Please refer to the questions on the attached file.

2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 1 EXPERIMENT CHEMICAL EQUILIBRIUM F5 AIMS OF THE EXPERIMENT • Application of Le Châtelier’s Principle in understanding and explaining the effects of changes in concentration and temperature on the position of an equilibrium reaction for a chemical system . • To determine the equilibrium constant for the formation of iron(III) thiocyanate complex ion using spectrophotometric technique. READING • Chemistry Human Activity, Chemical Reactivity , Mahaffy, Bucat, Tasker, Kotz, Treichel, Weaver and McMurry 2 nd ed. 2015: The reaction quotient and the equilibrium constant: Section 13.3, pages 494 – 497. Quantitative aspects of equilibrium constants: Section 13.4, pages 501 – 503. Disturbing reaction mixtures at equilibrium: Section 13.6, pages 511 – 514. • ChemCAL PreLab module INTRODUCTION Equilibrium and the Equilibrium Constant (K c) Chemical reaction s, in practice, do not go to completion, but they usually approach an equilibrium state. Using the collision theory model, as reactant molecules collide to form products and the product concentrations increase, collisions between product molecules can reform the reactant molecules. When the rate of the forward reaction equals the rate of the reverse reaction, the system is said to have r eached equilibrium. At equilibrium, the concentrations of the reactants and products remain constant . This does not mean that the reaction has stopped. In fact, the reaction continues to progress forward, as well as in reverse . It is said to be in a “dynamic state of equilibrium” , where ‘dynamic’ points to the fact that reaction has not stopped. A s the r ate of the forward reaction equals the rate of the reverse reaction, it means that as quickly a s we consume reactant molecules , they are reformed by the reverse reaction. Hence their concentration s no longer change. For the general reaction: aA + bB ⥫⥬ cC + dD this state of equilibrium is described by the equilibrium constant: Kc = [C]c[D]d [A]a[B]b For a given temperature, a constant value for the above ratio of concentrations will be reached at equilibrium . The equilibrium constant, Kc measures the extent of a reaction for an equilibrium system at a given temperature. The value may be determined from experimental data if the equilibrium concentrations of the reactants ([A] and [B]) and the products ([C] and [D]) are known. 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 2 Le Chatelier's Principle and Chemical Equilibrium Shifts A reaction system at equilibrium will remain at equilibrium unless disturbed by a change to the system . These changes may include: • changing the concentration of one of the components of the system • changing the pressure or volume • changing the temperature at which the reaction is run. The system will achieve equilibrium again, where the value of K will be equal to the previous ratio (unless there is a change in temperature), however the actual concentrations of A, B, C and D will be different. This is called a new equilibrium position. Le Chatelier's principle describes how the system will shift to establish a new equilibrium position, where the ra tes of the forward and reverse reactions are equal again. According to Le Chatelier’s Principle, in response to a disturbance to the equilibrium, the system will oppose the change in order to achieve equilibrium again. For instance, addition of more rea ctant A will disturb the equilibrium. The system will move to oppose this change by reducing the concentration of A. This can only happen by favouring more forward reaction to consume A, until the equilibrium condition is re -established. Example: Re-establishing equilibrium when concentration is changed for cases i) and ii) : i) ADDING A REACTANT Reaction shifts towards products – forward reaction favoured A + B ⥫⥬ C + D ii) ADDING A PRODUCT Reaction shifts towards reactants – reverse reaction is favoured Colour and the Absorbance of Light The colour of a