Topic 1.1 - Experimental Design [teacher edition].pdf

5086 SCIENCE (CHEMISTRY) SECONDARY THREE EXPRESS UNIT 1: EXPERIMENTAL CHEMISTRY TOPIC 1.1: EXPERIMENTAL DESIGN What You Will Learn 1. How are physical quantities measured? 2. How are gases co...

5086 SCIENCE (CHEMISTRY) SECONDARY THREE EXPRESS UNIT 1: EXPERIMENTAL CHEMISTRY TOPIC 1.1: EXPERIMENTAL DESIGN What You Will Learn 1. How are physical quantities measured? 2. How are gases collected? Learning Outcomes Candidates should be able to: (a) name appropriate apparatus for the measurement of time, temperature, mass and volume; including burettes, pipettes, measuring cylinders and gas syringes (b) suggest suitable apparatus, given relevant information, for a variety of simple experiments, including collection of gases and measurement of rates of reaction.  EXPERIMENTAL DESIGN (A) MEASURING MASS The mass of a substance is the measure of the amount of matter contained in it. Units for measuring masses of chemicals:- S.I. unit : kilogram (kg) other units : gram (g) / tonne (t) [1 tonne = 1 000 kg = 1 000 000 g] Apparatus for measuring masses of chemicals:- (i) beam balance (ii) electronic ‘top-pan’ balance Electronic balance Diagram Procedure: 1. Place an empty container on the balance pan. 2. Press the tare button to ‘zero’ the balance pan. 3. Remove the empty container, and add the chemical to be measured into it. 4. Place the container of chemical on the balance pan. 5. Read off the balance display for the mass of the chemical and record to the nearest 0.01 g. 2 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science (B) VOLUMES OF LIQUIDS Units for measuring the volume of a liquid :- S.I. Unit: cubic metre (m3) other units: cubic decimetre (dm3) cubic centimetre (cm3) [1 m3 = 1 000 dm3 = 1 000 000 cm3] The volume of a liquid can be measured with several different types of apparatus. The apparatus we choose for an experiment depends on: (a) the volume we are measuring, and (b) how accurate we need the measurement to be. Marked Beaker Diagram Characteristics: Can measure approximate volume of 100 cm3 or 250 cm3. Measuring Cylinder Diagram Characteristics: Can measure approximate volume of 10 cm3, 25 cm3 or 50 cm3. Measures to the nearest 0.5 cm3 (e.g. 41.5 cm3, 99.0 cm3). 3 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science  Pipette Diagram Characteristics: Can measure accurate fixed volumes of liquids (e.g. 20.0 cm3 or 25.0 cm3). Cannot measure an odd volume such as 26.0 cm3. Procedure: 1. Draw liquid up the pipette to a mark, using a pipette filler. 2. Deliver liquid into a container for use. Burette Diagram Characteristics: Has a long scale, from 0.00 cm3 to 50.00 cm3, graduated in 0.1 cm3 divisions. Can measure accurate volumes of liquids to the nearest 0.05 cm3 (e.g. 24.00 cm3 or 38.95 cm3). Procedure: 1. Using a filter funnel, pour the liquid into the burette, from the top. 2. Measure the level of liquid by reading off the scale. 3. Open the tap and allow the liquid to run out (into a container) from the bottom of the burette. 4. Measure the level of liquid again. 5. The difference in readings gives the volume of liquid released. 4 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science Example 1: Which apparatus is used to measure 25.0 cm3 of liquid most accurately? [D] Example 2: Which piece of apparatus is used to measure exactly 22.50 cm3 of a liquid? [D] Example 3: Which piece of apparatus would be most suitable to measure accurately the volume of acid needed to neutralise 25.0 cm3 of an alkali? [A] 5 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science (C) VOLUMES OF GASES Units for measuring volumes of gases: cubic centimetre (cm3) Apparatus for measuring volumes of gases: gas syringe Note: It is dangerous to collect gas in a gas syringe if the conical flask containing the reaction mixture is being heated with a Bunsen burner. Gas syringe Diagram Characteristics: Can measure up to 100 cm3 of gas on a graduated scale, with 2 cm3 divisions. A gas syringe is made up of two parts, the barrel and the plunger. At the start of an experiment, the plunger is pushed in fully to expel any gas in the syringe. As the gas from an external source enters the syringe, it pushes the plunger outwards. Example 4: Magnesium ribbon was reacted with excess dilute hydrochloric acid. Which piece of apparatus would be suitable for measuring the volume of hydrogen gas evolved? [B] 6 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science  Methods for Collecting Gases How we collect a gas depends on the physical properties of the gas: (a) solubility – how soluble the gas is in water (b) density – how dense the gas is compared to air Water displacement Diagram Characteristics: This method is suitable for collecting gases that are insoluble or slightly soluble in water. E.g.: H2, O2, CO2 Downward delivery Diagram Characteristics: This method is used to collect gases that are soluble in water and more dense / heavier than air. E.g.: Cl2 – soluble HCl – very soluble Upward delivery Diagram Characteristics: This method is used to collect gases that are soluble in water and less dense / lighter than air. E.g.: NH3 – extremely soluble The relative molecular mass, Mr, can be calculated as the sum of relative atomic