Le Chatelier's Principle Virtual Lab (3)

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LE CHATELIER’S PRINCIPLE FRQ (Virtual Lab) Le Chatelier’s Principle can best be observed using equilibrium systems that have different colored reactants and products. The shifting of the equilibrium can be seen by the color changes that occur. You will be watching videos representing three different equilibrium systems, and processing through the best way to justify your answers about the shifts that occur. EQUILIBRIUM SYSTEM #1 : The Equilibrium of Cobalt Complex Ions Watch the experiment at https://youtu.be/XqboaYTTxo8 At Equilibrium: PURPLE Co(H 2 O) 6 2+ (aq) + 4 Cl  (aq) ↔ CoCl 4 2  (aq) + 6 H 2 O (l) PINK BLUE 1. Write the equilibrium-constant expression (K c ) for the forward reaction shown above. K c =[CoCl 4 ]/{Co(H 2 O) 6 ][Cl] 4 2. ADDING WATER: Watch the video from the beginning until the 0:30 mark, and answer the statements below. a. When water was added to the system, did the molarity of each of the individual aqueous solutions in the test tube increase or decrease? Justify your answer. The molarities of each of the individual aqueous solutions in the test tube decreases because when water is added to the solutions the volume increases but the moles of the substance does not increase with it. Since the molarity is equal to moles over volume, the increasing of the denominator(volume) changes the mole to ratio volume and decreases the molarity b. Would this cause more of a change in the overall concentrations of reactants or of products, with regards to Q c ? Justify your answer. This would cause a greater change to the overall concentration of the reactants rather than the products since the total moles of aqueous solution on the reactants side is 5 moles while the products side is one c. What would the addition of water to the equilibrium system do to the value of Q c ? How would it compare to the value of K c ? Adding water would increase the Q c value since the denominator in the equation gets decreased more than the numerator does and since the denominator is much less the Q c constant will be more than the K c value d. Did the reaction shift towards the reactants or the products in order to re-establish equilibrium? Was this the side of the equation with more moles or less moles of aqueous ions? Did the observed color change from the video support this answer? The reaction was shifted towards the reactants (the side with more moles of the aqueous solution). The color change from the video supports this answer because the solution changed from its equilibrium color of purple to the reactant color which was pink. 3. ADDING CONCENTRATED HCl: Watch the video from the 0:30 mark to the 0:44 mark, and answer the statements below. a. When HCl is added to a solution, it will dissociate into ions. Write the dissociation equation for HCl below. HCl-> H + +Cl - b. Which ion did you notice was present in both the HCl (aq) dissociation, as well as in the reference equilibrium reaction? Cl -

c. The reference equilibrium reaction is affected by the addition of the HCl (aq) due to the addition of the ion you just identified. Was this identified ion a reactant or product in the equilibrium reaction? This ion was a reactant in the equilibrium reaction d. What would the addition of HCl (aq) to the equilibrium do to the value of Q c ? How would it compare to the value of K c ? The addition of HCl and the dissociation of the ions which followed increased the Chlorine concentration which was a reactant in the original equation which means that if the reactants increased then the Q c value would decrease to eventually be less than that of K c e. Did the reaction shift towards the reactants or products in order to re-establish equilibrium? Did the observed color change from the video support this answer? The reaction shifted towards the products and the observed color changed to blue9products color) which supports this answer. 4. CHANGING TEMPERATURE: Watch the video from the 0:44 mark to the 1:00 mark. a. What color is formed in the solution as the temperature increases? The color formed was blue b. Circle your choices : Increasing the temperature causes this reaction to experience a shift toward the ( reactants or products ), and the value of the equilibrium constant K has ( decreased or increased ). c. What color is formed in the solution as the temperature decreases? Pink d. Circle your choices : Decreasing the temperature causes this reaction to experience a shift toward the ( reactants or products ), and the value of the equilibrium constant K has ( decreased or increased ). e. Based on the information presented, should the forward chemical reaction be classified as endothermic or exothermic? Justify your answer. The forward chemical reaction should be classified as endothermic because when heat was added to the solution it shifted towards the products(color change to blue) which means the increase in heat affected the reactants side of the equation which made the reaction shift away from where the heat was added 5. ADDING NaCl: Watch the video from the 1:10 mark to the 1:35 mark, and answer the statements below. a. When NaCl is added to a solution, it will dissociate into ions. Write the dissociation equation for NaCl below. NaCl->Na + +Cl -- b. Which ion did you notice was present in both the NaCl (aq) dissociation, as well as in the reference equilibrium reaction? Cl - c. The reference equilibrium reaction is affected by the addition of the NaCl (aq) due to the addition of the ion you just identified. Was this identified ion a reactant or product in the equilibrium reaction? The ion was a reactant d. What would the addition of NaCl (aq) to the equilibrium do to the value of Q c ? How would it compare to the value of K c ? The addition of NaCl would decrease the Q c value to decrease making Q c less than K c

