Chem Chpt 1 PDF - Experimental Chemistry

# UNIT 1 EXPERIMENTAL CHEMISTRY ## 1.1 EXPERIMENTAL DESIGN ### Measurement 1. In the chemistry laboratory, specific apparatus are needed to measure: - time - temperature - mass - volume 2. The International System of Units (SI units) is used for recording measurements. ### Time...

# UNIT 1 EXPERIMENTAL CHEMISTRY ## 1.1 EXPERIMENTAL DESIGN ### Measurement 1. In the chemistry laboratory, specific apparatus are needed to measure: - time - temperature - mass - volume 2. The International System of Units (SI units) is used for recording measurements. ### Time 1. Time interval is measured using a stopwatch - analogue stopwatch or digital stopwatch. 2. The SI unit for time is the second (s). 3. Longer intervals of time are measured in minutes (min) or hours (h). #### Analogue stopwatch - measures time to an accuracy of ±0.1 s. #### Digital stopwatch - measures time to an accuracy of ±0.01 s. ### Temperature 1. The temperature of a substance is measured using a thermometer - analogue thermometer or digital thermometer. ### Key Study Guide O-LEVEL CHEMISTRY Revision Notes, Examples & Practice 2. The SI unit for temperature is the kelvin (K). 3. Another common unit for temperature is degree Celsius (°C). 4. Kelvin scale has its zero point at absolute zero (0K) which is the lowest temperature possible. Thus, a temperature on the kelvin scale will never be a negative value. 5. Celsius scale is based on 0 °C for the freezing point of water and 100 °C for the boiling point of water. 6. Converting temperature in degree Celsius to kelvin: $Temperature\ in\ kelvin\ (K) = temperature\ in\ degree\ Celsius\ (°C) + 273$ #### Worked Example: The boiling point of pure water at atmospheric pressure is 100 °C. What is its value on the kelvin scale? **Solution:** $Conversion\ factor: 1 °C = (1 + 273) K$ $Hence, 100 °C = 100 + 273 = 373 K$. ### Mass 1. The mass of a substance is measured using an electronic balance, a chemical balance or a beam balance. 2. The SI unit for mass is the kilogram (kg). 3. Smaller masses are measured in grams (g) or milligrams (mg). 4. Very large masses are measured in tonnes (t). 5. Electronic balance has an accuracy of about ±0.01 g. ### Volume 1. The SI unit for volume is the cubic metre (m³). 2. Smaller volumes are measured in cubic decimetres (dm³) or cubic centimetres (cm³). $1\ m² = 1000\ dm²; 1\ dm² = 1000\ cm³]$ 3. Volume is measured using the following apparatus: - burette - measuring cylinder - pipette - syringe - volumetric flask 4. The choice of apparatus to measure the volume of a liquid or gas depends on: - the volume to be measured; and - the accuracy required. #### Worked Example: Fizzy drinks are packed in bottles of different volumes. What is the volume (in cm³) of the drink contained in a 1.25 l bottle? **Solution:** $1.25\ l\ means\ 1.25\ litres.$ $1\ litre\ is\ the\ same\ as\ 1\ dm³.$ $Hence, 1.25\ l = 1.25 × 1000\ cm³ = 1250\ cm³$ ## Choosing Suitable Apparatus for Carrying out Simple Experiments 1. Measuring the melting point of a pure solid. 2. Measuring the freezing point of a pure solid. 3. Measuring the boiling point of a pure liquid. 4. Measuring the volume of a gas liberated during a chemical reaction. ### Methods for collecting gases liberated during chemical reactions. - By displacement of water - By downward delivery - By upward delivery ## 1.2 METHODS OF PURIFICATION AND ANALYSIS ### Separating a Solid-Solid Mixture 1. **Use of a suitable solvent (dissolution)** - This method is used to separate a mixture of a soluble solid and an insoluble solid by using a suitable solvent. - Example: A mixture of common salt (soluble solid) and sand (insoluble solid) can be separated using water as the solvent. **Procedure:** - Add distilled water to the mixture and stir. The salt will dissolve in the water whereas sand will not dissolve. - Filter the mixture. The insoluble sand remains in the filter funnel as residue. The soluble salt passes through the filter paper and is collected as filtrate. - The filtrate is then evaporated to dryness to obtain the salt. 2. **Sublimation** - Sublimation is used for the separation of a solid-solid mixture where one of the solids sublimes upon heating (changes directly from solid state to gaseous state), leaving behind the other solid that is non-sublimable. - Example: Separating a mixture of dry ice and common salt. Sodium chloride is non-sublimable; dry ice sublimes. On heating the mixture, dry ice changes directly into a gas and leaves the salt behind. ### Separating a Solid-Liquid Mixture 1. **Filtration** - Filtration is used to separate an insoluble solid from a liquid. - Example: A mixture of limestone (insoluble solid) and water can be separated by filtration. The insoluble limestone remains in the filter funnel as residue. The liquid water passes through the filter paper and is collected as filtrate. 