This laboratory session delves into the chemical reactions between potassium thiocyanate (KSCN) and iron nitrate (Fe(NO₃)₃), focusing on observing the color changes and precipitate formation that occur under varying conditions, including temperature changes and the addition of different reagents.
Educational Goals
- Chemical Reactions: Students will explore the interaction between iron and thiocyanate ions to form colorful complexes, enhancing their understanding of reaction mechanisms.
- Temperature Effects: The experiment allows observation of how temperature variations impact the speed and direction of chemical reactions, demonstrating the influence of thermal energy on chemical processes.
- Analytical Chemistry Applications: Participants will learn about the application of complexation reactions in chemical analysis, gaining insights into analytical techniques.
- Development of Experimental Skills: Students will refine laboratory techniques, including solution handling, experimental condition adjustment, and qualitative reaction observation, improving their practical chemistry skills.
Through this experiment, students will gain a practical understanding of complex chemistry, observing firsthand how variables such as reagent concentration and temperature can affect chemical reactions. This hands-on experience enhances knowledge of inorganic and analytical chemistry’s fundamental principles, illustrating the dynamic nature of chemical interactions and the critical role of experimental conditions in determining reaction outcomes.
Protocol
1. Preparation of base solutions
a) Measure 50 mL of 0.001M KSCN solution using a graduated cylinder.
b) Pour the measured solution into a 50 mL beaker.
c) Add 12 drops of 0.5M Fe(NO3)3 solution to the beaker.
d) Stir the mixture with a glass rod.
e) Distribute the obtained solution into eight test tubes (about 6 mL per test tube).
2. Changing the equilibrium point
f) Add about 1.5 to 2g of KSCN powder to test tube #2 using a spatula.
g) Add between 1.5 and 2g of iron nitrate crystals to test tube #3.
h) Add between 1.5 and 2g of KSCN to test tube #4.
i) Shake test tubes #2, #3, and #4.
j) Add between 1.5 and 2g of iron nitrate crystals to test tube #4.
k) Shake test tube #4.
l) Add 1 or 2 drops of KOH to test tube #5 and shake.
m) Add between 1.5 and 2g of Na2HPO4 to test tube #6 and shake.
n) Prepare an ice bath by filling a 250ml beaker halfway with water and ice and then place test tube #7 in it.
o) Fill another 250 mL beaker with water and place it on a heating plate to immerse test tube #8.
p) Insert the magnetic stirrer into the beaker on the heating plate and start the stirrer.
q) Heat the water to about 80°C.
r) Stir test tube #7 while it is immersed in the ice bath.
s) Observe the changes in each test tube, noting differences in color or precipitation.
t) Turn off the heating plate and magnetic stirrer once the experiment is completed.
u) Shake all the test tubes one last time to homogenize the reactions.
v) Record the colors of test tubes #2 to #8, referencing control test tube #1.
w) Rinse the used equipment with distilled water after retrieving the magnetic stirrer.
Anticipated Outcomes
- Test tube #1: Serves as the reference color, consistently described as reddish or reddish-brown, for the equilibrium mixture of Fe³⁺, SCN⁻, and FeSCN²⁺.
- Test tube #2: Changes for a bit more reddish-brown color, indicating approx. 50% more FeSCN²⁺ formation, as the direct formation is favored.
- Test tube #3: Shows a reddish-brown color between Test Tube #1 and #2, indicating an increase in FeSCN²⁺ concentration when Fe(NO₃)₃ is added, demonstrating a shift in equilibrium toward product formation.
- Test tube #4: Exhibits a very dark reddish-brown or darkest red color, the most intense among all, suggesting a significant increase in FeSCN²⁺ due to the addition of both Fe(NO₃)₃ and KSCN, marking a substantial shift toward products.
- Test tube #5: Presents a light brown color with a red precipitate, indicating the formation of Fe (OH)₃ from Fe³⁺ reacting with OH⁻.