substance arises when white light strikes a substance (or solution of a substance) and some wavelengths of light are absorbed. Those energies, or wav elengths, promote electrons in the species to higher energy levels. The remaining wavelengths of light are reflected and transmitted through the solution , and are recognised as the colour of the so lution . For instance , if the wavelengths of light absorb ed are at the blue e nd of the visible spectrum, the solution will appear red to the eye. Spectrophotometry and the Spectrophotometer Spectrophotometry is the study of how different chemical species absorb light, looking at which wavelengths have been absorbed. T he amount of light absorbed by a solution can be related to the amount of the species present in solution . This technique is u sed extensively in the analysis of nutrient and contaminant levels, for example to determine nitrate and phosphate levels in water. A spectrophotometer is an instrument designed to expose a sample to wavelengths of light in the visible and UV region of the electromagnetic spectrum. An analysis of the amount of substance present exploit s the Beer -Lambert Law (F5.1 ) which relates the absorbance A ( how much light is absorbed at a given wavelength) to the concentration (c) of the absorbing species. One form of this law is: A = ε c l (F 5.1 ) where ε is the molar absorptivity (cm -1.mol -1.L), c is the concentration (mol.L -1) and l is the distance the light travels through the solution (cm) (usually the width of the sample container which is called a cuvette) . Both ε and l are constant s for a given pure substance and spectrophotometer (and cuvette) at a particular wavelength . A calibration graph of A against c (that is a plot of A vs c) should give a straight line, with gradient = ε.l . 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 3 FOLLOW THE EXPERIMENT ON THE FILM (Full Experimental Procedure p 7) Data Tables and Questions Use the data recorded in the Results Sheet for the calculations. Answer all questions in the Results and Discussion section of your report and show working for your calculations. Part A: Chemical Equilibrium Shifts for the Iron(III) Thiocyanate (FeSCN) 2+ Complex Ion System Iron(III) (Fe 3+) ions react with thiocyanate (SCN –) ions to form iron(III) thiocyanate [Fe(SCN)] 2+ complexes. It is an exothermic reaction and the ionic equation for this equilibrium re action is given in equation F5.2 . Fe 3+(aq) + SCN –(aq) ⥫⥬ [Fe(SCN)] 2+(aq)  H = -ve (F 5.2) 1. Preparation of iron(III) thiocyanate complex ion Record the colour of iron(III) nitrate solution in Results Sheet Record the colour of potassium thiocyanate solution in Results Sheet. Record the colour of the mixture in Results Sheet. 2. Test tube reactions Record your observations of each test tube reaction in Results Sheet. Is there any colour change? Is there any precipitation? Answer the following questions on the Results Sheet in the table provided : Question 1 Using the colours in the table above, for each test tube reaction 2 - 9, after disturbance, identify which reaction is favoured, forward or reverse. Provide an explanation (see example in table ). (Tips: Fe(OH) 3, AgSCN are insoluble in water, NH 3 is good base - what reaction doe s it undergo with water?) Question 2 Write an ionic equation for any precipitation reaction you have observed. Part B: Determination of the Equilibrium Constant for Iron(III) Thiocyanate Complex Ion Step (1 ) Standard solution An ICE table can be constructed to show the relation between the initial concentrations and equilibrium concentrations of all species in this reaction, show ing the amounts used for Part B: Fe 3+(aq) + SCN –(aq) ⥫⥫ [Fe(SCN)] 2+(aq) 0.200M 0.00 02M large excess limiting reactant Initial 0.200M 0.0002M 0 Change -x -x +x Equilibrium 0.200 -x 0.0002 -x +x For x= 0.002M 0.200 -0.0002 0 0.0002 As the [Fe 3+]initial concentration is in large excess (100 fold) , according Le Chatelier’s Principle, the equilibrium will shift to the product side until all the SCN – is converted to [Fe(SCN)] 2+. Thus the equilibrium concentration [Fe(SCN)] 2+ in the standard solutio n (now called [Fe(SCN)] 2+eq. standard ) is approximately equal to the initial concentration of SCN – (i.e. [SCN –]initial ) in the solution. [Fe(SCN)] 2+eq. standard = [ SCN –]initial (F 5.3) Calculations: a) The absorbance of the standard solution (Astandard ) at a wavelength of 460 nm has been recorded in the Results Sheet. 