2. **Evaporation** - This method is used to separate a dissolved solid from its solvent. - Example: Sea salt (dissolved solid) is obtained from seawater by evaporation. **Procedure:** - Seawater is poured into an evaporating dish. The seawater is heated to evaporate off all the water (evaporate to dryness). Sea salt is left behind in the evaporating dish. 3. **Crystallisation** - Crystallisation is used to separate a soluble solid from its solution. - This method is based on the different solubilities of solids in water. - The solubility of most solids increases as temperature rises. - Hence, when a saturated solution is cooled, the solubility of the solid will decrease and the excess solid will be crystallised out. **Example: Obtaining copper(II) sulfate from copper(II) sulfate solution.** **Procedure:** - The copper(II) sulphate solution is poured into an evaporating dish. - The solution is heated to evaporate most of the water to obtain a saturated solution. - Allow the hot saturated solution to cool. Crystallisation occurs and crystals are formed. - Filter to remove the crystals. Wash the crystals with a little cold water and dry the crystals by pressing them between sheets of filter paper. ### Separating a Liquid-Liquid Mixture 1. **Simple distillation** - Simple distillation is carried out to separate a volatile liquid from a non-volatile solute or two miscible liquids with widely differing boiling points (greater than 50 °C). **Procedure:** - On heating, the volatile liquid boils and turns into a vapour. - The vapour rises and enters the condenser. - In the condenser, the vapour condenses back into a pure liquid. - Hence, the volatile liquid will distil over as the distillate. - The non-volatile solute has a much higher boiling point compared to the volatile liquid and will not distil over during the distillation. - The non-volatile solute remains as a residue in the distillation flask. - In a simple distillation, the boiling and condensation process occurs only once. ### Example of simple distillation: Obtaining pure water from a salt solution **Procedure:** - Pour the salt solution into a round-bottom flask (distillation flask). - Add a few pieces of boiling chips to smoothen the boiling. - Heat the solution until it boils. - The water vaporises. - The vapour rises and enters the condenser. - In the condenser, the water vapour condenses to water. - Pure water is collected as the distillate. - The salt remains in the round-bottom flask. - In a simple distillation, the boiling and condensation process occurs only once. 3. **Fractional distillation** - Fractional distillation is used to separate two or more miscible liquids with different boiling points (separated by less than 40–50 °C). - The liquids are distilled out in order of boiling points. - The liquid with the lowest boiling point is distilled first, while the liquid with the highest boiling point is distilled last. **In industries, fractional distillation is used to:** - separate petroleum into various useful fractions such as petrol, diesel, kerosene, lubricating oil and bitumen; - obtain nitrogen from liquid air to be used in the Haber process for the manufacture of ammonia; and - obtain wine, beer, whiskey and other liquor from fermented liquor. ### Example of fractional distillation: Separating a mixture of ethanol (boiling point: 78 °C) and water (boiling point: 100 °C) **Procedure:** - The liquid mixture is poured into the distilling flask. - Add a few pieces of boiling chips to smoothen the boiling. - The liquid mixture is heated and brought to boil. - Both ethanol and water change to vapour. - The more volatile ethanol vaporises more easily, producing more ethanol vapour in the vapour phase. - The vapour moves up the fractionating column. - The glass beads provide a large surface area for the vapour to undergo repeated condensation and vaporisation as it moves up the fractionating column. - The first vapour to reach the top of the fractionating column is the more volatile one (i.e., the liquid with the lowest boiling point). - Ethanol will distil over when the thermometer reads a constant 78 °C. - When all the ethanol has distilled over, the temperature will rise again. - The receiver is changed to collect water which will distil over at 100 °C as shown in the heating curve below. 