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Experiment 5
Le Châtelier’s Principle
Pre-lab Assignment
Before coming to lab:
- Read the lab thoroughly.
- Answer the pre-lab questions that appear at the end of this lab exercise. The questions should be answered on a separate (new) page of your lab notebook. Be sure to show all work, round answers, and include units on all answers. Background information can be found in Chapter 15, especially sections 15.6 your textbook (Brown and LeMay).
- Follow the guidelines in the "Lab Notebook Policy and Format for Lab Reports" section of the lab manual to complete in your lab notebook the following sections of the report for this lab exercise: Title, Lab Purpose, Procedure and Data Section. For this lab, there will be no data table since your will not be recording any measurements. You will need space to record your observations however.
Purpose
In this experiment you will observe shifts in four equilibrium systems, and learn to explain the observed changes in terms of molecular/ionic interactions and Le Châtlier’s Principle.
Background
Chemical Equilibrium All chemical reactions eventually reach a state in which the rate of the reaction in the forward direction is equal to the rate of the reaction in the reverse direction. When a reaction reaches this state, it is said to be at chemical equilibrium. The concentrations of reactants and products will remain constant. For the generic reaction equation below aA(aq) + bB(s) à cC(aq )+ dD(l) Equation (1) We can express the equilibrium-constant expression for this equation as, Kc = [C]c Equation (2) [A]a where the values of [A] and [C] are the concentration of A and C at equilibrium in molarity and a and c are their respective stoichiometric coefficients. What about B and D in the reaction? Only aqueous species and gases appear in the equilibrium expressions. Pure liquids and solids do not appear in the equilibrium-constant expression; therefore B and D do not appear in the equilibrium constant expression in this example.
Le Châtelier’s Principle It has been observed that when a reaction at equilibrium is disturbed by changing either the concentration of one of the chemical components, the total pressure, or the temperature, the reaction will respond by shifting its equilibrium position so as to counteract the effect of the disturbance. This idea was first proposed by Henri-Louis Le Châtelier and has since been referred to as, “Le Châtelier’s principle”. When the reaction makes more products as a response to the disturbance, we call it a right-shift. When the reaction makes more reactants in response to the disturbance, we call it a left- shift. These shifts often produce observable results. For example, the color of the solution may change or a precipitate may form or dissolve. To understand how changes in concentration might shift a chemical reaction at equilibrium consider the following generic equation: A+ B C+ D How might the equilibrium be shifted in the direction of the products? Consider these three possibilities:
- Add More Reactants to the System. As additional reactants are added (the stress), the reaction system attempts to remove them by shifting to the right (relief of the stress), forming more products. This occurs because the addition of reactants increases the reactants concentration which increases the frequency of collisions between reactant particles- thus increasing the rate of the forward reaction. As the rate of the forward reaction increases, it forms more products. Eventually the rate of the forward and reverse reaction will again be equal in rate and a new equilibrium will be reached.
- Remove Products from the System. This is less intuitive. Removal of a product (the stress) results in a lower concentration of products and therefore fewer collisions between product particles—hence the reverse reaction decreases in rate relative to the forward reaction. Figure 1 Note that at equilibrium, the concentrations of reactants and are products are not equal; however they are not changing although the reaction is continuing in both the forward and reverse direction. Figure 2 LeChatelier's Principle can be illustrated by this teeter totter analogy. a) Two equal weight children are balanced (equilibrium). b) Some weight is added to the boy at the right, which causes the teeter totter to lower at the right end. c) If some of the weight is moved to the left, then the teeter totter is balance again (equilibrium). This shift to the left results in products returning back to reactant to achieve equilibrium again.
Part A Formation of the Fe(SCN)2+^ Complex Ion In this part of the experiment, ferric ion, Fe3+,^ reacts with thiocyanate ion, SCN–^ , to form the deep red, complex ion, FeSCN^2 +^. The intensity of the red color will tell you if [FeSCN2+] changes. You will make three changes to this equilibrium system:
- Adding solid potassium thiocyanate (KSCN). KSCN is a soluble solid which will dissociate in water to form K+^ ions and SCN-^ ions
- Adding silver nitrate (AgNO 3 ) which removes SCN-^ ions from the equilibrium system by forming an insoluble white precipitate.
