To observe the effect of different concentrations of ferric nitrate on the equilibrium between ferric chloride and sodium thiocyanate solutions.

Part 3:

Independent Variable:

• Concentration of Potassium and thiocyanate  ions added to the system i.e. volume of Potassium thiocyanate solution added to the solution

Dependent Variable:

• Optical Density of the solution which shows the shift in the equilibrium of the system i.e. the Spectrophotometric Reading

Controlled Variables:

• Temperature at which the system is
• Pressure applied to the system
• Concentration of Ferric Chloride Solution
• Concentration of Potassium Thiocyanate Solution
• Volume of Ferric Chloride Solution
• Volume of Potassium Thiocyanate Solution
• Concentration of Potassium thiocyanate Solution
• Spectrophotometric Reading for the Fe(SCN)3 2+ complex solution
• Size of the test tubes
• Physical states of the reactants

Materials:

• 0.32g Ferric Chloride
• 1.20g Ferric Nitrate
• 0.05g Potassium Thiocyanate
• 0.5g Potassium Chloride
• Spectrophotometer
• 4 250mL beakers
• 4 pipettes
• 4 test tubes
• Distilled Water
• 1 Test tube rack
• 50 mL graduated cylinder
• 10 mL Graduated measuring flask
• 3 Spatula
• Weighing Scale
• Spectrophotometer
• Distilled Water
• Wax Paper
• Safety goggles
• Gloves
• Protective apron

Method to control variables:

Part 1:

Independent Variable:

• Concentration of Ferric Ions and nitrate ions added to the system i.e. volume of Ferric Nitrate Solution added – this can be controlled by measuring the volume of 0.1M ferric nitrate added to the solution using a pipette.

Dependent Variable:

• Optical Density of the solution which shows the shift in the equilibrium of the system i.e. the Spectrophotometric Reading – this is dependant upon the volume of the independent variable added.

Controlled Variables:

• Temperature at which the system is – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at room temperature.
• Pressure applied to the system – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at standard atmospheric pressure of 1 atm.
• Concentration of Ferric Chloride Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Concentration of Potassium Thiocyanate Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Volume of Ferric Chloride Solution – this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Volume of Potassium Thiocyanate Solution– this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Concentration of Ferric Nitrate Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Spectrophotometric Reading for the Fe(SCN)3 2+ complex solution – by first preparing a large amount of solution and then using parts, we can ensure that the solution is the same and so the Spectrophotometric readings will also be constant.
• Size of the test tubes – this can be controlled by using test tubes that have been manufactured and calibrated so that their volumes are the same.
• Physical states of the reactants – by keeping temperature and pressure constant, the physical states can also be kept constant.

Part 2:

Independent Variable:

• Concentration of potassium and chloride Ions and nitrate ions added to the system i.e. volume of potassium chloride Solution added – this can be controlled by measuring the volume of 0.1M potassium chloride added to the solution using a pipette.

Dependent Variable:

• Optical Density of the solution which shows the shift in the equilibrium of the system i.e. the Spectrophotometric Reading – this is dependant upon the volume of the independent variable added.

Controlled Variables:

• Temperature at which the system is – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at room temperature.
• Pressure applied to the system – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at standard atmospheric pressure of 1 atm.
• Concentration of Ferric Chloride Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Concentration of Potassium Thiocyanate Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Volume of Ferric Chloride Solution – this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Volume of Potassium Thiocyanate Solution– this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Concentration of potassium chloride Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Spectrophotometric Reading for the Fe(SCN)3 2+ complex solution – by first preparing a large amount of solution and then using parts, we can ensure that the solution is the same and so the Spectrophotometric readings will also be constant.
• Size of the test tubes – this can be controlled by using test tubes that have been manufactured and calibrated so that their volumes are the same.
• Physical states of the reactants – by keeping temperature and pressure constant, the physical states can also be kept constant.

Part 1:

Independent Variable:

• Concentration of potassium ions and thiocyanate ions added to the system i.e. volume of potassium thiocyanate Solution added – this can be controlled by measuring the volume of 0.1M potassium thiocyanate added to the solution using a pipette.

Dependent Variable:

• Optical Density of the solution which shows the shift in the equilibrium of the system i.e. the Spectrophotometric Reading – this is dependant upon the volume of the independent variable added.