e. Did the reaction shift towards the reactants or products in order to re-establish equilibrium? Did the observed color change from the video support this answer? ( Hint: Be sure to compare it to the control to see the color change.) The reaction would shift towards to products and this was supported by the video because of how it began to take on a dark shade of purple towards blue and away from the pale red/pink which was the hue of the reactants f. The color change observed here was not as dramatic as the color change observed when concentrated HCl was added, even though the same ion was introduced in both scenarios. What could account for the less dramatic color change in this situation? This could be a result of a smaller amount of NaCl being added in comparison to the amount of HCl which would make a less amount of chlorine ions being added to the reactants EQUILIBRIUM SYSTEM #2 : The Equilibrium of Iron (III) Thiocyanate At Equilibrium: YELLOW Fe 3+ (aq) + SCN  (aq) ↔ FeSCN 2+ (aq) COLORLESS ORANGE For this equilibrium system, you will be addressing similar changes as seen in the previous equilibrium system. However, this time, you will have to describe the changes, and explain how equilibrium would shift to relieve the stress! Just like you did in the previous questions, make sure to address Q c vs. K c in your explanations. 6. Write the equilibrium-constant expression (K c ) for the forward reaction shown above. K c =[FeSCN 2+ ]/[Fe 3+ ][SCN - ] At Equilibrium: YELLOW Fe 3+ (aq) + SCN  (aq) ↔ FeSCN 2+ (aq) COLORLESS ORANGE 7. ADDING IRON (III) NITRATE: When Iron (III) Nitrate, Fe(NO 3 ) 3 , is added to the solution, it will fully dissociate into ions. How will the system respond to the stress to re-establish equilibrium? What color would be seen in the solution once equilibrium is re-established? Since the reactant was added the Q c value would be less than the K c value and to equate them the reaction would have to shift to the products and since the products were orange the solution would begin to turn orange as well. 8. ADDING POTASSIUM THIOCYANATE: When Potassium Thiocyanate, KSCN, is added to the solution, it will fully dissociate into ions. How will the system respond to the stress to re-establish equilibrium? What color would be seen in the solution once equilibrium is re-established? In this scenario since the reactant was added, the Q c value would be less than the K c value which would mean a reaction shift to the products and since the reactants are more orange the solutions would begin to become orange as well. 9. ADDING SODIUM SULFITE: When Sodium Sulfite is added to the solution, it will change the Fe 3+ into Fe 2+ , effectively removing Fe 3+ out of the equilibrium system. How will the system respond to the stress to re-establish equilibrium? What color would be seen in the solution once equilibrium is re-established? In this scenario since the reactant was removed, the Q c value would be greater than the K c value which would mean a reaction shift to the reactants and since the reactants are colorless the solutions would begin to become colorless as well.