- Heating and cooling the system. Prepare the equilibrium reaction system
- Add approximately 50 mL of 0.001 M potassium thiocyanate (KSCN) solution to a beaker or flask. Add about 10 drops of 0.1 0 M ferric nitrate [Fe(NO 3 ) 3 ] solution, and stir. This should give a red color which is intense enough to see but not so intense that changes in the color cannot be noticed.
- Pour approximately 1/5 of the solution into each of five large test tubes. Test tube #1 will be your control. You will not be adding anything else to it.
- Effect of adding KSCN 3a. Write the equilibrium chemical reaction you are studying in your notebook. 3 b. In a moment, you will be adding KSCN to your reaction system. Adding KSCN should affect the concentration of which species in the equilibrium reaction you are studying? 3c. Should this speed up the forward reaction or reverse reaction? 3d. According to Le Chatelier’s Principle, predict which way should the reaction shift (left or right). Now to test your prediction, to test tube #2 add a few crystals of solid potassium thiocyanate (KSCN).
Fe
3+
+ SCN
FeSCN
2+ Ferric ion thiocyanate ion complex (light yellow) (colorless) (deep red)
3e. What do you observe happening? Record any color changes or other changes you observe. 3f. Do your observations support your prediction based on Le Chatelier’s Principle? Explain clearly using your color observations as evidence.
4. Effect of adding AgNO 3 4a. In a moment, you will be adding AgNO 3 to your reaction system. Adding AgNO 3 should affect the concentration of which species in the equilibrium reaction you are studying? Would it increase or decrease the concentration? 4b. According to Le Chatelier’s Principle, predict which way should the reaction shift? Now to test your prediction, to test tube #3 add several drops of 0.1 M silver nitrate solution (AgNO 3 ). 4 c. What do you observe happening? 4d. Do your observations support your prediction based on Le Chatelier’s Principle? Explain clearly using your color observations as evidence. 5. Effect of temperature change To see the effect of temperature changes (heat), place test tube #4 in a 250 mL beaker filled with ice and water. Place test tube #5 in a 150 mL beaker half filled with water, and then heat the water to near boiling with a Bunsen burner while supporting the beaker on a ring stand. 5 a. What do you observe happening to the solution in ice water? To the solution in hot water? 5 b. Based on the color change you observe, is the equilibrium for the solution in the ice water shifting left or right? 5 c. Based on the color change you observe, is the equilibrium for the solution in the hot water shifting left or right? 5 d. Do your observations indicate that the reaction is exothermic or endothermic? That is, based on your observations, is heat a product or a reactant? 5e. Write the net ionic equation for the reaction you are studying including heat in the equation.
- To see if equilibrium shifts are reversible, move test tube #4 from the ice water to the hot water and test tube #5 from the hot water to the ice water. 6a. What do you observe? 6b. Are equilibrium shifts reversible?
- Dispose of all solutions in the proper container, not down the sink. Note to instructors: This procedure has been modified from the Miracosta, Chem 30 lab manual,
- To the same test tube, add deionized water drop-wise until a color change is observed. 2 a. How does the equilibrium react when water is added? (shift left, right or no effect) 2 b. Is the water just diluting the solution or is it reacting chemically with the solute? How do you know? What is the evidence? Effect of temperature change
- Use a hot plate to heat a 150 mL beaker of water to near boiling.
- Half fill a new, large test tube with a new portion of the purple Co(H 2 O) 6 2+^ solution and add 12 M HCl drop-wise until the color is between pink and blue. Gently heat the test tube by placing it in the boiling water bath. 4 a. What color change do you see?