Controlled Variables:

• Temperature at which the system is – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at room temperature.
• Pressure applied to the system – conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at standard atmospheric pressure of 1 atm.
• Concentration of Ferric Chloride Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Concentration of Potassium Thiocyanate Solution – this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Volume of Ferric Chloride Solution – this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Volume of Potassium Thiocyanate Solution– this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder
• Concentration of potassium thiocyanate Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water.  
• Spectrophotometric Reading for the Fe(SCN)3 2+ complex solution – by first preparing a large amount of solution and then using parts, we can ensure that the solution is the same and so the Spectrophotometric readings will also be constant.
• Size of the test tubes – this can be controlled by using test tubes that have been manufactured and calibrated so that their volumes are the same.
• Physical states of the reactants – by keeping temperature and pressure constant, the physical states can also be kept constant.

Procedure:

1. Rinse all apparatus with distilled water 
2. Take 0.32g of Ferric Chloride and dissolve it in a beaker containing 20mL of water. This is your solution of 0.1M FeCl3. 
3. Take 0.05g of Potassium Thiocyanate and dissolve it in a beaker containing 20mL of water. This is your solution of 0.1M KSCN 
4. Take a third beaker and fill it with 50mL of water 
5. Add 1mL of 0.1M FeCl3 and 1mL of 0.1M KSCN to the 50mL of water 
6. This will be your standard solution of Fe(SCN)2+ 
7. Take 5mL of Fe(SCN)2+ solution and put it in a test tube 
8. Repeat this with the other 3 test tubes 
9. Take one test tube and measure the wavelength of the solution using a spectrometer 
10. Set this wavelength to be constant on the spectrophotometer
11. Rinse the beaker containing FeCl3 
12. Prepare a solution of 0.1M Fe(NO3)2 by taking 1.20g of Fe(NO3)2 and dissolving it in a beaker containing 20mL of water 
13. Prepare a solution of 0.1M KCl by taking 0.5g of KCl and dissolving it in a beaker containing 20mL of water 
14. Rinse all the pipettes 
15. Take one test tube containing Fe(SCN)2+ and add 0.1M Fe(NO3)2 using a pipette till a color change can be noted. Remember to count the number of drops added 
16. Check the optical density of this solution using the spectrophotometer 
17. Take another test tube containing Fe(SCN)2+ and add 10 drops of 0.1M KCl using a pipette 
18. Take another test tube containing Fe(SCN)2+ and add 0.1M KSCN using a pipette till a color change is noted. Remember to count the number of drops added 
19. Check the optical density of this solution using the spectrophotometer

Data Collection:

Wavelength of original Fe(SCN)22+ solution = 620 nm

Color of FeCl3 = pale yellow

Color of KSCN = colorless

Data Analysis:

Chemical reactions may be envisioned in terms of reactants and products and written in the general form

The reaction may be spontaneous, in which case the reactants would continue to react until they are exhausted with the end composition being nearly all products. In other cases a significant amount of energy must be supplied to get the reaction to go at all, so that the reactants, left to them, would remain in their current form. In intermediate cases, there may be some particular mixture which will exist at equilibrium. It is in those cases where the idea of an equilibrium constant is of greatest value. The equilibrium constant may be expressed in the form

where [C] represents the molar concentration of C at equilibrium. For a given reaction, the concentrations at equilibrium would have to be determined experimentally.

If a stress is applied to a system that is in equilibrium, the equilibrium constant shifts in order to minimize the effect of the stress. This was enshrined by Le Chatelier, who said that "Any change in one of the variables that determines the state of a system in equilibrium causes a shift in the position of equilibrium in a direction that tends to counteract the change in the variable under consideration."

In this experiment, we observed the equilibrium that was formed between Fe+++ ions and SCN- ions which combine to form a Fe(SCN)3 2+ complex. The reaction can be represented as:

FCl3(aq) + 3KSCN(aq) ⬄ Fe(SCN)3(aq) + 3KCl(aq)

In part 1, we add ferric nitrate to the ferric thiocyanate solution that we had prepared. The color of the solution changes from amber to dark red. This is because the addition of ferric nitrate solution causes an increase in the number of ferric ions i.e. It increases the concentration of ferric ions. These ferric ions are reactants in the system at equilibrium and therefore, increasing its concentration causes a shift in equilibrium in accordance with Le Chatelier’s Principle. An increase in reactants causes an increase in the rate of the chemical reaction. This increase in the rate of the reaction causes an increase in the yield of the product. This increase in the yield of the product is what causes the color change from amber to dark red. The magnitude of this color change can be seen when a sample of the new solution is placed into the spectrophotometer. The equilibrium will now shift to the right as there will be more products will be formed. The change in the color of the solution is visible to the naked eye. But to get more accurate results we must place a sample of the new solution inside the spectrophotometer. The optical density of this solution is 0.197 ± 0.001. The Spectronic 20 spectrophotometer us used to measure the amount of light being absorbed at 450 nm, the wavelength at which the thiocyanatoiron(III) complex absorbs visible light. The spectrophotometer shows us the color change by showing us the change in optical densities.