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10. ADDING SILVER NITRATE: When Silver Nitrate is added to the solution, it will form a precipitate with SCN  , effectively removing SCN  out of the equilibrium system. How will the system respond to the stress to re- establish equilibrium? What color would be seen in the solution once equilibrium is re-established? Since the reactant was removed the Q c value was greater than that of the K c value and in order to get them equal the reaction would shift towards the reactants which are colorless which means the solution is getting colorless as well 11. Watch the video at the following link: https://youtu.be/T97YyD72smk . How did the color changes you predicted in the above three questions compare to the results shown in the video? When making the comparison between my answers and the results my answers were correct in the the Potassium Thiocyanate and iron nitrate made it a darker orange whikle the sodium sulfate and the silver nitrate made the solution colorless EQUILIBRIUM SYSTEM #3 : The Equilibrium of NO 2 and N 2 O 4 2 NO 2 (g) ↔ N 2 O 4 (g) RED-BROWN COLORLESS 12. Write the equilibrium-pressure expression (K p ) for the forward reaction shown above. K c =[N 2 O 4 ]/[NO 2 ] 2 13. When an equilibrium mixture of NO 2 (g) and N 2 O 4 (g) at 298 K is placed in a beaker of water at 350 K, the color of the gas mixture changes from a light brown color to a darker shade of brown. a. If the color changed to a darker brown, does this indicate a shift to reactants or products? This indicates a shift to the reactants b. At the higher temperature of 350 K, would the value of K c be larger or smaller than it is at the lower temperature of 298 K? Justify your answer. If heat is added to the products side of the reaction then the K c value would decrease and be smaller than it is at the lower temperature because when the temperature increases the reaction shifts towards the reactants indicating that it is exothermic(decrease in K if temp increases) c. Is this reaction endothermic or exothermic? Justify your answer. The reaction is exothermic since heat is released and also because the K value decreases when the temperature increases since these are two things that correlate with heat being added to an exothermic reaction. d. If the temperature was decreased below 298 K, how would equilibrium shift in response to the stress? Justify your answer. Since heat is a product of this reaction and the product is being removed, the reaction would shift towards the products side to accommodate for that and the K value will increase. e. Watch the video at the following link: https://youtu.be/ScWBj0hqOLE . How did your answers compare to the results shown in the video? In comparison to the results shown in the video, my answers were accurate because the reaction in fact was exothermic which was my main observation and the reaction in fact did shift towards th reactants when in hot water and towards the products in water which was cold 14. When the above reaction is at equilibrium, the volume of the container was increased. a. When the volume of the container holding the reaction was increased, would the pressure of each of the individual gases in the system increase or decrease? Justify your answer. The pressure would increase for both of the individual gases in the system because increasing the volume makes there have to be less interaction between each of the gases as well as the wall which would reduce the overall pressure by giving them more space to move around.

b. Would this cause more of a change in the overall pressure of reactants or of products, with regards to Q p ? Justify your answer. Increasing the overall volume of the container would shift the reaction to the reactants side because it has more moles than the products side so while they both do decrease the pressure of the reactants changes more dramatically because of their 2 coefficients (decreases by “double”) c. What would the increase of the volume of the container holding the reaction do to the value of Q p ? How would it compare to the value of K p ? The denominator of Q p decreases twice as much as the numerator which makes the value of Q p go up since there isn’t ‘as much’ of the numerator in comparison to the denominator as there was before and as a result in comparison to K p the Q p would be larger. d. Would the reaction shift towards the reactants or the products in order to re-establish equilibrium? Was this the side of the equation with more moles or less moles of gas? The reaction would shift to the reactants to re-establish equilibrium. This side of the equation has more moles of gas. e. When the volume of the container holding the reaction was increased, would the color of the gas mixture become a lighter brown color or a darker brown color? The color of the gas mixture would turn a darker brown color when the volume of the container which contained the reaction was increased.

Le Chatelier's Principle Virtual Lab (3)

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