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 4 b) Calculate and record in Results Sheet (show your working): the equilibrium concentration of iron(III) thiocyanate complex ion, [Fe (SCN )]2+eq. standard , in the 20 mL standard solution, using equation F 5.3 as a guide. (NOTE : consider the dilution factor of 0.002 M KSCN in making the standard solution). Step (2) Finding the concentration of [Fe(SCN)] 2+eq. sol ution n using Absorbance measurements. Four different mixtures of Fe( NO 3)3 and KSCN are prepared according to the table below: Table F5.1 Reaction mixture in each flask . The iron(III) thiocyanate complex absorbs light at 460nm. The Beer -Lambert Law given in equation F5.1 for the absorption of light by [Fe(SCN)] 2+ at 460nm (using the solution in PartB( 1)) may be written as: Absorbance (A standard ) = ε x l x c = ε x l x [Fe(SCN)] 2+eq. standard (F 5.4 ) showing a linear relationship between A and [Fe(SCN)] 2+eq. Therefore , for each solution A to D : [ ( )]+. = . = [ ( )]+. (F 5.5) Hence the equilibrium concentration of iron(III) thiocyanate [Fe (SCN )]2+eq for each sol ution, n, can be determined by measuring the absorbance (A) of these solutions at equilibrium and using the relationship g iven in equation F 5.5 : i.e. For so lution n, the equilibrium concentration [ ( )]+. = [ ( )]+. × (F 5.6) using the relationship from Part B (a) above: [Fe (SCN )]2+eq. standard = [ SCN –]initial = 0.0002M (F 5.3) Absorbance readings : The absorbance readings for each reaction solution (A to D) are provided in the Results Sheet. Calculations : Using the concentration [Fe(SCN)] 2+eq. standard and absorbance (A standard ) of standard solution from PartB(1 ), calculate the iron(III) thiocyanate concentration ([FeSCN 2+]eq solution n ) at equilibrium in each reaction flask (equation F 5.6). Conical Flask Fe(NO 3)3 (mL) KSCN (mL) Distilled Water (mL) Total Volume (mL) A 5.0 2.0 3.0 10.0 B 5.0 3.0 2.0 10.0 C 5.0 4.0 1.0 10.0 D 5.0 5.0 0.0 10.0 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 5 Step (3) Calculating the va lue of K for each of the four so lutions , A to D For each so lution, the equilibrium can be expressed in the following ICE table: Fe 3+(aq) + SCN –(aq) ⥫⥫ [Fe(SCN)] 2+ (aq) Initial [Fe 3+]initial [SCN –]initial 0 Change -x -x +x Equilibrium [Fe 3+]initial - x [SCN –]initial - x +x =[Fe 3+]eq =[SCN –]eq = [Fe(SCN)] 2+eq. = [ ( )]+. [ +] . [ −] (F 5.7) Write the expression for K for this reaction (F 5.2) into your Results Sheet. Calculations a) Use data on the Results Sheet provided ; show all working for calculations . b) Calculate the concentration of iron(III) ions ([Fe 3+]initial ) initially in each re action flask using equation F5.8. Concentration (solution 1) x Volume (solution 1) = Concentration (solu tion 2) x Volume (solution 2) (F 5.8 ) c) Calculate the concentration of thiocyanate ions ([ SCN –]initial ) initially in each re action flask using equation F5.8. d) Calculate the concentration of Fe 3+ at equilibrium in each reaction flask using equation F5. 9. [Fe 3+]eq = [Fe 3+]initial ̶ [Fe (SCN )]2+eq. sol ution (F5.9) e) Calculate the concentration of SCN – at equilibrium in each reaction flask using equation F5.10 . [SCN –]eq = [ SCN –]initial ̶ [Fe (SCN )]2+eq. solution (F5.10) f) Calculate the equilibrium constant (K c) for each reaction mixture in flask using your expression written in equation F5.7. g) Calculate an average equilibrium constant for the iron(III) thiocyanate complex ion over the four reaction mixtures A to D. Question 3 Why is it necessary to use DRY co nical flasks for the preparation of reaction mixtures? Question 4 Inspect the values for K obtained for reaction mixtures A to D. (i) Does the value of K depend on the initial concentrations of Fe 3+ and SCN – ions? (ii) From the average value of K obtained for this reaction, would you describe it as a product –favoured or a reactant –favoured reaction? 