Concentration on Chemical Equilibrium

The system reaches a condition of chemical equilibrium whenever the forward reaction rate equals the rate of the backward reaction. The system will react somehow when a chemical equilibrium state is abruptly interrupted until chemical equilibrium is restored. When a compound is added, a reaction absorbs it, reducing the reactants or products or reactant concentration added. The chemical reaction that occurs in the path that replaces the substance that is withdrawn relieves the products or reactants concentration eliminated. Thus, the concentration could change the reaction's value by either adding or removing the compound.

Table of Content

• Aim

• Apparatus Required

• Theory 

• Procedure

• Result

Aim

To study the effect of change in concentration of chemical equilibrium shift of ferric and thiocyanate ions.

Apparatus Required

1. Ferric Chloride

2. Potassium Chloride

3. Potassium Thiocyanate 

4. Dropper

5. Test Tube Stands

6. Test Tubes 

7. Test Tube Holder 

8. Glass Rod

9. Beakers

10. Measuring Flasks and Cylinders

Theory

To reduce the impact of concentration variations, the chemical equilibrium mixture composition varies whenever the concentrations of the products or reactants in a chemical reaction at chemical equilibrium are modified.

Look at the example below to see how a change in concentration can affect equilibrium.

Fe3++SCN-→[Fe(SCN)2+]

The equation for the constants of chemical equilibrium for the reaction mentioned above is,

${K_c} = \dfrac{{\left[ {Fe{{\left( {SCN} \right)}^{2 + }}} \right]}}{{\left( {F{e^{3 + }}} \right)(SC{N^ - })}}$

Here Kc is the equilibrium constant, [Fe3+], [SCN-], and [Fe(SCN)2+], are the chemical equilibrium concentrations of the corresponding species.

The chemical equilibrium is disturbed whenever the concentrations of either of these species change, as well as the reaction quotient, persists below the equilibrium value. The ions connect in a way that makes the reaction quotient equivalent to the chemical equilibrium constant once more to restore equilibrium.

Procedure

1. In a 250mL clean and dry beaker add 10 mL of ferric chloride (FeCl3) using a measuring cylinder and glass rod.

2. Then add 10mL of potassium thiocyanate (KSCN) to the beaker containing FeCl3. The colour of the solution in the beaker will be blood-red.

3. Take 4 test tubes and label them as A, B, C, and D. In each of the 4 test tubes add 10mL of blood-red solution and keep them in the test tube stand.

4. In a test tube, A containing red blood solution, add 5mL of distilled water and keep it as a reference. 

5. In test tube B add 5mL of FeCl3, in test tube C add 5mL of KSCN and in test tube D add 5mL of KCl.

6. Now examine the colours in test tubes B, C, as well as D against test tube A containing the red colour solution, which has been used as a reference. Record the observation.

Observations

Result

The results of the chemistry experiment on the effect of change in concentration of equilibrium verified that:-

• The increase in reactant (Fe3+ or SCN-) concentration shifts the equilibrium towards the right.

• The decrease in the reactant (Fe3+ or SCN-) concentration shifts the equilibrium towards the left.

Precautions

• Handle chemicals and apparatus with caution.

• Every apparatus should be rinsed with distilled water and dried before starting the experiment.

• Utilize test tubes of nearly the same diameter.

• Thiocyanate concentrations should be diluted.

• A solution's colour intensity should indeed be evaluated by placing the reference and the solution side-by-side and subsequently viewing from the top.

• After the completion of the experiment, wash your hands thoroughly.

Lab Manual Questions

1. Why are concentrated acids not used in this experiment?

Ans: When using concentrated acids, the reaction is extremely violent and the mixture gets hot. This speeds up the process much more and the liquid within may foam. The process is significantly under control when the acid is diluted.

2. Why were test tubes of the same diameter used in this experiment?

Ans: To obtain a precise measurement of the solutions that were introduced for reference for comparison study.

3. What is the aim of transferring solutions employing glass rods?

Ans: Solutions are poured using glass rods to prevent spillage. A glass rod placed against a beaker's pouring edge enables the solution to run down the glass rod and into the collecting vessel, as opposed to splashing beyond the lip.

4. What will happen if solid KCl is added to the system when it is in equilibrium?

Ans: If solid KCl is added, then the solubility gets decreased. The composition of the equilibrium is changed by the addition of a common ion; however, the ionization constant is unaffected.

Viva Questions

1. What is chemical equilibrium?

Ans: Chemical equilibrium is the condition that occurs in a chemical process when the forward reaction rate equals the backward reaction rate.