masses, Ar, of the elements in a molecule of an element or compound. e.g.: Mr of H2 = 2(1) = 2 Mr of HCl = 1 + 35.5 = 36.5 Mr of NH3 = 14 + 3(1) = 17 In general, ❖ relative molecular mass < 17 [i.e. less dense than air, hence upward delivery] ❖ relative molecular mass > 17 [i.e. more dense than air, hence downward delivery] 7 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science Example 5: A gas, X, is less dense than air and insoluble in water. Which method cannot be used to collect the gas? [A] Example 6: The diagrams show two methods of collecting gases. Which row gives the properties of a gas which can be collected by both methods? [B] property 1 property 2 A insoluble in water denser than air B insoluble in water less dense than air C soluble in water denser than air D soluble in water less dense than air 8 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science  Methods for Drying Gases Sometimes, we need to use a dry gas in an experiment. We can dry a gas by passing it through a drying agent. Concentrated Sulfuric Acid Diagram Characteristics: Nature: acidic It is used to dry most gases. E.g.: Cl2, HCl Quicklime (calcium oxide) Diagram Characteristics: Nature: basic It is used to dry ammonia. Fused Calcium Chloride (calcium chloride that has been heated to Diagram remove all traces of water) Characteristics: Nature: neutral It is used to dry most gases. 9 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science Example 7: A gas, Y, is less dense than air, very soluble in water and is an alkali. Which method is used to collect a dry sample of the gas? [A] Example 8: A student is provided with two drying agents: concentrated sulfuric acid and calcium oxide. Which method should he use to collect a sample of dry ammonia? [D] 10 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science Example 9: A gas X is insoluble in water and less dense than air. An impure supply of X contains water vapour and a water-soluble impurity. In which order should the pieces of apparatus shown joined together to collect a pure, dry sample of X? [D] A 1,2,3,4 B 1,2,3,5 C 1,3,2,5 D 1,3,2,4 Example 10: HCl is very soluble in water, whereas Cl2 is only slightly soluble in water. Both gases can be dried using conc. H2SO4. Which diagram represents the correct method of obtaining dry Cl2 from damp Cl2 containing a small amount of HCl? [B] Example 11: The diagram shows apparatus used to obtain carbon monoxide. What is the main purpose of Y? [C] A to dry the gas B to remove hydrogen chloride from the gas C to remove carbon dioxide from the gas D to prevent water from being sucked back on to the hot carbon 11 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science (D) MEASURING TEMPERATURE Units for measuring temperature:- S.I. unit : kelvin (K) other unit : degree Celsius (oC) Apparatus for measuring temperature:(a) mercury-in-glass thermometer (b) alcohol-in-glass thermometer (c) electronic temperature sensor Mercury / Alcohol Thermometer Diagram Characteristics: These thermometers are commonly used in the school laboratory. Can measure temperature from -10oC to +110oC. Each division on the scale of this thermometer is 1oC. Temperature readings are recorded to the nearest 1 d.p. (e.g. 27.0oC, 35.5oC). Data Logger Diagram Characteristics: A temperature sensor can be connected to a data logger to measure temperature. This gives more accurate readings of temperature than a mercury or alcohol thermometer. Useful for recording temperature changes continuously over a period of time. Commonly used for taking measurements outdoors. 12 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science (E) MEASURING TIME Units for measuring time: S.I. unit : second (s) other units : minute (min) / hour (h) Apparatus for measuring time: stopwatch [Time readings are recorded to the nearest second.] (F) SUMMARY physical quantities units apparatus remarks gram (g) beam balance recorded to the mass kilogram (kg) electronic balance nearest 0.01 g 1 m3 measure up to = 1 000 dm3 50.00 cm3, burette = 1 000 000 cm3 accurate to 0.05 cm3 very accurate set volumes, such as pipette exactly 20.0 cm3 liquid or 25.0 cm3 volume more accurate measuring than beaker, but cylinder accurate only to the nearest cm3 approximate beaker volume of 100 cm3 or 250 cm3. gas gas syringe degrees Celsius (oC) thermometer recorded to the temperature temperature sensor nearest 1 d.p. second (s) recorded to the time stopwatch minute (min) nearest second 13 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science Example 12: Name the pieces of apparatus best used to carry out the following procedures. (a) Measure the melting point of a solid. : thermometer (b) Add 250 cm3 of liquid to a beaker. : measuring cylinder (c) Collect and measure the volume of a water-soluble gas. : gas syringe (d) Add 17.30 cm3 of solution to a flask. : burette Example 13: Name the pieces of apparatus most suitable to complete the following laboratory actions: (a) measure exactly 22.70 cm3 of solution into a beaker, : burette (b) collect and measure the volume of a water-soluble gas, : gas syringe (c) add exactly 25.0 cm3 of solution to each of several beakers. : pipette 14 Queenstown Secondary School 5086 Topic 1.1: Experimental Design Department of Science