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 6 REPORT AND CALCULATIONS YOUR REPORT Save your report, either scanned or photographed pages, as ONE compressed pdf file (less than 10 MB) in Portrait Format before submission. You can use the scanning apps Scannable (iOS device) or Genius Scan (Android) which will collate your pages. Make sure you clearly show your demonstrator’s name of your practical allocation in your report. The DUE DATE for the report submission will be given on the LMS on release of this assignment and by your demonstrator. You are still permitted to submit your report after the due date but there will be a penalty of ONE MARK for each late day. Your demonstrator will provide you with your summary mark and feedback by the given FEEDBACK DATE. Scientific Report Students can complete their report on the Results Sheet provided. You should submit this scientific report together with the signed Cover Sheet. Your scientific report needs to be CONCISE and should contain the following sections. A guideline to the contents of each section is provided. Aim: 1. 2 to 3 sentences 2. Brief and concise in your own words with references to the aim of experiment given in laboratory manual Experimental Method: In this experiment, it is sufficient to note: “Refer to First Year Chemistry Laboratory Online Assignment, Experiment F5 , 2020.” Results and Discussion: Complete all questions and all calculations in this section, showing your working. Conclusion: 1. 1 to 2 sentences 2. Use past tense and passive voice 3. Conclusion should be a short summary of the main results obtained from the experiment 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 7 EXPERIMENTAL PROCED URE Iron(III) (Fe 3+) ions react with thiocyanate (SCN –) ions to form iron(III) thiocyanate [ Fe (SC N)] 2+ complexes. It is an exothermic reaction and the ionic equation for this equilibrium reaction is given in equation F5.5. Fe 3+(aq) + SCN –(aq) ⥫⥬ [Fe(SCN)] 2+(aq)  H = -ve (F 5.2 ) Part A: Chemical Equilibrium Shifts for the Iron(III) Thiocyanate (FeSCN) 2+ Complex Ion System 1. Preparation of iron(III) thiocyanate complex ion a) With an auto -zippette, deliver 1 mL of the 0.100 M iron(III) nitrate (Fe(NO 3)3) solution to a 50 mL conical flask. • Record the colour of iron(III) nitrate solution in Results Sheet b) With a second auto -zippette add 1 mL of 0.100 M potassium thiocyanate (KSCN) solution to the above iron(III) solution in the conical flask. • Recor d the colour of potassium thiocyanate solution in Results Sheet. • Record the colour of the mixture in Results Sheet. a) Add 30 mL of distilled water using a 100 mL measuring cylinder to the mixture. Mix it thoroughly using a plastic dropping pipette. 2. Test t ube reactions a) Place 0.5 mL of iron(III) thiocyanate complex ion solution (prepared above in Step 1) using a plastic dropping pipette into nine micro test tubes. b) Label the micro test tubes from 1 to 9: i. Test tube 1 will serve as reference for results obtained in followin g steps (ii) to (ix). ii. Add 4 5 drops of 0.200 M Fe(NO 3)3 to test tube 2. iii. Add 4 5 drops of 0.200 M KSCN to test tube 3. iv. Add 4 5 drops of 0.1 M AgNO 3 to test tube 4. v. Add 4 5 drops of 0.2 M KCl to test tube 5. vi. Add 6 7 drops of 1.0 M N H3 to test tube 6. vii. Add 6 7 drops of 2.0 M NaOH to test tube 7. viii. Place test tube 8 in an ice bath for 15 minutes. ix. Place test tube 9 in a boiling water bath for 15 minutes. c) Allow the mixture in each test tube from (ii) to (vii) to react undisturbed for 15 min utes. • Record your observations of each test tube reaction in Results Sheet. Is there any colour change? Is there any precipitation? Record observations into Table and answer the