Experiment #8. LeChatelier s Principle

Transcription

1 Experiment #8. LeChatelier s Principle Aim: To study the effect of concentration and temperature on chemical equilibrium When the rate of the forward reaction becomes equal to the rate of the reverse reaction, we say that the reaction is at equilibrium. We can disturb a reaction at equilibrium by changing the concentration, pressure/ volume or temperature. According to LeChatelier s Principle, when a system at equilibrium is disturbed by a change in temperature, pressure or concentration, the system s equilibrium will shift so as to counteract the effect of the disturbance. Consider the general equilibrium a A + b B c C + d D (1) If we add a reactant, the equilibrium will shift towards the product side. This is because a portion of the added reactant will be used up to form more products, until the equilibrium is re- established. If we remove a product, then to compensate for the loss of the products, more product will be formed from the reactants and the equilibrium will shift towards the products. If we remove a reactant, then to compensate for the loss of the reactants, the product will decompose to form reactants and therefore the equilibrium will shift towards the reactants. If we add a product, then the equilibrium will shift to the reactants in order to consume the excess product. We can also change the concentration of the reactants/products by carrying out a side reaction. For example, to the equilibrium reaction in equation 1, we could add some compound F so that F reacts with C c C + f F x X () Due to reaction, the amount of C will decrease (because C is being used up in reaction ) which means that the equilibrium in reaction 1 will shift towards the product side to form more C. Pressure changes can affect an equilibrium involving gases. If the pressure on the system increases (or the volume decreases), then the equilibrium will shift towards the side with fewer total gas moles to decrease the pressure. The opposite effect is observed when the pressure is decreased (or volume is increased). The system shifts in the direction of the side with more gas moles to increase the pressure. Pressure can also be changed by adding an inert gas (gas not participating in the equilibrium). However, this does not affect the equilibrium. If the pressure is increased/decreased by adding/removing one of the gases participating in the equilibrium, then the effect will be the same as adding /removing reactants/products. Temperature also affects an equilibrium. An endothermic process will be favored (shifting towards the formation of products) by an increase in temperature. An exothermic process will be favored (shifting towards the formation of products) by a decrease in temperature. Concentration, Pressure and Volume changes shift the equilibrium but do not change the value of the equilibrium constant. Temperature changes, by contrast, not only shift the equilibrium but also change the value of the equilibrium constant

2 Equipment 1 small test tube 1 large test tube Chemicals 0.10 M CuSO 4 (aq) 0.10 M CaCl (aq) Concentrated NH 3 (aq) (ammonia solution) 0.10 M Na CO 3 (aq) 1 M HCl (aq) (hydrochloric acid) 6 M HCl (aq) (hydrochloric acid) 6 M NaOH (aq) (sodium hydroxide) Procedure Part I: Change in concentration 1. Place 1 ml (about 10 drops) of 0.1 M CuSO 4 in a test tube and note its initial color. To this, add 15 M NH 3 (concentrated NH 3 ) dropwise until the solution changes color and is not cloudy. Note the final color of the solution.. To the solution from step 1, add 1 M HCl dropwise until there is a color change. Note down the color change. DISPOSAL: Dispose the contents of the test tube from Part I in the appropriately labeled waste container. Part II Sparingly soluble salt 1. Take 10 drops 0.1 M Na CO 3 in a big test tube and add to it 10 drops of 0.1 M CaCl. Note down your observations in the data sheet.. Add 6 M HCl dropwise to the solution from step 1 until a change is observed. Note down your observations. 3. To this solution (in step ), add 6 M NaOH solution dropwise till you see a change. Note your observations in the data sheet. 4. Add 6 M HCl dropwise to the solution from step 3 till a change is observed. Note down your observations. DISPOSAL: The contents of the test tube from Part II can be disposed of in the sink. Part III Temperature change 1. Collect a sealed test tube containing cobalt ions as mentioned by your instructor.. Note down the color of the solution in the test tube. 3. Now place the test tube in a beaker containing boiling water. Continue keeping the test tube in the hot water until you observe a color change. Note down this new color of the solution. 4. Remove the test tube from the hot water, cool it for a minute or so, and now place the test tube in a beaker containing ice water. Continue keeping the test tube in the cold water until you observe a color change. Note down this new color of the solution. 5. Return the test tube to its original location.

