Volumetric analysis involving acids and alkalis

Transcription

1 Chapter 19 Volumetric analysis involving acids and alkalis 19.1 Standard solutions 19.2 Acid-alkali titrations 19.3 Calculations on volumetric analysis 19.4 Writing a laboratory report on volumetric analysis Key terms Progress check Summary Concept map P. 1 / 74

2 19.1 Standard solutions A solution of accurately known molar concentration or molarity is called a standard solution. Key point A standard solution is a solution of accurately known molar concentration or molarity. Concept check P. 2 / 74

3 Apparatus for preparing standard solutions Name 1. Weighing bottle (a) Glass weighing bottle (b) Plastic weighing bottle cap cap Use It is used to hold the sample to be weighed. (a) Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions Standard solutions (b) P. 3 / 74

4 Name Use 2. Electronic balance It is used to weigh the solid quickly and accurately (accuracy up to (a) 0.01 g or (b) g). (a) (b) Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions Standard solutions P. 4 / 74

5 Name Use 3. Volumetric flask It is used to make up a solution to a specific volume (e.g cm 3 ) accurately. * Before use, rinse the volumetric flask with distilled water. Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions Standard solutions P. 5 / 74

6 Name Use 4. Pipette It is used to deliver a specific volume (e.g cm 3 ) of a solution accurately. * Before use, rinse bulb the pipette with distilled water, then with the solution it is going to deliver. Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions Standard solutions P. 6 / 74

7 Name 5. Pipette filler (a) The simple type (b) The traditional type with valves valves Use It is used to suck up a solution into a pipette. * As a safety precaution, always fill a pipette with a pipette filler. Do NOT use your mouth. Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions Standard solutions P. 7 / 74

8 Name Use 6. Beaker It is used to hold solutions. Table 19.1 Names and uses of the apparatus commonly used in the preparation of standard solutions. Class practice Standard solutions P. 8 / 74

9 Preparing standard solutions Primary standard A primary standard is a substance which can be used to prepare a standard solution directly. Learning tip Only very few substances can be used as a primary standard. Examples include anhydrous sodium carbonate and oxalic acid crystals Standard solutions P. 9 / 74

10 A good primary standard should have the following properties: readily available with high purity chemically stable (should not react with oxygen, carbon dioxide and water vapour in air) high solubility in water does not absorb water vapour from air. does not lose water of crystallization to the atmosphere on exposure to air if it is a hydrated substance involatile and non-toxic Example Standard solutions P. 10 / 74

11 Preparing a standard solution by dissolving a pure solid Steps for preparing cm 3 of an approximately 0.10 M sodium carbonate solution Stage A: Weighing the pure solid 1. Calculate the mass of anhydrous sodium carbonate required. Mass of Na 2 CO 3 required = number of moles of Na 2 CO 3 molar mass of Na 2 CO 3 = 0.10 mol dm dm g mol 1 = 2.65 g 19.1 Standard solutions P. 11 / 74

12 2. Use an electronic balance to weigh out accurately the required mass of anhydrous sodium carbonate. anhydrous sodium carbonate weighing bottle Figure 19.1 Steps for preparing a standard solution by dissolving a pure solid in distilled water. (1) and (2) electronic balance SBA note Do not waste time trying to weigh out exactly 2.65 g of the solid. However, the mass weighed out should be accurate (e.g. to be at least 0.01 g) Standard solutions P. 12 / 74

13 Stage B: Dissolving the pure solid 3. Dissolve the solid in about 100 cm 3 of distilled water in a beaker. Stir the solution with a glass rod. 100 cm 3 of distilled water glass rod beaker anhydrous sodium carbonate sodium carbonate solution (3) 19.1 Standard solutions P. 13 / 74

14 Stage C: Making up the solution to cm 3 in a volumetric flask 4. Pour the solution to a cm 3 volumetric flask through a filter funnel. glass rod filter funnel sodium carbonate solution volumetric flask (250.0 cm 3 type) 19.1 Standard solutions (4) P. 14 / 74

