Expt.9: Chemical Equilibrium Claudine Rosario Barlaan Lukban Albert Mitchell Lopez Yap
TCD3, Group 9, Prof. M.L. Rubio March 16, 2010
Abstract In understanding Chemical Equilibrium and Le Châtelier’s Principle, students are able to analyze the factors that can cause a shift in the equilibrium or affect a system. Students will be able to further understand the dynamic nature in chemical reactions. For the experiment, simple tools and substances were used to formulate an observation. For the first experiment, testing concentrations and interactions of the substances, the change in color intensity were used to create an observation on the shift on the equilibrium. Utilizing the same method for observation, the second experiment tested the effect of temperature on the system. It was generalized, integrating Le Châtelier’s Principles, that the darker color intensity signified a direction of shift forward. This shift is associated to a decrease in temperature or an increase in concentration. The opposite applies to the lightening of color intensity. Keywords: 1. Le Châtelier’s Principle 2. Stress 3. Shift 4. Common Ion Effect 5. Exothermic Reaction 6. Endothermic Reaction Introduction Some chemical reactions do not readily go into completion. In the reaction process, there will be an instance wherein the rate of formation of the product will be equal to the rate of formation of the reactants. This point is the state of equilibrium. Some conditions affect the equilibrium like temperature or concentration. With the use of Le Châtelier’s Principle, one can predict the shift of the equilibrium in the system. The experiment aims to test this principle with regards to observable conditions and to prove the concepts with the means of a simple set-up. Conducting the experiment aims for the students to be able to: “to evaluate the effect of change in concentration and change in temperature on the equilibrium system”, “to explain the effect of the changes on the equilibrium system” and “to interpret the results based on Le Châtelier’s Principles”. Experimental Instead of following the procedures in the manual, the experiment was conducted using spot plates for the students to better observe the changes and difference in color of the substances. 20 drops of 1 M ferric nitrate Fe(NO 3)3 solution was placed in a 10-mL test tube along with 20 drops of 1 M potassium thiocyanate KCNS. 7 mL of water was then added to the test tube before shaking well. The color of the mixture was noted thereafter. Six depressions on the spot plate were labeled from 16. Five drops of the solution were then placed in the eight labeled depressions. Five drops of different solutions were placed in the first six depressions. Distilled water was added to the first depression,
0.1 M of Fe(NO3)3 for the second, 0.1 M KCNS for the third, 0.1 M KCl for the fourth, 0.1 M AgNO3 for the fifth and a pinch of NaF for the sixth depression. Changes in the intensity of color were noted, using the substance in the first depression as reference. Observations and reactions after adding each reagent was recorded as well. For the second part of the experiment, ten drops of solution was placed in two test tubes each. After this, ten drops of distilled water was added to each of the test tubes. The first test tube was placed in ice water, while the other over a low flame using an alcohol lamp. Observations of both substances were then compared to the substance in the first depression of the spot plate. Results Reagent 0.1M Fe(NO3)3 0.1M KCNS 0.1M KCl
Observation
Direction of Shift Forward Forward Backward
Pinch of NaF
Darker Slightly darker Slightly lighter Formation of white precipitate; light yellow Almost transparent w/ hint of yellow
Inc. in temp. Dec. in temp.
Lighter Darker
Backward Forward
0.1M AgNO3
Backward Backward
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Discussion There is a special kind of Le Châtelier’s Principle wherein the system is diluted in water. This special case is called the “common ion effect”. Regarding the first and second reagent, there are ions the reactants share common ions with the reagents. In the first one, the common ion is Fe 3+ in Fe(NO3)3. In the second one, the common ion is CNS- in KCNS. What happens is when the reagents were added, the number of moles of the reactants increased, thereby resulting in a higher concentration (one of the causes for a shift in the system). So because the reactants exhibited a higher concentration than the products, the free Fe3+ ions and the free CNS- ions now combine and shift towards the product side, thereby placing the system again at equilibrium. For the third, fourth, and fifth reagents, since they are electrolytes, they dissociate into ions and pair up with the free ions originally in the reactant sides. Therefore, the number of free ions in the reactant side decreased. And as a result, the stress in the product side increased. So, to balance the system, some molecules of the product dissociate once more and move towards the reactant side to balance out the stress. Lastly for the sixth and the seventh cases wherein there was a change in temperature, the system shifted forward or backward depending on what type the reaction is. Since the reaction is exothermic, once the temperature is increased, the system will be forced to move backward and vise versa. Guide Questions and Answers: 1. Explain your observations on the basis of Le Châtelier’s Principle. The “shift” that the system underwent was to even out the external stress applied to it. 2. Which species (ions) in the reagents are effective in altering the state of the system? Fe3+ in Fe(NO3)3 and CNS- in KCNS (due to common ion effect)
3. Is the reaction endothermic or exothermic? Why? Exothermic. In an exothermic reaction, if there’s an increase in the temperature, the direction of the shift goes backward. Whereas if there’s a decrease in temperature, the shift goes forward. Conclusion and Recommendation Generally, the observations with the integration of Le Châtelier’s Principle show the shift and how each substance is affected. Given the test tubes in the final portion of the experiment, the observations clearly show that when there is a decrease in temperature, the shift goes forward, therefore resulting in a dark shade of color for the substance. On the other hand, an increase in temperature causes a shift backward and results in lighter shade of color. The group would like to recommend the employment of substances in test tubes or in a clearer container to be able to easily observe the changes and differences the substances for comparison. References Chang, R. (2005). Chemistry (8th ed). DPSM. (2006). Laboratory Manual in General Chemistry I. Masterson and Hurley (2005) Chemistry: Principles and Reactions (5th ed).
I hereby certify that I have given substantial contribution to this report.
Claudine Rosario B. Lukban
Albert Mitchell L. Yap
Page 2 of 2 Chem 14.1 – Chemical Equilibrium