- Half fill a new, large test tube with a new portion of the purple Co(H 2 O) 6 2+^ solution and add 12 M HCl drop-wise until the color is between pink and blue. Cool the test tube by placing it a 250mL beaker filled with ice and water. 5 a. What color change do you see? 5 b. Is the equilibrium reaction endothermic or exothermic? 5 c. Write the net ionic equation for the reaction you are studying including heat in the equation. Part C. Acid-Base Equilibrium In the next several parts of the experiment you will make use of coupled equilibria to change the equilibrium position of reactions. This is sometime known as the common ion effect. Let’s see how such coupled equilibria work. Suppose we have the two reactions described by the chemical equations below: A(aq) B(aq) Equation (3) B(aq) +C(aq) D(aq) Equation (4) Notice that the species B(aq) is common to both reaction equations. The presence of a common species couples these two reactions. We can disturb the equilibrium position of the reaction described by Equation (3) by the addition of some C(aq). The addition of C(aq) to this system will cause the equilibrium position of the reaction described by Equation (4) to shift right, in accordance with Le Châtelier’s principle. This right shift in the equilibrium position of Equation (4) will result a corresponding decrease in the
concentration of B(aq). Because B(aq) is also present in Equation (3), the decrease in the concentration of B(aq) will in turn result in a right shift in the equilibrium position of the reaction described by Equation (3). Thus, the addition of C(aq) to the reaction described by Equation (4) results in a right shift in the equilibrium position of the reaction described by Equation (3) because these two equilibria are coupled. Many of the reactions that we observe in this experiment will also involve the use of coupled equilibria especially involving the reaction of acids and bases to form water. The net ionic equation for this reaction is shown below:
H+^ (aq) + OH-^ (aq) H 2 O Equation (5)
(from an acid) (from a base) Equilibrium of Indicators The indicator we use most often in lab is phenolphthalein which is a weak acid. As a weak acid, it loses H+^ and turns a different color in the process. It should be noted that all acid-base indicators work on this principle. We can represent this process below where HIn is a shorthand for the formula of an indicator .
- Prepare a phenolphthalein solution by adding one drop of phenolphthalein into 5 mL of water in a test tube and mixing. Effect of adding NaOH 2a. The concentration of which species in Equation 5 would be affected by adding NaOH? Would it increase or decrease in concentration? 2 b. According to Le Chatelier’s Principle, predict which way should the reaction in Equation (5) shift? 2c. The shift you predict for Equation 5 will affect the equilibrium of Equation 6. Using your knowledge of the common ion effect, which direction will the equilibrium of Equation 6 shift as NaOH is added? Now to test you prediction, add 6 M NaOH until there is a color change 2d. What color change did you observe? 2e. Do your observations support your prediction based on Le Chatelier’s Principle? Explain clearly using your color observations as evidence. 2f. For phenolphthalein, what is the main species present in basic solution? HIn or In-? 2g. In contrast, what is the main species present in an acidic solution?
HIn (aq) H
(aq) + In
(aq) Equation (6)
colorless pink
To see the effect of temperature changes (heat), place test tube #3 in a 250 mL beaker filled with ice and water. Place test tube #4 in a 150 mL beaker half filled with water, and then heat the water to near boiling while supporting the beaker on a ring stand. 6 a. What is the initial color and appearance of the mixtures in Test tubes 3 and 4. If a precipitate is present, note that fact and the color of the precipitate. 6 b. What color change do you observe happing to the solution in hot water? 6 c. What color change do you observe happing to the solution in ice water? 6 d. Based on the color change you observe, is the reaction exothermic or endothermic? 6 e. Write the net ionic equation for the reaction you are studying including heat in the equation.
Post-Lab Questions
- Experience teaches us that most solids are more soluble in warm water than in cold water. Does the solubility of Mg(OH) 2 fit this pattern?
- Considering Part B of the lab, when the test tube was placed in hot water, did the value of Kf get bigger, smaller or remain constant?
- Dinitrogen tetraoxide (colorless gas) is converted to nitrogen dioxide (dark reddish brown gas) and is endothermic in the forward direction: a. Write a balanced equation for this reaction. b. If a closed container containing a mixture of these two gases at equilibrium is a light brown in color when it is at room temperature (20.0°C), what change in color would you expect to observe if the container is placed in boiling water (100.0 °C)? c. If a closed container containing a mixture of these two gases at equilibrium is a light brown in color when it is at room temperature (20.0°C), what change in color would you expect to observe if the container is placed in ice water (0.0 °C)?
- Hydrogen gas (colorless) reacts with pure iodine vapor (purple) to give hydrogen iodide gas (colorless): a. Write a balanced equation for this reaction. b. If the equilibrium mixture of these three gases is light purple in color, what change in color would you expect to observe if more hydrogen gas were added to the system? c. If the equilibrium mixture of these three gases is light purple in color, what change in color would you expect to observe if some of the hydrogen iodide gas were removed from the system?
- Consider the following equilibrium heat + CaCO3(s) à CaO(s) + CO2(g) a. If the pressure of CO2(g) is increases, in which direction will the equilibrium shift? b. If the temperature is increased, in which direction will the equilibrium shift? c. If the temperature is decreased, in which direction will the equilibrium shift?