15 5. Wash the beaker and the glass rod with distilled water several times. Pour all the washing to the volumetric flask. plastic wash bottle distilled water washing sodium carbonate solution distilled water 19.1 Standard solutions (5) P. 15 / 74

16 6. Add distilled water to the volumetric flask but stop about 2 cm below the graduation mark. 7. Use a dropper to add distilled water until the bottom of the meniscus reaches the graduation mark. beaker distilled water dropper distilled water cm 3 graduation mark (6) (7) 19.1 Standard solutions P. 16 / 74

17 8. Stopper the flask and invert it several times to mix the contents well. stopper invert it several times meniscus (8) standard sodium carbonate solution Example Standard solutions P. 17 / 74

18 Preparing a standard solution by diluting a concentrated solution of known molarity Steps for preparing cm 3 of M sodium carbonate solution from M sodium carbonate solution 1. Calculate the volume of the original solution required for dilution. number of moles of Na 2 CO 3 = number of moles of Na 2 CO 3 before dilution after dilution 19.1 Standard solutions M 1 V 1 = M 2 V 2 V = V 1 = 25.0 cm P. 18 / 74

19 2. Use a pipette to transfer 25.0 cm 3 of the original solution to a cm 3 volumetric flask. pipette filler 25.0 cm 3 pipette meniscus 25.0 cm 3 pipette M sodium carbonate solution M sodium carbonate solution M sodium carbonate solution cm 3 volumetric flask (1) and (2) Figure 19.1 Steps for preparing a standard solution by diluting a concentrated solution Standard solutions P. 19 / 74

20 SBA note You should use the index finger instead of the thumb to control the flow of solution in a pipette Standard solutions P. 20 / 74

21 3. Add distilled water to the volumetric flask but stop about 2 cm below the graduation mark. 4. Use a dropper to add distilled water until the bottom of the meniscus reaches the graduation mark. beaker distilled water dropper distilled water cm 3 graduation mark (3) (4) 19.1 Standard solutions P. 21 / 74

22 5. Stopper the flask and then invert it several times to mix the contents well. stopper invert it several times meniscus (5) M sodium carbonate solution Example 19.3 Example 19.4 Class practice 19.2 Experiment 19.1 Experiment Standard solutions P. 22 / 74

23 19.2 Acid-alkali titrations Volumetric analysis and titration Volumetric analysis is a method to determine the amount (or concentration) of a substance in a sample. requires the measurement of volumes of the solutions involved. is also known as titrimetric analysis as it is done by titration. P. 23 / 74

24 Titration is an experimental technique used for determining the amount (or concentration) of a substance in a solution. In a titration, a solution of known concentration (i.e. a standard solution) is slowly added to another solution of known volume, until the two solutions have just reacted completely Acid-alkali titrations P. 24 / 74

25 Apparatus for titration Name Use 1. Burette It is used to deliver variable volumes (ranging from 0.05 to cm 3 ) of a solution accurately. * Before use, rinse the stopcock burette with distilled water and then with the solution it is going to deliver. Table19.2 Names and uses of some apparatus used in acid-alkali titration Acid-alkali titrations P. 25 / 74

26 Name Use 2. Conical flask It is used to hold a solution to be titrated. Its conical shape allows it to be swirled gently without spilling out the solution. * Before use, rinse the conical flask with distilled water. Table19.2 Names and uses of some apparatus used in acid-alkali titration Acid-alkali titrations P. 26 / 74

27 Name Use 3. Stand and clamp They are used to support the burette. Table19.2 Names and uses of some apparatus used in acid-alkali titration Acid-alkali titrations P. 27 / 74

28 Name Use 4. White tile It is used to allow clear observation of the colour change of the indicator. Table19.2 Names and uses of some apparatus used in acid-alkali titration Acid-alkali titrations P. 28 / 74

29 Procedure for acid-alkali titration A standard solution of an acid (or an alkali) is slowly added to a known volume of a solution of an alkali (or an acid), until the acid and alkali have completely reacted with each other. The stage at which the acid and the alkali have just reacted completely during titration is called the equivalence point. Learning tip The procedure for an acid-carbonate (e.g. HCl and Na 2 CO 3 ) titration is more or less the same as that for the acid-alkali titration Acid-alkali titrations P. 29 / 74

