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When a system in equilibrium is suddenly disturbed, it will react in the opposite way until equilibrium is re-established. The equilibrium constant of the equilibrium reaction between ferric chloride and potassium thiocyanate can be studied through a chemistry lab experiment with a step by step procedure and sufficient precautions.
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Key Terms: Shift in equilibrium, ferric ions, thiocyanate ions, chemistry experiment
Principle of Experiment
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The reaction and equilibrium constant of the equilibrium reaction between ferric chloride and potassium thiocyanate is given by:
The equilibrium constant is given by the following formula
The equilibrium constant for the reaction between potassium thiocyanate and ferric chloride is given as follows
At a given constant temperature, the value of K will also remain constant.
If you Increase the concentration of Fe3+ ions or thiocyanate ions, there will be an increase in the concentration of [Fe(SCN)]²? ions also.
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Experiment
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Aim
It is to understand the shift in equilibrium reaction between ferric ions and thiocyanate ions by increase or decrease in the concentration of the ions participating in the reaction.
Chemicals Required
0.100g Ferric chloride and 0.100g potassium thiocyanate,
Apparatus Required
2 Beakers of 100 mL capacity, 1 beaker of 250 mL capacity, 6 boiling tubes, 4 burettes with stands, 2 glass droppers, 1 Test tube stand, and 1 glass rod
Experimental Setup in the Lab
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Procedure
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- Dissolve 0.100g of ferric chloride salt in 100 mL of water in one baker. Dissolve 0.100 g potassium thiocyanate in 100 mL of water in another beaker.
- 20 mL of ferric chloride solution will be mixed with 20 mL of potassium thiocyanate solution to get a bright blood red colour solution.
- Take the bright blood red colour solution into a burette.
- Take five boiling tubes of equal size to label them as a, b,c,d, and e.
- Drop 2.5 mL of blood red solution into each boiling tube from the burette.
- Add 17.5 mL of water to the boiling tube ‘a’ to make it upto 20 mL as a reference.
- Take three burettes to label them as A, B, and C.
- Fill ferric chloride solution in burette A, fIll potassium thiocyanate solution in burette B, and fill water in burette C.
- Add 1.0 mL, 2.0 mL, 3.0 mL and 4.0 mL of ferric chloride solution to the remaining four boiling tubes b, c, d, and e from burette A.
- Add 16.5 mL, 15.5 mL, 14.5 mL, and 13.5 mL of water from burette C to boiling tubes b, c, d, and e respectively.
- You can notice a change in colour intensity in the solution of the boiling tubes b,c,d,and e with the colour of the reference solution in the boiling tube ‘a’.
- Take one more set of four boiling tubes and fill with 2.5 mL of blood red solution from the burette.
- Repeat the same experiment by taking 1.0 mL, 2.0 mL, 3.0 mL and 4.0 mL of potassium thiocyanate solution from burette B to the boiling tubes b′, c′, d′, and e′ respectively followed by the addition of 16.5 mL, 15.5 mL, 14.5 mL and 13.5 mL of water.
- Once again compare the colour intensity of the solution of b.c.d.e test tubes with the reference solution in the tube ‘a’.
- Record all the observations in the observation table given below.
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Observation Table
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The study of the equilibrium shift when the concentration of ferric ions is increased
| Boiling Tube No. | Amount of ferric chloride solution | Change in colour intensity as compared with the reference solution in boiling tube “a” | Direction of shift in equilibrium |
|---|---|---|---|
| a | Reference solution 2.5 mL blood red solution + 17.5 mL water | Equilibrium position | |
| b | 1 | Colour deepens | Towards right |
| c | 2 | Colour deepens | Towards right |
| d | 3 | Colour deepens | Towards right |
| e | 4 | Colour becomes lighter | Towards left |
The study of the equilibrium shift when the concentration of thiocyanate ions is increased
| Boiling Tube No | Volume of thiocyanate | Change in colour intensity as compared with the reference solution in boiling tube “a” | Direction of shift in equilibrium |
|---|---|---|---|
| a | Reference solution 2.5 mL blood red solution + 17.5 mL water | Equilibrium position | |
| b | 1.0 | Colour deepens | Towards right |
| c | 2.0 | Colour deepens | Towards right |
| d | 3.0 | Colour deepens | Towards right |
| e | 4.0 | Colour becomes lighter | Towards left |
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Precautions
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- Use only diluted solutions of ferric chloride and potassium thiocyanate.