following questions: Question 1 Using the colours in the table above, for each test tube reaction 2 - 9, after disturbance, identify which reaction is favoured, forward or reverse. Provide an explanation (see example in table ). (Tips: Fe(OH) 3, AgSCN are insoluble in water, NH 3 is good base - what reaction does it undergo with water?) Question 2 Write an ionic equation for any precipitation reaction you have observed. Part B: Determination of the Equilibrium Constant for Iron(III) Thiocyanate Complex Ion Absorbance (A) measurements using spectrophotometer a) Your demonstrator will explain the use of the spectrophotometer to make absorbance measurements at a wavelength of 460 nm. b) Always use the reference solution to zero the spectrophotometer at a given wavelength before tak ing any absorbance measurement. 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 8 c) A set of cuvettes, each with an optical path length of 1.00 cm, is provided for measurement. d) Use of spectrophotometer cuvette: i. The cuvettes should only be handled by top edge of the two ribbed sides; fingers must not touch the optically clear sides. ii. Rinse cuvette three times with distilled water from a wash bottle and drain on a tissue. iii. ALWAYS rinse cuvette twice with the solutio n to be measured. iv. Fill the cuvette (only to two -third full) with the solution to be measured, away from the spectrophotometer and over a waste container. v. Carefully wipe the clear sides with a tissue to remove dust or liquid droplets. vi. Ensure there are no ai r bubbles on the inside walls of the cuvette by gently tapping the cuvette on a hard surface . vii. Check all sides are properly clean before inserting the cuvette in the spectrophotometer cuvette holder. Always position the cuvette so the light passes through t he clear sides. e) When the experiment is concluded, rinse with distilled water and drain on a tissue. NOTE : Cuvettes must never be placed in or removed from the cuvette holder when the holder is in the spectrophotometer. Notify your demonstrator if any liqui d is spilt inside the spectrophotometer. (a) Standard solution An ICE table can be constructed to show the relation between the initial concentrations and equilibrium concentrations of all species in this reaction , show ing the amounts used for Part B : Fe 3+(aq) + SCN –(aq) ⥫⥫ [Fe(SCN)] 2+(aq) 0.200M 0.00 02M large excess limiting reactant Initial 0.200 M 0.00 02M 0 Change -x -x +x Equilibrium 0.200 -x 0.00 02-x +x For x= 0.002M 0.200 -0.00 02 0 0.00 02 As the [Fe 3+]initial concentration is in large excess (100 fold) , according Le Chatelier’s Princ iple, the equilibrium will shift to the product side until all the SCN – is converted to [Fe(SCN)] 2+. Thus the equilibrium concentration [Fe(SCN)] 2+ (now called [Fe(SCN)] 2+eq. standard ) in the standard solution is approximately equal to the initial concentration of SCN – (i.e. [SCN –]initial ) in the solution. [Fe(SCN)] 2+eq. standard = [ SCN –]initial (F 5.3) (b) Finding the concentration of [Fe(SCN)] 2+eq. using Absorbance measurements. The iron(III) thiocyanate complex absorbs light at 460nm. The Beer -Lambert Law given in equation F5.1 for the absorption of light by [Fe(SCN)] 2+ at 460nm (using solution in PartB(a) ) may be written as : Absorbance (A standard ) = ε x l x c = ε x l x [Fe(SCN)] 2+eq. standard (F 5.4 ) showing a li near relationship between A and [Fe(SCN)] 2+eq. Therefore: [ ( )]+. = . = [ ( )]+. (F 5.5 ) Hence the equilibrium concentration of iron(III) thiocyanate [FeSCN 2+]eq for each so lution , n, can be determined by measuring the absorbance (A) of these solution s at equilibrium and using the relationship g iven in equation F 5.5 : 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 9 i.e . For so lution n , the equilibrium concentration [ ( )]+. = [ ( )]+. × (F 5.6 ) and using the relationship from Part B (a) above: [FeSCN 2+]eq. s tandard = [ SCN –]initial = 0.00 02M (F5. 