30 Key point The equivalence point of an acid-alkali titration is the point at which an acid and an alkali have just completely reacted with each other. The equivalence point of an acid-alkali titration can be estimated by using a ph meter or a datalogger connected with a ph sensor. It is not possible to detect the equivalence point using an acid-base indicator Acid-alkali titrations P. 30 / 74

31 During the acid-alkali titration, a suitable acid-base indicator is usually added. The colour of the indicator changes sharply at or very near the equivalence point, then the end point of titration is reached. Key point End point is the point at which the indicator changes colour sharply Acid-alkali titrations P. 31 / 74

32 Detection of end point by using an acid-base indicator 1. Fill the 25.0 cm 3 pipette (using a pipette filler) with the alkali until the graduation mark is reached. Transfer 25.0 cm 3 of the alkali to a conical flask. meniscus alkali 25.0 cm 3 pipette alkali conical flask (1) alkali Figure 19.3 Procedure for performing an acid-alkali titration Acid-alkali titrations P. 32 / 74

33 2. Fill the burette with the acid. Make sure that the jet is filled completely (no air bubbles inside the jet). stand clamp acid burette Learning tip white tile (2) The solution in the burette is often known as titrant Acid-alkali titrations P. 33 / 74

34 3. Add a few drops of an acid-base indicator to the alkali. Note the colour of the solution. 4. Take the initial burette reading, with your eyes on the same level as the meniscus. (3) indicator alkali meniscus (4) read as 0.6 cm 3 eye at proper level 19.2 Acid-alkali titrations P. 34 / 74

35 5. Slowly add the acid to the alkali until the mixture just turns to another persistent colour. This is the end point of titration. burette acid 19.2 Acid-alkali titrations solution mixture + indicator (5) P. 35 / 74

36 6. Take the final burette reading. Calculate the volume of the acid added to reach the end point. 7. Repeat the titration (steps 1 to 6) at least three times, until we get three sets of data for calculating the average volume of the acid used. Learning tip The volume of titrant used to react completely with the solution in the conical flask is called titre. Skill corner 19.1 Example 19.5 Class practice Acid-alkali titrations P. 36 / 74

37 Choosing a suitable acid-base indicator Methyl orange and phenolphthalein can give a sharp colour change at the end point of titration. Litmus is not used because it does not give a sharp colour change. methyl orange phenolphthalein (a) ph = 2 ph = 12 (b) ph = 2 ph = 12 Figure 19.4 The sharp colour changes of (a) methyl orange and (b) phenolphthalein in solutions of ph = 2 and ph = 12. (There is often a marked change in ph at the end point of titration. The change from ph = 2 to ph = 12 is used as an example here.) 19.2 Acid-alkali titrations P. 37 / 74

38 Titration type Strong acid strong alkali Example HCl(aq) vs. NaOH(aq) Suitable indicator(s) methyl orange; phenolphthalein Strong acid weak alkali (Strong acid carbonate) HNO 3 (aq) vs. NH 3 (aq) HCl(aq) vs. Na 2 CO 3 (aq) methyl orange Weak acid strong alkali CH 3 COOH(aq) vs. KOH(aq) phenolphthalein Weak acid weak alkali CH 3 COOH(aq) vs. NH 3 (aq) (no indicator would be suitable) Table 19.3 The correct choice of indicators for different types of acid-alkali titrations Acid-alkali titrations Example 19.6 P. 38 / 74 Class practice 19.4

39 Estimation of equivalence point by measuring the change in ph of the mixture The change in ph of the mixture during titration can be followed by using a ph meter or a data-logger connected with a ph sensor 19.2 Acid-alkali titrations P. 39 / 74