5. **Using a separating funnel** - A separating funnel is used to separate two immiscible liquids – liquids that do not mix. - On standing, the two liquids separate into two layers – the denser liquid sinks to the bottom and the other is at the top. - Each layer can be removed by opening the tap at the bottom of the funnel and collecting it in separate containers. - Oil and water can be separated by using a separating funnel. #### Worked Example 1 How would you separate a mixture of copper(II) oxide and copper(II) sulfate? **Solution:** Copper(II) oxide is insoluble in water while copper(II) sulfate is soluble in water. Add distilled water to the mixture. Stir the mixture to dissolve the copper(II) sulfate. Filter the mixture to remove the copper(II) oxide, then wash and dry it with filter paper. Evaporate the filtrate to evaporate until the solution is saturated. Cool the hot saturated solution for crystallisation to occur. Filter off the copper(II) sulfate crystals and dry them between filter papers. #### Worked Example 2 The boiling points of some gases found in air are given below. | Gas | Boiling Point (°C) | |---|---| | Krypton | -152 | | Neon | -246 | | Nitrogen | -196 | | Oxygen | -183 | (a) A mixture of the above gases is cooled. Which gas will change into a liquid first? (b) A liquid mixture of the above gases is fractionally distilled. In what order will these gases distil out? **Solution:** (a) Krypton gas will liquefy first because the gas with the highest boiling point will be the first to liquefy. (b) The gases will distil out in order of increasing boiling points – lowest boiling point first and highest boiling point last. Hence, neon will distil out first, followed by nitrogen, oxygen, and finally, krypton. ## 1.3 CHROMATOGRAPHY 1. Chromatography is a technique used to: - identify a substance - determine the purity of a substance - separate two or more substances in a mixture. 2. **Principles of chromatography** - Different substances have different solubilities in the same solvent. - Different substances travel at different rates. - More soluble substances move at a faster rate. - Less soluble substances move at a slower rate. - This causes the substances to spread apart. 3. Paper chromatography is a simple chromatography technique employed in the laboratory to deal with small quantities of substances and for mixtures with many substances. 4. The figure below (left) shows the apparatus set-up for a paper chromatography experiment. 5. The figure below (right) shows two developed chromatograms. 6. A pure substance will only produce one spot on the chromatogram. 7. An impure substance will produce more than one spot on the chromatogram. Each substance in the impure sample will appear as a spot on the chromatogram. 8. Paper chromatography can be used to separate and identify the various components in black ink or a mixture of amino acids. 9. **Example: Separating and identifying the components in a sample of black ink (by comparing with known pure dye samples)** **Procedure:** - Cut a 20-cm strip of filter paper. - Use a pencil to draw a line 2 cm from one end of the paper. - Use a capillary tube to place a drop of black ink on the line and allow it to dry. - Besides the spot of black ink, spot the following standard colours using pure dye samples: red, blue, orange and green. - After the spots have dried, lower the paper into a glass tank containing a solvent until the spots are just above the surface of the solvent. - Leave it to develop. - Remove the paper when the solvent has almost reached the top of the paper but not over the paper. - Allow the paper to dry. - The developed chromatogram is shown below. 10. **Separating and identifying a mixture of amino acids (using R, values)** - A mixture of amino acids can be separated by using paper chromatography. - Amino acids are colourless substances. - To locate the colourless spots of amino acids on the chromatogram, a locating agent is used. - The chromatogram is sprayed with a chemical called ninhydrin. - The locating agent reacts with the components to form coloured compounds. - A developed chromatogram is shown below. - The amino acids can be identified by its retention factor (R) values. - The retention factor is defined as the ratio of the distance travelled by the centre of a spot to the distance travelled by the solvent front. $R₁= \frac{Distance\ moved\ by\ component}{Distance\ moved\ by\ solvent}$ - R, values for amino acids A, B and C: $Amino\ acid\ A: R₁ = \frac{a}{y}$ $Amino\ acid\ B: R₁ = \frac{b}{y}$ $Amino\ acid\ C: R₁ = \frac{c}{y}$ - Compare the calculated R, values to those in a database using the same solvent to identify the amino acids in the mixture. 