3) 1. Preparation of standard solution for iron(III) thiocyanate complex ion a) With an auto -zippette, dispense 18 mL of the 0.200 M iron(III) nitrate, Fe(NO 3)3, in 1 M HNO 3 solution into a DRY 50 mL conical flask. b) Collect 10 mL of 0.002 M potassium thiocyanate (KSCN) solution, via an auto - zippette, into a DRY 50 mL conical flask. c) Rinse a 2.00 mL pipette twice with distilled water and twice with the 0.002 M potassium thiocyanate (KSCN) solution. d) Pipette a 2.00 mL aliquot of the 0.002 M KSCN solution i nto the above conical flask containing the Fe(NO 3)3 in 1 M HNO 3 solution. Mix it thoroughly by swirling the contents gently. e) Allow the mixture to equilibrate for 1 minute. f) Fill the cuvette carefully (only to two -third full) with the reaction mixture, away from the spectrophotometer and over a waste container . g) Measure the absorbance (A standard ) of this standard solution at a wavelength of 460 nm. Use distilled water as the reference. • Record the absorbance (A standard ) in the Results Sheet provided. h) Calculate and record in Results Sheet the concentration of iron(III) thiocyanate complex ion, [FeSCN 2+]standard , in the 20 mL standard solution. Use equation F5. 3 as guidelines. (NOTES: consider the dilution factor of 0.002 M KSCN in s tandard solution). 2. Determination of equilibrium constant for iron(III) thiocyanate complex ion a) Label four DRY 25.0 mL conical flasks “A” to “D”. b) Table F5.1 shows the amount of each following solution required to provide a total of 10 mL in each reacti on conical flask: i. Add 0.002 M iron(III) nitrate (Fe(NO 3)3) in 1 M HNO 3, via an auto -zippette. ii. Add 0.002 M potassium thiocyanate (KSCN) solution using a DRY 10 mL measuring cylinder. iii. Add distilled water using a 10 mL measuring cylinder. Table F5.1 Reaction mixture in each flask . c) Mix each reaction mixture thoroughly by swirling the contents gently. d) Allow the mixture to equilibrate for 1 minute. e) Fill the cuvette carefully (only to two -third full) with the reaction mixture, away from the spectrophotometer and over a waste container . f) Measure the absorb ance (A) of each reaction mixture at the wavelength 460 nm. Use distilled water as the reference. • Record the absorbance (A) of each reaction mixture in the Results Sheet provided. 3. Calculations a) Use data on the Results Sheet provided ; show all working for calculations. Conical Flask Fe(NO 3)3 (mL) KSCN (mL) Distilled Water (mL) Total Volume (mL) A 5.0 2.0 3.0 10.0 B 5.0 3.0 2.0 10.0 C 5.0 4.0 1.0 10.0 D 5.0 5.0 0.0 10.0 Pipette 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 10 b) Using the concentration [Fe(SCN)] 2+eq. standard and absorbance (A standard ) of standard solution from PartB(1), calculate the iron(III) thiocyanate concentration ([FeSCN 2+]eq solution n) at equilibrium in each reaction flask (equation F 5.6). c) Write the equilibrium constant expression (K c) for equation F5.2. d) Calculate the concentration of iron(III) ions ([Fe 3+]initial ) initially in each re action flask using equation F5.8. Concentration (1) x Volume (1) = Concentration (1) x Volume (2) (F 5.8 ) e) Calculate the concentration of thiocyanate ions ([ SCN –]initial ) initially in each re action flask using equation F5.8. f) Calculate the concentration of Fe 3+ at equilibrium in each reaction flask using equation F5. 9. [Fe 3+]eq = [Fe 3+]initial ̶ [FeSCN 2+]eq. solution (F5.9) g) Calculate the concentration of SCN – at equilibrium in each reaction flask using equation F5.10 . [SCN –]eq = [ SCN –]initial ̶ [FeSCN 2+]eq. solution (F5.10) h) Calculate the equilibrium constant (K c) for each reaction mixture in flask using your expression written in Step 3 . i) Calculate an average equilibrium constant for the iron(III) thiocyanate complex ion over the four reaction mixtures A to D. Question 3 Why is it necessary to use DRY conical flasks for the preparation of reaction mixtures? Question 4 Inspect the values for K obtained for reaction mixtures A to D. (iii) Does the value of K depend on the initial concentrations of Fe 3+ and SCN – ions? (iv) From the average value of K obtained for this reaction, would you describe it as a product –favoured or a reactant –favoured reaction? 