40 Steps for estimating equivalence point by measuring the change in ph of the mixture 1. Fill the 25.0 cm 3 pipette with 0.1 M HCl(aq). Transfer 25.0 cm 3 of the acid to a beaker placed on a magnetic stirrer. 2. Put the electrode of a ph meter or a ph sensor connected with a data-logger into the beaker of acid. 3. Fill the burette with 0.1 M NaOH(aq). Make sure that the jet is filled completely (no air bubbles inside the jet). 4. Slowly add the alkali to the acid. The ph of the mixture is measured immediately after each addition of the alkali Acid-alkali titrations P. 40 / 74

41 0.1 M NaOH(aq) burette ph meter magnetic stirring bar magnetic stirrer electrode of ph meter 0.1 M HCl(aq) Figure 19.5 The experimental set-up for the estimation of equivalence point by ph measurement Acid-alkali titrations P. 41 / 74

42 If the ph reading is plotted against the volume of the sodium hydroxide solution added, a titration curve is obtained. There is a marked change in the ph at the equivalence point of titration. Learning tip The ph of the mixture at the equivalence point may NOT be 7 for some acid-alkali titrations. Experiment 19.2 Experiment Acid-alkali titrations P. 42 / 74

43 ph ph of the mixture when the equivalence point is reached equivalence point volume of NaOH(aq) required to reach the equivalence point Volume of NaOH(aq) added (cm 3 ) Figure 19.6 Titration curve for the titration of 0.1 M sodium hydroxide solution with 25.0 cm 3 of 0.1 M hydrochloric acid Acid-alkali titrations P. 43 / 74 Think about

44 Estimation of equivalence point by measuring the change in temperature of the mixture Neutralization between an acid and an alkali is an exothermic reaction. The reaction gives out heat Acid-alkali titrations P. 44 / 74

45 Steps for estimating equivalence point by measuring the change in temperature of the mixture 1. Place 25 cm 3 of 0.1 M HCl(aq) and a thermometer in a polystyrene cup. 2. Record the initial temperature of the acid. 3. Add 0.1 M NaOH(aq) slowly to the acid. 4. Record the temperature of the mixture immediately after each addition of NaOH(aq). Learning tip The set-up for the experiment here is very similar to the one shown in Figure Except that: a polystyrene cup is used instead of a beaker. a thermometer is used instead of a ph meter Acid-alkali titrations P. 45 / 74

46 equivalence point E Temperature ( C) A volume of NaOH(aq) needed to reach the equivalence point B Volume of NaOH(aq) added (cm 3 ) Figure 19.7 A plot of the temperature change of the mixture against the volume of NaOH(aq) added Acid-alkali titrations P. 46 / 74

47 Temperature change during an acid-alkali titration From A to E: The temperature of the mixture rises because heat is given out when NaOH(aq) is added to dilute HCl(aq). At E: The temperature of the mixture reaches the maximum because the equivalence point is reached. From E to B: The temperature of the mixture falls because addition of excess NaOH(aq) will not give out heat but cool the mixture. Class practice Acid-alkali titrations P. 47 / 74

48 19.3 Calculations on volumetric analysis Standardization To standardize (i.e. to find the molarity of) a solution of unknown molarity by titrating it with a standard solution. Problem-solving strategy 19.1 Example 19.7 Experiment 19.3 Experiment 19.3 P. 48 / 74

49 Basicity of an acid To determine the basicity of an acid by finding out the number of moles of sodium hydroxide required to neutralize one mole of the acid. Example 19.8 Class practice 19.6 Molar mass of a substance To determine the molar mass of an acid, a metal or a base by volumetric analysis. Example Calculations on volumetric analysis P. 49 / 74

50 Relative atomic mass of an element To determine the relative atomic mass of an element by volumetric analysis. Example Class practice 19.7 Number of molecules of water of crystallization To determine the number of molecules of water of crystallization in a formula unit of a hydrated compound by volumetric analysis. Example Class practice Calculations on volumetric analysis P. 50 / 74

51 Percentage by mass of a substance in a sample Key point Percentage by mass of a substance in a sample (%) mass of the substance (g) = 100% mass of the sample (g) Figure 19.8 The percentage by mass of anhydrous sodium sulphate in this bottle is 99% Calculations on volumetric analysis Example Class practice 19.9 P. 51 / 74