- Test tube #6: Presents a light brown color with a brownish precipitate, indicating the formation of FePO4 from Fe³⁺ reacting with PO4-.
- Test tube #7: Displays a darker reddish-brown or paler red color upon cooling, suggesting the favored formation of FeSCN²⁺, consistent with an exothermic process.
- Test tube #8: Shows a lighter reddish-brown or lighter brown color upon heating, indicating a shift toward reactants, favoring the endothermic dissociation of FeSCN²⁺ into Fe³⁺ and SCN⁻.
The experiment vividly demonstrates Le Chatelier’s Principle, showing how the system responds to changes in concentration, temperature, and the presence of additional reactants or products. The color changes in each test tube provide a qualitative measure of the shifts in equilibrium, highlighting the dynamic nature of chemical equilibria and the factors that influence them. This approach allows for a visual understanding of equilibrium shifts, reinforcing the theoretical concepts with tangible evidence.
Adding reactants (KSCN or Fe(NO₃)₃) shifts the equilibrium toward more product formation (FeSCN²⁺), as evidenced by the darker color.
Removing a reactant or product (as in test tube #5 and #6) shifts the equilibrium to compensate, here reducing FeSCN²⁺ concentration.
Temperature changes also affect the equilibrium; cooling favors exothermic reactions, while heating favors endothermic reactions.
Lessons learned
Le Chatelier’s Principle: the experiment vividly demonstrates how a system at equilibrium responds to external changes to maintain balance.
Equilibrium shifts: understanding that the addition of a reactant or product shifts the equilibrium toward one side, while its removal shifts it toward the other.
Effect of temperature: observing how temperature changes influence equilibrium, offering insight into the exothermic or endothermic nature of reactions.
Chemistry principles behind
Chemical equilibrium: the dynamic balance where the rate of the forward reaction equals the rate of the reverse reaction.
- Color change as an indicator: the change in color intensity serves as a qualitative indicator of the shift in equilibrium concentrations.
- Precipitation reactions: the formation of Fe(OH)₃ demonstrates how precipitate formation can be used to deduce changes in ion concentrations in a reaction mixture.
This experiment provides a hands-on understanding of how equilibria respond to changes in conditions, illustrating the adaptability of chemical systems to maintain balance, aligning with Le Chatelier’s Principle.
Summary of Assignment by Grade Range
Grades 3-5 (Ages 8-10)
- Focus: Basic introduction to chemical reactions and observation of color changes.
- Activities: Simple observations of color changes when mixing potassium thiocyanate and iron nitrate, understanding basic concepts of chemical reactions, basic safety instructions.
Grades 6-8 (Ages 11-13)
- Focus: Intermediate understanding of reaction mechanisms, temperature effects, and basic analytical chemistry.
- Activities: Conducting experiments to observe color changes and precipitate formation, measuring temperature effects on reaction rates, exploring basic analytical techniques, following detailed safety protocols.
Grades 9-12 (Ages 14-18)
- Focus: Advanced understanding of Le Chatelier’s Principle, complexation reactions, and analytical chemistry applications.
- Activities: Accurately conducting experiments with potassium thiocyanate and iron nitrate, observing and recording the impact of varying conditions, analyzing the effects of temperature and reagent concentration, detailed recording and interpretation of results, adhering to advanced safety protocols, reinforcing concepts of chemical equilibrium and reaction mechanisms.
Laboratory essentials
Instruments
Beakers (50ml, 250ml & 1000ml)
Droppers
Electronic Scale
Glass Rod
Graduated Cylinders (10ml & 70ml)
Hot plate
Lab Stand & Clamps
Magnetic stirrer
Spatulas
Test Tubes
Thermometers
Products
Iron nitrate (solution)
Iron nitrate (crystals)
Potassium hydroxide (solution)
Potassium thiocyanate (solution)
Potassium thiocyanate (powder)
Sodium hydrogen phosphate (powder)