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 11 EXPERIMENT F5 – RESULTS SHEET CHEMICAL EQUILIBRIUM Name: ______________________________________Day/AM / PM/ Group No.: _____________ Demonstrator: ________________________________ _ AIM ________________________________ ________________________________ ______ ________________________________ ________________________________ ______ _______________________________________________________________________ METHOD ________________________________ ________________________________ ______ PART A: CHEMICAL EQUILIBRIUM SHIFTS FOR [Fe (SCN )]2+ COMPLEX IONS SYSTEM 1. The Equilibrium System of [Fe (SCN )]2+ Complex Ions Fe 3+(aq) + SCN – (aq) ⥫⥬ [Fe(SCN)] 2+(aq) iron(III) thiocyanate iron(III) thiocyanate ions ions complex ions 2. Chemical Equilibrium Shifts: Record observations into Table and answer the following questions: Question 1 Using the colours in the table above, for each test tube reaction 2 - 9, after disturbance, identify which reaction is favoured, forward or reverse. Provide an explanation (see example in table ). (Tips: Fe(OH) 3, AgSCN are insoluble in water, NH 3 is good base - what reaction does it undergo with water?) Question 2 Write an ionic equation for any precipitation reaction you have observed. ________________________________ ________________________________ ______ SOLUTION COLOUR Fe (NO 3)3 KSCN [Fe (SCN )]2+ 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 12 2. Table 1. Chemical Equilibrium Shifts: Fe 3+(aq) + SCN –(aq) ⥫⥫ [Fe(SCN)] 2+(aq)  H -ve TEST TUBE NUMBER TEST SOLUTION ADDED (4 to 5 drops) OBSERVATIONS (e.g. colour changes/precipitation/ colour of precipitates) QUESTION 1 Shifts towards products or reactants? Explain. QUESTION 2 (ionic equations for any precipitation reaction) example 0.5M K 3PO 4 Solution becomes light yellow Reaction shifts to the left. Phosphate PO 43- reacts with Fe 3+ forming insoluble FePO 4, and system moves left to form more Fe 3+to get back to equilibrium. Fe 3+(aq) + PO 4 3–(aq) → FePO 4 (s) 2 0.2 M Fe(NO 3)3 3 0.2 M KSCN 4 0.1 M AgNO 3 5 0.2 M KCl 6 1.0 M NH 3 7 2.0 M NaOH 8 ice bath 9 hot water bath 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 13 PART B: DETERMINATION OF EQUILIBRIUM CONSTANT (K c) 1. Standard Solution for [Fe (SCN )]2+ Complex Ions Absorbance of [Fe (SCN )]2+ standard solution ( Astandard ) = 0.883 Concentration of [Fe(SCN)] 2+ in the standard solution ([Fe(SCN)] 2+eq. standard ), using equation F 5.3 = _________________________________________ (Space for calculations) (Tips: consider the dilution factor of 0.002 M KSCN in standard solution). (Eqn F 5.3 [Fe(SCN)] 2+eq. standard = [ SCN –]initial ) 2. Table 2. Determination of Equilibrium Concentrations for [Fe(SCN)] 2+ Complex Ions [ ( )]+. = [ ( )]+. × REACTION FLASK ABSORBANCE (Asolution ) [ ( )]+. = × (answer from Q1 above) A 0.195 B 0.295 C 0.410 D 0.500 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 14 3. Equilibrium Constant Expression for [Fe (SCN )]2+ Complex Ions Kc = Table 3. Calculation of Equilibrium Constant for [Fe(SCN)] 2+ Complex Ions REACTION FLASK A B C D [Fe 3+]initia l= C 1 x [SCN –]initial = C 1 x [Fe (SCN )]2+eq solution n from above data table (in Step 2 ) [Fe 3+]eq = [Fe 3+]initial ̶ [Fe (SCN )]2+eq. solution [SCN –]eq = [SCN –]initial ̶ [Fe (SCN )]2+eq. soilution Equilibrium constant (K c) Average Equilibrium Constant (K c) = 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 15 ANSWERS TO QUESTIONS Question 3 Why is it necessary to use DRY conical flasks for the preparation of reaction mixtures? ________________________________ ________________________________ ______ ________________________________ ________________________________ ______ ________________________________ ________________________________ ______ _______________________________________________________________________ Question 4 Inspect the values for K obtained for reaction mixtures A to D. (i) Does the