In part 2, we add potassium chloride solution. There is no color change observed. There is an addition of potassium and chloride ions. These ions do not, however, form part of the thiocyanatoiron(III) complex. Therefore, the addition of potassium chloride does not change the concentrations of any of the reactants that form the complex. It therefore, does not change the rate of the reaction or the equilibrium constant either. An increase in potassium and chloride ions does not affect the equilibrium formed by the complex because they do not affect the complex. This is why no color change is visible. However, when put into the spectrophotometer, a 0.001 change is observed. This could be because of the solution that is formed, which is more dilute than the original solution.

In part 3, we add potassium thiocyanate solution to the original solution of the thiocyanatoiron(III) complex. The color of the solution changes from amber to dark red, similar to the color change in the first part. This is because the addition

of potassium thiocyanate solution causes an increase in the number of thiocyanate ions i.e. It increases the concentration of thiocyanate ions. These thiocyanate ions are reactants in the system at equilibrium and therefore, increasing its concentration causes a shift in equilibrium in accordance with Le Chatelier’s Principle. An increase in reactants causes an increase in the rate of the chemical reaction. This increase in the rate of the reaction causes an increase in the yield of the product. This increase in the yield of the product is what causes the color change from amber to dark red. The magnitude of this color change can be seen when a sample of the new solution is placed into the spectrophotometer. The equilibrium will now shift to the right as there will be more products will be formed. The change in the color of the solution is visible to the naked eye. But to get more accurate results we must place a sample of the new solution inside the spectrophotometer. The optical density of this solution is 0.192 ± 0.001. The Spectronic 20 spectrophotometer us used to measure the amount of light being absorbed at 450 nm, the wavelength at which the thiocyanatoiron(III) complex absorbs visible light. The spectrophotometer shows us the color change by showing us the change in optical densities.

Adding more of the reactant ions in parts 1 and 3 increases the reactant concentration. In accordance with Le Chatelier’s principle, this increases the rates of the reactions because of the increase in the number of collisions caused among the reactant ions. This, in turn, increases the number of successful collisions which causes an increased yield in the product molecules formed. This is caused by the tendency of the system at equilibrium to shift so that the effect of the stress is minimized.

Evaluation and Conclusion:

This lab was designed to observe the effect that the stress of changing concentration has on the equilibrium that is formed. For this, we chose the reaction that involved the equilibrium complex formed between ferric ions and thiocyanate ions. The effect of changing concentrations can be seen by the color change that it causes. This color change is accurately seen using a spectrophotometer.

On performing the experiment, I felt that there were a number of areas in the procedure that could have been improved upon so as to yield more accurate and more precise results. Firstly, the spectrophotometer must be considered. The original value that is set on the instrument was 450 nm. However, this could be inaccurate and therefore, the spectrophotometer readings must be considered with an error of 0.001. If the test tubes used have not been mechanically tested, the light that is emitted by the spectrophotometer could be absorbed to a certain extent by the walls of the test tube, thereby affecting the readings obtained from the spectrophotometer.

Secondly, we must consider the human errors associated with measuring the amount of salt, the volume of the solutions added and the amounts of solution taken for measurement. These human errors can be reduced to a large extent by using instruments that are more sensitive. For example, using a thinner calibrated cylinder would reduce the error associated with it because a small change in amount would be seen by a large change in the cylinder.

Thirdly, the salts and solution used could have been impure. There could have been a number of substances chemically combined with them that would have affected the readings. These could have been reduced by using chemicals that have been stored in conditions that prevent them from coming in contact with any impurities.

Fourthly, the weights of the salts used could have been faulty and inaccurate. These could have been negated by measuring the salts repeatedly till constant values are obtained. Fifthly, the chemicals that were used might not have been of the specified concentrations and strengths. Again, these could have been negated by using chemicals stored in good conditions. Also, the tests tubes used could have had impurities which would have affected the readings. These could have been avoided by cleaning the test tubes well.

Furthermore, impurities from the air could have reacted with the solutions when the experiment was being conducted. This could have been avoided by conducting the experiment. In general, the accuracy of results could have been increased by repeating the experiment many times. However, time constraints might prevent this from being possible.