11. **Uses of paper chromatography:** - Detect the purity of products. - Detect contamination in various samples, like food and beverages. - Used for the separation of impurities from various industrial products. - Analysis of reaction mixtures in chemical laboratories. #### Worked Example 1 In preparing a chromatogram, the following precautions are taken. (a) The start line must be drawn with a pencil rather than with a ballpoint pen. (b) The spots of samples on the start line should be small. (c) The solvent front must not be allowed to reach the top of the chromatography paper. **Solution:** (a) Ballpoint pen ink contains a mixture of coloured dyes. The dyes from the ballpoint pen will be separated and may overlap with spots from the original unknown sample, complicating the results obtained. (b) Small spots are used to prevent the dyes from spreading sideways and hence overlapping with spots next to them. (c) When the solvent overshoots the top of the paper, the distance travelled by the solvent cannot be determined accurately. Hence, the R, values cannot be calculated to enable identification of the spots. #### Worked Example 2 The figure below shows a chromatogram obtained using three single dyes, red, green and blue, and four unknown dye samples, P, Q, R and S. **Solution:** Use the matching method - same substance travelled the same distance. - Sample P contains green dye and one unknown dye. - Sample Q contains only one dye - blue. - Sample R contains all three dyes - red, green and blue. - Sample S contains two dyes - green and red. #### Worked Example 3 The chromatogram below was obtained from a paper chromatography experiment to analyse the component dyes in an ink sample using ethanol solvent. (a) Why is ethanol used as a solvent instead of water? (b) Calculate the R, values of the two spots. (c) Are the two spots made up of pure substances? Give your reasons. **Solution:** (a) The dyes are organic compounds - soluble in organic solvents but not in water. (b) Blue spot: $R₁ = \frac{8}{10} = 0.8$ Red spot: $R₁ = \frac{2}{10} = 0.2$ (c) Yes. Each spot is produced by one pure substance. The two spots are far apart with no overlapping. ## 1.4 CRITERIA OF PURITY 1. The purity of a substance can be determined by: - chromatography technique - melting point and boiling point determinations. 2. **Chromatography technique** - A pure substance can be determined by paper chromatography. - A pure substance will produce only one spot on the developed chromatogram. 3. **Melting point and boiling point determinations** - A pure substance has a sharp melting point. It melts at a constant fixed temperature. - An impure substance melts over a range of temperatures. - A pure substance has a sharp boiling point. It boils at a constant fixed temperature. - An impure substance boils over a range of temperatures. - Impurities lower the melting point of a pure substance. - Adding salt to ice will lower its melting point to below 0 °C. - Impurities elevate the boiling point of a pure substance. Example: Seawater boils above 100°C. 4. By comparing the melting and boiling points of an unknown substance with those found in a database, the identity of the substance can be determined. Example: A substance that melts at 0 °C and boils at 100 °C under normal atmospheric pressure is most likely to be water. 5. **Importance of purity in substances** - Foodstuffs that are impure, i.e., contaminated with harmful substances, can cause adverse side effects on the human body and in extreme cases may even lead to death. - Drugs that are impure can cause adverse allergic reactions. - Pure silicon is used to make silicon chips for making microprocessors. - Pure copper is needed to make copper wires used in electrical wiring. #### Worked Example Various mixtures of tin and lead were prepared and their melting points determined. The graph below shows the melting points of various mixtures of tin and lead. (a) What is the melting point of pure tin? (b) What is the melting point of pure lead? (c) What is the melting point of any mixture of tin and lead? Explain your answer. (d) Which mixture of tin and lead has the lowest melting point? **Solution:** (a) 232 °C. (b) 328 °C. (c) Below 328 °C. The presence of an impure substance will lower the melting point of a pure substance. The melting point of pure tin (232 °C) is lower than the melting point of pure lead (328 °C). (d) 64% tin and 36% lead. ## REVISION CHECKLIST You should be able to: - name appropriate apparatus for the measurement of time, temperature, mass and volume; including burettes, pipettes, measuring cylinders and gas syringes, - suggest suitable apparatus, given relevant information, for a variety of simple experiments, including drying and collection of gases and measurement of rates of reaction, - describe methods of separation and purification for the components of mixtures, to include: - use of a suitable solvent, filtration and crystallisation or evaporation, - sublimation, - distillation and fractional distillation, - use of a separating funnel, and - paper chromatography, - suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: - solid-solid, - solid-liquid, and - liquid-liquid (miscible and immiscible), - interpret paper chromatograms including comparison with “known" samples and the use of R values, - explain the need to use locating agents in the chromatography of colourless compounds, - deduce from given melting point and boiling point data the identities of substances and their purity, and - explain the importance of measuring the purity in substances used in everyday life.