Beatty 6092 Practical Revision 2023

PURE CHEMISTRY (6092) PRACTICAL REVISION NOTES 2023 Prepared by: Mr. Yeo CK Page | 1 Beatty Secondary School Secondary 4 Express Name: ______________________ ( ) Class: 4E1 Date: ________ Practical Revision: Volumetric Analysis (Titration) Key Concepts: Titrate solution Q with solution P / Titrate solution Q against solution P. solution P in the burette A. General Tips 1. Rinse the funnel and burette with deionised water before the start of the titration. 2. Check for air bubbles at the tip of the burette / leaking of solution before rinsing with chemicals. 3. Remove the filter funnel before the start of titration. 4. Rinse the pipette with deionised water before placing into the reagent bottle. 5. Check if there is liquid in the pipette filler. Def late it completely to ensure no liquid is trapped inside. 6. Tap the tip of the pipette 3 times into the conical flask to ensure that the maximum volume of solution is pipetted. B. Presentation of Data and Measurement Measurement and Accuracy of Data: apparatus accuracy burette 0.05 cm3 (2 decimal places) pipette 25.0 cm3 (1 decimal place) Presentation of Data (Titre volumes should be consistent within ± 0.20 cm3): titration number rough 1 2 3 final burette reading / cm3 25.10 28.60 24.60 49.20 initial burette reading / cm3 0.00 4.30 0.00 24.60 volume of liquid Q added / cm3 25.10 24.30 24.60 24.60 best titration results ✓ ✓ Results: Average volume of Q used = 24.60 24.60 2 + = 24.60 cm3 (2 decimal places) Volume of P used = 25.0 cm3

PURE CHEMISTRY (6092) PRACTICAL REVISION NOTES 2023 Prepared by: Mr. Yeo CK Page | 2 Key Concepts: C. Types of Indicators and Colour Changes indicators acidic end-point alkaline methyl orange red orange yellow screened methyl orange purple grey green thymolphthalein colourless colourless blue D. Calculations involving titration 1. Percentage composition by mass 2. Percentage yield determines the percentage of product formed [mole of reactant]. 3. Percentage purity determines how much pure substance is found within the sample [mole of product]. 4. Chemical Calculations Formulae Note calculations on dilution [mole in 25 cm3 to mole in 1 dm3 and vice-versa] E. Sources of Error, Effects and Improvement 1. Source of Experimental Error: There may be presence of other soluble impurities in the acid solution pipetted into the conical flask which may react with the alkali from the burette. Effect: The volume of solution used in the burette may be larger than expected. Or There may be presence of other soluble impurities in the acid solution pipetted into the conical flask which may react with the acid itself. Effect: The volume of solution used in the burette may be smaller than expected. Improvement: Remove the soluble impurities by the precipitation followed by filtration. Number of atoms of element × Ar of element Mr of compound % by mass of an element = × 100% experimental or actual yield theoretical yield % yield = × 100% mass of pure substance mass of sample % purity = × 100% Note: Parallax Error is not a source of experimental error. It is considered as a human error.

PURE CHEMISTRY (6092) PRACTICAL REVISION NOTES 2023 Prepared by: Mr. Yeo CK Page | 3 F. Rationale behind experimental procedures 1. General Titrations (Acid-Base / Acid-Carbonate) Q(a) The filter funnel was left on top of the burette during the course of titration. Explain the effect on the titration results. A(a) Excess solution from the funnel may be added to the burette. The volume of solu tion added to the conical flask may be larger than expected / The volume of solution recorded in the burette will be smaller than expected volume used. Improvement: Remove the filter funnel and repeat the titration several times to values ± 0.20 cm3. Q(b) A student suggested that the investigation could be improved by making the titrations more accurate. He said that the concentrations of solution in the burette should be reduced. State and explain whether or not this suggestion would make the titrations more accurate. A(b) The titration results would be more accurate since the titre volume is bigger. Hence there will be a smaller percentage error. 2. Redox Titrations Q(a) In a titration involving aqueous iron(II) sulfate against potassium manganate(VII), 25 cm3 of sulfuric acid was added to the conical flask. Explain why sulfuric acid was added and why the volume of acid does not need to be measured accurately. A(a) The purpose of the acid is to remove soluble carbonate i mpurities present, hence accurate volumes of the acid added is not necessary. Q(b) Dilute sulfuric acid is added to acidify aqueous potassium manganate( VII) as it is stable to oxidation and provides hydrogen ions as a catalyst to speed up the reaction. Suggest why dilute hydrochloric acid is not used in the reaction. A(b) Aqueous potassium manganate( VII) is a strong oxidising agent and will oxidise hydrochloric acid to liberate chlorine gas, causing a side reaction. 3. Iodometric Titrations (sodium thiosulfate with potassium iodide/iodate) Q(a) Suggest why starch solution is added as an indicator. A(a) The resulting mixture is a pale yellow solution which is difficult to determine the end - point visually. Starch solution detects any free iodine and reacts with iodine to form a complex to give a dark blue colour. The colour change from blue to colourless is more visible and gives a sharp end-point. Q(b) Suggest why the starch indicator is not added at the start of the titration. A(b) The starch-iodine complex formed at high iodine concentrations is relatively stable , which is difficult to decompose. The correct titre volume will be higher than expected volume used when starch indicator was added at the start.

PURE CHEMISTRY (6092) PRACTICAL REVISION NOTES 2023 Prepared by: Mr. Yeo CK Page | 4 Practical Revision: Thermochemistry (Energy Changes) Key Concepts: A. Presentation of Data and Measurement Measurement and Accuracy of Data: apparatus accuracy thermometer 0.5 °C (1 decimal place) measuring cylinder 0.5 cm3 (1 decimal place) electronic balance 0.01 g (2 decimal places) Presentation of Data: initial temperature measured / °C highest temperature reached / °C total temperature change / °C 28.0 28.0 0.0 28.0 29.0 + 1.0 28.0 31.0 + 3.0 28.0 32.0 + 4.0 28.0 33.5 + 4.5 28.0 34.5 + 6.5 28.0 35.0 + 7.0 28.0 34.5 + 6.5 28.0 34.0 + 6.0 B. Sources of Error, Effects and Improvement BI. Thermometric Titration (measuring heat change involving burette and styrofoam cup) 1. Source of Experimental Error: There may be heat loss to the surroundings (for exothermic reaction) / heat gained from the surroundings (for endothermic reaction) Effect: The temperature measured will be lower than expected (exothermic) / higher than expected (endothermic). Improvement to minimise heat loss: Use a styrofoam cup covered with lid to reduce heat loss to the surroundings. Nest the styrofoam cup in a beaker to ensure stability. Improvements to improve accuracy: Measure the volume of solution using a more precise burette instead of a measuring cylinder / Use a more precise thermometer e.g. more subdivisions, reading to 0.1 °C / Take the initial temperature of both solutions and calc