52 Back titration Two ways to perform a titration direct titration back titration 1. Direct titration Add just the right amount of titrant (the solution in the burette) to the solution in the conical flask Calculations on volumetric analysis P. 52 / 74

53 2. Back titration Direct titrations are impossible in some cases. E.g. percentage by mass of CaCO 3 (s) in a sample cannot be found by titrating standard HCl(aq) with the sample directly. CaCO 3 (s) is insoluble in water and it is unable to make an aqueous solution of it. Learning tip Back titration is also performed if the reaction between the substance being analysed and the reagent involved is too slow Calculations on volumetric analysis P. 53 / 74

54 Steps for back titration 1. Add a known excess amount of standard HCl(aq) to a known mass of the CaCO 3 (s) sample. CaCO 3 (s) reacts with HCl(aq), dissolving completely to form a solution. 2. Back titrated the unreacted acid in the solution against a standard NaOH(aq). Problem-solving strategy 19.2 Class practice Calculations on volumetric analysis P. 54 / 74

55 19.4 Writing a laboratory report on volumetric analysis Format of laboratory reports Use the correct tenses Write in the third person and passive voice Write with proper grammar You should use past tense to describe specific experimental methods and observations. Because scientific experiments demonstrate facts that do not depend on the observer, reports should not be written in the first or the second person. Besides, passive voice is used. Complete sentences with no grammatical mistakes should be used. Table 19.4 Some useful guidelines in writing a laboratory report. Class practice P. 55 / 74

56 1 Title 2 Objective 3 Apparatus and materials Write the title of the experiment as given in the laboratory manual. (e.g. Determining the concentration of ethanoic acid in commercial vinegar)) Use a single sentence to state clearly the objective of the experiment. (e.g. To determine the concentration of ethanoic acid in commercial vinegar) List the apparatus and materials used in the experiment as specifically as possible. This allows others to repeat your experiments. (e.g. electronic balance beaker (100 cm 3 ) volumetric flask (250.0 cm 3 ) anhydrous sodium carbonate powder (6.30 g)) Table 19.5 Parts of a formal laboratory report Writing a laboratory report on volumetric analysis P. 56 / 74

57 4 Procedure 5 Results and observations Describe clearly the procedure of the experiment so that someone else can repeat what you did. For example, describe clearly how solutions are obtained or prepared and used in the analysis; how measurements are done; state the amounts of substances used, the volumes and concentrations of solutions used. In general, write the procedure in a step-by-step format rather than in essay form. In most cases, draw diagrams of the experimental set-ups for better presentation. Summarize your numerical data using tables or graphs. Express numerical results to the appropriate number of significant figures and in the correct units. Record all observations clearly and fully using appropriate terms in chemistry. Table 19.5 Parts of a formal laboratory report Writing a laboratory report on volumetric analysis P. 57 / 74

58 6 7 Analysis/ Calculations (if any) Conclusions and evaluation Give the equations used to work out the answers. Show your calculation steps clearly. Provide definitions of every variable used. Draw valid conclusions based on the observations and data collected. Describe the problems encountered in the experiment. Discuss the sources of errors and uncertainties, especially those due to error in laboratory techniques and those due to imprecision of the apparatus. Give any suggestions for improvement to avoid these errors. Answer the questions (if any) in the laboratory manual at the end of the laboratory report. Table 19.5 Parts of a formal laboratory report Writing a laboratory report on volumetric analysis P. 58 / 74

59 Attitude of writing laboratory reports We should not just report good data. Try to find out why irregularities arise, whether we have made some mistakes while performing experiments or we have to revise the experimental procedures. Experiment 19.4 Experiment Writing a laboratory report on volumetric analysis P. 59 / 74

60 Key terms 1. back titration 返滴定 2. end point 終點 3. equivalence point 當量點 4. laboratory report 實驗報告 5. primary standard 基本標準 6. standard solution 標準溶液 7. titration 滴定 8. titration curve 滴定曲線 9. volumetric analysis 容量分析 P. 60 / 74