- Keenly observe and compare the colour of the solutions in the boiling tubes and reference test tube.
- All boiling tubes should be of the same size.
Things to Remember
- When a system in equilibrium is suddenly disturbed, it will react in the opposite way until equilibrium is reestablished.
- The reaction and equilibrium constant are given by the following equation
- The equilibrium constant is given by the following formula
- The equilibrium constant for the reaction between potassium thiocyanate and ferric chloride.
- The equilibrium constant of the equilibrium reaction between ferric chloride and potassium thiocyanate can be studied through a chemistry lab experiment with a step by step procedure and sufficient precautions.
- At a given constant temperature, the value of K will also remain constant.
- When ferric chloride solution is added to the red solution containing ferric ions, thiocyanate ions and ferric-thiocyanate ions, concentration of ferric ions increases and, the concentration of thiocyanate ions decreases.
- Use only diluted solutions of ferric chloride and potassium thiocyanate.
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Sample Questions
Ques: Do you accept that the constancy in colour intensity indicates the dynamic nature of equilibrium? What could be the reasons.? (2 Marks)
Ans: No, the constancy in colour intensity does not indicate the dynamic nature of equilibrium, since the colour becomes constant even after the reaction stops at equilibrium.
Ques: What is the equilibrium constant? How does it differ from the rate constant? (2 Marks)
Ans: The equilibrium constant is given by the following equation:
The equilibrium constant is always independent of the initial concentration of reactants. It depends on temperature. It remains constant at a constant temperature.
Ques: Why do you carry the equilibrium experiment with dilute solutions only? (2 Marks)
Ans: The equilibrium experiment is carried out with dilute solutions only because dilute solutions are safer to handle and do not do not cause damage in lab conditions on heating.
Ques: Does temperature affect equilibrium reactions? (2 Marks)
Ans: Yes. If the temperature is increased, equilibrium shifts in the direction of the endothermic reaction. If the temperature is decreased, the equilibrium shifts in the direction of the exothermic reaction.
Ques: Why are boiling tubes of the same size used in the equilibrium experiment? (2 Marks)
Ans: Boiling tubes of the same size are used in the equilibrium experiments to get the precise measurement of the solution added for reference.
Ques: What are the three precautions taken during the equilibrium experiment? (2 Marks)
Ans:
- Use only diluted solutions of ferric chloride and potassium thiocyanate.
- Keenly observe and compare the colour of the solutions in the boiling tubes and reference test tube.
- All boiling tubes should be of the same size.
Ques 7: What are the different chemicals and apparatus required to do experiments on equilibrium reactions? (2 Marks)
Ans:
Chemicals Required: 0.100g Ferric chloride and 0.100g potassium thiocyanate,
Apparatus required: 2 Beakers of 100 mL capacity, 1 beaker of 250 mL capacity, 6 boiling tubes, 4 burettes with stands, 2 glass droppers, 1 Test tube stand, and 1 glass rod
Ques: 1. A liquid substance is in equilibrium with its vapour in a sealed container at a constant temperature. If the volume of the container is suddenly increased. a) What would be the initial effect on the change on vapour pressure? b) How would rates of evaporation and condensation change in the beginning? c) What would happen when equilibrium is restored at the end and what would be the final vapour pressure? (5 Marks)
Ans:
(a) On increasing the volume of the container, the vapour pressure will initially decrease as the container size is more and the same amount of vapours have to be distributed over a large space.
(b) On increasing the volume of the container, the rates of evaporation will increase in the beginning as there is more exposed surface area. As the amount of the vapours per unit volume decreases on increasing the volume of the container, the rate of condensation will also decrease in the beginning.
(c) Equilibrium will be restored when the rates of the forward and backward reactions become equal. But, the vapour pressure will remain unchanged as it depends on the temperature and not on the volume of the container.
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