2. What happens when inert gas is added during chemical equilibrium?

Ans: Introduction of inert gas has no effect on concentration on chemical equilibrium.

3. What are the types of chemical equilibrium?

Ans: Homogeneous and heterogeneous equilibrium.

4. What are the factors affecting chemical equilibrium?

Ans: Concentration, temperature, and pressure of the system affect the chemical equilibrium.

5. Does the consistency of colour intensity imply that equilibrium is dynamic?

Ans: No, since even if the process completely halted at equilibrium, the colour would remain constant.

6. Define forward reaction.

Ans: A forward reaction is one wherein the reactants are solely transformed into products.

7. Define backward reaction.

Ans: Backward reactions are those wherein the products are transformed back into the reactants.

8. What are the industrial applications of chemical equilibrium?

Ans:  Haber’s and contacts process.

9. What complex is produced when a thiocyanate ion is added to a ferric salt solution?

Ans: [Fe (SCN)2+]- Thiocyanato (III) ion

10. What is the chemical formula of the complex ion when HCl is added to a solution comprising [Co (H2O)6]2+ ions?

Ans:  [CoCl4]2-

Practical-Based Questions

1. Which of the following statements is false?

1. The chemical equilibrium is affected by the catalyst

2. The chemical equilibrium is not affected by the catalyst

3. A catalyst speeds up the reaction

4. A catalyst speeds up both forward and backward reaction

Ans: The chemical equilibrium is affected by the catalyst.

2. In a chemical equilibrium, what happens to the composition of the mixture when the reactant or product concentration changes?

1. The composition of the mixture does not change

2. The composition of the mixture changes

3. The composition of the mixture remains constant

4. None of the three options

Ans: The composition of the mixture changes while the product or reactant concentration changes.

3. Why does the concentration of colour intensity increase?

1. Due to an increase in the concentration of SCN-

2. Due to the decrease in the concentration of SCN-

3. Maintaining the concentration of solution without any increase or decrease 

4. None of the three options

Ans: Due to the increase in the concentration of SCN- the concentration of colour intensity increases.

4. What happens when KCl is added?

1. The concentration of Cl- increases

2. The concentration of Cl- decreases

3. The concentration of K+ increases

4. The concentration of K+ decreases

Ans: When KCl is added, the concentration of K+ increases.

5. What is the colour of [Fe (SCN)2+] complex?

1. Pink

2. Blue

3. Red

4. Blood-red

Ans: The colour of [Fe (SCN)2+] is blood-red.

6. What influences potassium and thiocyanate ions to combine to generate additional potassium thiocyanate, which causes the chemical equilibrium to move too backwards?

1. Increase in SCN- ion concentration

2. Increase in K+ ion concentration

3. Decrease in K+ ion concentration

4. None of the three options

Ans: Increase in K+ ion concentration.

7. What happens when thiocyanate ion concentration decreases?

1. The reaction remains constant

2. The reaction shifts toward the left

3. The reaction shifts towards the right

4. None of the three options

Ans: The reaction shifts towards the left when the thiocyanate ion concentration decreases. 

8. How does distilled water act in chemical laboratories?

1. Indicator

2. Solvent 

3. Universal solvent

4. None of the three options

Ans: Distilled water acts as a universal solvent in chemical laboratories.

9. What happens if the chemical equilibrium reaction mixture' temperature is raised?

1. Equilibrium remains constant

2. Equilibrium shifts towards an exothermic direction

3. Equilibrium shifts towards an endothermic direction

4. None of the three options

Ans: The equilibrium shifts towards the endothermic direction if the temperature rises.

10.  What happens when a potassium ferrocyanide solution is combined with a ferric salt solution?

1. Blue colour deepens

2. Changes to blue colour

3. Red colour deepens

4. No colour change

Ans: The red colour deepens when a potassium ferrocyanide solution is combined with a ferric salt solution.

Conclusion

This chemistry experiment on the effect of change in concentration on chemical equilibrium infers that the Fe3+ or SCN- ion reactant concentration changes cause the equilibrium to shift towards the right (forward reaction), while changes in either of the reactant's concentrations cause the equilibrium to shift towards the left (backward reaction). 

As the ferric ions' concentration rises, more thiocyanate ions combine with ferric ions to form more [Fe (SCN)]2+ complex, and red colour intensity increases. As a result, raising Fe3+ ions concentration causes the aforementioned equilibrium to change.