61 Progress check 1. What is a standard solution? 2. Can you name the essential apparatus used in the preparation of a standard solution? 3. What is a primary standard? 4. What are the characteristics of primary standards? 5. Do you know how to prepare a standard solution by dissolving a pure solid? 6. Do you know how to prepare a standard solution by diluting a concentrated solution of known molarity? P. 61 / 74

62 7. What is volumetric analysis? 8. Can you name the essential apparatus used in titration? 9. What is the equivalence point of an acid-alkali titration? 10.What is the end point of an acid-alkali titration? 11.Do you know how to perform a simple acid-alkali titration? 12.Do you know how to detect the end point in an acid-alkali titration? 13.Can you name two common acid-base indicators used in titration? Progress check P. 62 / 74

63 14.Do you know how to choose the correct indicator(s) for different types of acid-alkali titrations? 15.Do you know how to estimate the equivalence point in an acid-alkali titration? Progress check P. 63 / 74

64 16.Can you perform the following calculations using the concepts of concentration of solution and the results of acid-alkali titration? (a) Standardization (b) Basicity of an acid (c) Molar mass of a substance (d) Relative atomic mass of an element (e) Number of molecules of water of crystallization of a hydrated compound (f) Percentage by mass of a substance in a sample Progress check P. 64 / 74

65 17.What are direct titration and back titration? 18.Do you know how to solve stoichiometric problems involved in back titration? 19.Do you know how to write a laboratory report? Progress check P. 65 / 74

66 Summary 19.1 Standard solutions 1. A standard solution is a solution of accurately known molar concentration or molarity. 2. The essential apparatus used in the preparation of a standard solution include volumetric flask, pipette and pipette filler. 3. A primary standard is a substance which can be used to prepare a standard solution directly. P. 66 / 74

67 4. A standard solution can be prepared by dissolving a known mass of a pure solid or by diluting a concentrated standard solution to a specific volume Acid-alkali titrations 5. Volumetric analysis is a method to determine the amount (or concentration) of a substance in a sample. Titration is one of the techniques of volumetric analysis. 6. The essential apparatus used in titration include burette, pipette and conical flask. Summary P. 67 / 74

68 7. The equivalence point of an acid-alkali titration is the point at which an acid and an alkali have completely reacted with each other. 8. The equivalence point of an acid-alkali titration can be estimated by measuring the change in ph and the change in temperature of the mixture. 9. End point is the point at which the indicator changes colour sharply. Summary P. 68 / 74

69 10. The end point of titration can be detected using the appropriate acid-base indicator(s). The choices of indicators in acid-alkali titration are summarized in Table 19.3 on p.15. Summary P. 69 / 74

70 19.3 Calculations on volumetric analysis 11. We can apply the concepts of concentration of solution and acid-alkali titration results to solve stoichiometric problems such as (a) Standardization (b) Molar mass of a substance (c) Basicity of an acid (d) Relative atomic mass of an element (e) Number of molecules of water of crystallization of a hydrated compound (f) Percentage by mass of a substance in a sample Summary P. 70 / 74

71 19.4 Writing a laboratory report on volumetric analysis 12. A laboratory report can help to communicate the procedure and results of an experiment on volumetric analysis. 13. A formal laboratory report should consist of the following sections: (a) Title (b) Objective (c) Apparatus and materials (d) Procedure (e) Results and observations (f) Analysis and calculations (if any) (g) Conclusions and evaluation Summary P. 71 / 74

72 Concept map determines VOLUMETRIC ANALYSIS apparatus concentration of solution basicity of an acid molar mass of a substance relative atomic mass of an element number of molecules of water of crystallization percentage by mass of a substance in a sample weighing bottle electronic balance volumetric flask pipette pipette filler beaker burette conical flask P. 72 / 74

73 VOLUMETRIC ANALYSIS Preparation of a standard solution Titration methods Dissolve a known mass of a pure solid Dilute a solution of known molarity Concept map P. 73 / 74

74 Titration example Acid-alkali titration detection of end point detection of equivalence point Use an acid-base indicator Measure the change in ph of the mixture Measure the change in temperature of the mixture Concept map P. 74 / 74