value of K depend on the initial concentrations of Fe 3+ and SCN – ions? (ii) From the average value of K obtained for this reaction, would you describe it as a product – favoured or a reactant –favoured reaction? _______________________________________________________________________ _______________________________________________________________________ ___ ____________________________________________________________________ ___________________________________________ ____________________________ _____________________________________________ __________________________ CONCLUSION _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ ________________________________________ _______________________________ _______________________________________________________________________ 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 16 REPORT COVER SHEET AND FEEDBACK SHEET School of Chemistry The University of Melbourne PLEASE COMPLETE AND ATTACH AS THE FRONT PAGE OF YOUR REPORT Laboratory Report Cover Sheet Student Name: Student Number: Subject Name & Code: Demonstrator: Experiment Title: F5: Chemical Equilibrium In cases where group work is involved, give details of your group here: Due Date: _____________________________ Special Consideration application submitted: YES/NO Plagiarism: Plagiarism is the act of representing as one's own original work the creative works of another, without appropriate acknowledgment of the author or source. Collusion: Collusion is the presentation by a student of an assignment as his or her own work, but which is in fact the result in whole or in part of unauthorised collaboration with another person or persons. Collusion involves the cooperation of two or more students in plagiarism or other forms of academic misconduct. Both collusion and plagiarism can even occur in group work. For examples of plagiarism, collusion and academic misconduct in group work please see the University’s policy on Academic Honesty and Plagiarism: https://academichonesty.unimelb.edu.au Plagiarism and collusion constitute cheating. Disciplinary action will be taken against students who engage in plagiarism and collusion as outlined in University policy. Proven involvement in plagiarism or collusion may be recorded on your academic file in accordance with Statute 13.1.18. 2020 Experiment F5 result sheet: Chemical equilibrium Copyright: School of Chemistry, The University of Melbourne. 17 COMPULSORY STUDENT DECLARATION DETAILS: Please tick to indicate that you understand the following statements: I declare that:  This laboratory report is my own original work and do es not involve plagiarism or unauthorised collusion , except where due credit is given to the work of others. The report is based on results and spectra obtained by me during my laboratory session.  This laboratory report has not previously been submitted for assessment in this or any other subject. For the purposes of assessment, I give the assessor of this assignment the permission to:  Reproduce this laboratory report and provide a copy to another member of staff; and  Take steps to authenticate the as signment/laboratory report, including communicating a copy of this assignment to a checking service (which may retain a copy of the assignment on its database for future plagiarism checking). Signed: ………………………………………………………… Date: ………………………………….……. Feedbac k Report: Feedback on your report and the mark you received will be returned by the Demonstrator using the Feedback Report below. You are invited to discuss your report and mark with your demonstrator upon return of this Feedback Report. F5: CHEMICAL EQ UILIBRIUM COMMENTS MARK Chemical Equilibrium Shifts 1. [Fe(SCN )]2+ complex ions obs . 1 mark 2. Test tube reactions 8 marks /9 Equilibrium Constant (K c) Calculations for: 1.[Fe(SCN )]2+ standard 1 mark 2.[ Fe(SCN )]2+ eq 1 mark 3.Initial concentrations 2 marks Equilibrium Concentrations 2 marks Equilibrium constants 2 marks Average K 1 mark /9 Questions Q3 1 mark Q4 1 mark /2 TOTAL Demonstrator Signature: /20