3 Name: CHM11 Lab LeChatelier s Principle Grading Rubric Criteria Points possible Points earned Lab Performance Printed lab handout and rubric was brought to lab 3 Safety and proper waste disposal procedures observed Followed procedure correctly without depending too much on instructor or lab partner 3 Work space and glassware was cleaned up 1 Lab Report Part I: Observations correctly recorded 1 Part I: Analysis (1 pt each question), (shift direction correctly identified, reason for shift clearly explained) Part II: Observations correctly recorded 1 Part II: Reactions (balanced with correct formulas, phases, and charges) Part II: Analysis (1 pt each question), (shift direction correctly identified, reason for shift clearly explained) Part III: Observations correctly recorded 1 Part III: Analysis (1 pt each question), (shift direction and endo/exo correctly identified, reason for shift clearly explained) Total 0 Subject to additional penalties at the discretion of the instructor.

4

5 LeChatelier s Principle: Observations and Discussion Name Part I: Change in concentration Procedure Observation Initial color of the CuSO 4 solution Color of the solution after NH 3 is added Color of the solution after adding HCl When CuSO 4 is mixed with NH 3, the following equilibrium is established (3) Cu + (aq) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] + (aq) [Cu(NH 3 ) 4 ] + (aq) is dark blue in color a) Which direction did the equilibrium shift when NH 3 was added to CuSO 4? Describe the type of stress this applied to the equilibrium in equation (3) and how the observed shift relieves that stress. Be specific in your answer. When HCl is added it reacts according the following equation: (4) H + (aq) + NH 3 (aq) à NH 4 + (aq) b) Which direction did the equilibrium in equation (3) shift when HCl was added? Describe the type of stress this applied to the equilibrium in equation (3) and how the observed shift relieves that stress. Be specific in your answer.

6 Part II: Sparingly Soluble Salt Step Procedure Observation 1 Mixing 0.1 M Na CO 3 and 0.1 M CaCl a) Write the balanced chemical reaction taking place when Na CO 3 is mixed with CaCl (this is a double replacement reaction). Include all phases and write correct product formulas. b) Write the net ionic equation taking place for the reaction in a). (This is your equilibrium reaction) Step Procedure Observation Adding 6 M HCl The added HCl reacts according to the following equations: (5) H + (aq) + CO 3 - H CO 3 (aq) H O (l) + CO (g) c) Which direction did the equilibrium in b) shift when HCl was added? Describe the type of stress this applied to the equilibrium you wrote for b) and how the observed shift relieves that stress. Be specific in your answer.

7 Step Procedure Observation 3 Adding 6M NaOH 4 Adding 6 M HCl d) Explain your observations for steps 3 & 4 in terms of LeChatelier s principle and your equilibrium from b). Part III: Change in Temperature Initial color of solution in the sealed test tube Color of the solution on heating in hot water bath Color of the solution on cooling in ice water The equilibrium reaction taking place in the sealed test tube is (6) [Co(H O) 6 ] + (aq) + 4 Cl - (aq) [CoCl 4 ] - (aq) + 6 H O (l) [Co(H O) 6 ] + (aq) is pink in color [CoCl 4 ] - (aq) is blue in color a) Which direction did the equilibrium in (6) shift when added to hot water? Which direction did the equilibrium in (6) shift when added to cold water? b) Based on your observations, is reaction (6) endothermic or exothermic? (hint try writing in heat as a product or reactant and see which fits) Describe how the observed shift in a) relieves the stress (heat) applied to equilibrium (6).