PRACTICAL METHODS Flashcards
enthalpy change practical - enthalpy change when A is dissolved in B
• measure 50cm3 of B using a burette/pipette
• transfer to a polystyrene cup with a lid (provides insulation to minimise heat loss)
• measure 4.00g of A into a weighing boat using an accurate (e.g. 2dp) balance
• use a precise thermometer to take the temperature of B each minute for 4 minutes (to establish an accurate initial temperature)
• pour A into B at minute 5
• reweigh the weighing boat to determine the accurate, exact mass transferred
• replace lid (to insulate) and stir
• continue to record temperature at minute intervals up to 15 minutes
• plot temperature (y-axis) against time (x-axis) to obtain two lines of best fit
• extrapolate the lines back to the point of addition to determine ΔT
• calculate q using q = mcΔT (m is the mass of B)
• ΔH = q (in kJ) / moles of A
disappearing cross practical - investigating the effect of temperature on initial rates by reacting sodium thiosulfate and dilute HCl
• use water baths to heat separate samples of 10cm3 of sodium thiosulfate and 1cm3 of HCl to 25°C, 35°C, 45°C, and 55°C
• put water bath over the cross
• add first 25°C sample of HCl to first 25°C sample of sodium thiosulfate and start timer
• record time taken for the cross to disappear
• repeat experiment using water baths of 35°C, 45°C, and 55°C, keeping volumes and concentrations the same
• calculate 1/time for each temperature (as a surrogate measure of rate)
• plot 1/time (y-axis) against temperature (x-axis)
distillation of organic compounds practical to separate A from a mixture of A and B
• A and B have different boiling points (due to IMF)
• transfer reaction mixture into a round-bottom flask with a side arm
• add anti-bumping granules (to prevent large bubbles forming)
• clamp round-bottom flask over electric heater (safer than naked flame as organics are flammable)
• add bung with integrated thermometer that sits at the height of the side-arm, ensuring the bung fully seals the flask to allow the collection of A
• attach condenser to side-arm (water in at the bottom, out at the top)
• apply heat using electric heater
• collect the distillate when thermometer shows boiling point of A
iodine clock continuous rates practical to determine order - adding varying concentrations of A, which has iodide ions to B, which has hydrogen peroxide, and starch solution
• add 5cm3 of B and 1cm3 of starch to beaker
• add 5cm3 of A and start timer
• record time taken for blue colour to appear
• repeat experiment with varying known concentrations of A but keep concentrations and volumes of B and starch the same, and controlling temperature using a water bath
• calculate 1/time for each concentration of A (as a surrogate measure of rate)
• plot a graph of rate (y-axis) against concentration of A (x-axis)
• if when [A] doubles, rate is unchanged, then zero order
• if when [A] doubles, rate doubles, then first order
• if when [A] doubles, rate x4, then second order
gas syringe rate practical
• use burette to measure 50cm3 of hydrogen peroxide into a conical flask
• use high precision balance to measure out to measure 1g of manganese dioxide (catalyst) - significant excess so concentration is effectively constant
• prepare bung with delivery tube attached to gas syringe
• add catalyst to flask, insert bung, and start timer
• record volume of gas produced every 10 seconds until syringe is full
• molar ratios from equation, calculate the concentration of hydrogen peroxide for each time point
• plot concentration of hydrogen peroxide (y-axis) against time (x-axis)
• draw tangents at each time point to find rates at each concentration
• plot rate (y-axis) against concentration (x-axis)
purifying aspirin
• (filter/decant to remove from reaction mixture)
• dissolve crude product in minimum volume of hot solvent
• filter hot through fluted fluted filter paper (to remove insoluble impurities)
• slowly cool until aspirin recrystallises (by transferring vessel to ice bath)
• filter under reduced pressure using Buchner funnel (speeds up filtering)
• wash with cold solvent
• dry
TLC to separate a mixture of amino acids
• wear gloves to prevent contaminating the plate
• draw pencil line parallel to the bottom of the plate, 1.5cm from the bottom
• use a capillary tube to add a small spot of the mixture to the pencil line
• allow plate to completely air dry
• add <1.5cm depth of solvent to the tank
• place TLC plate in the tank, ensuring solvent does not reach the pencil line
• replace lid of tank to prevent solvent dissolving
• remove plate when solvent front is ~1cm away from the top of the plate
• mark position of solvent front with a pencil line
• dry the plate in a fume cupboard (solvent is toxic)
• visualise plate under a UV lamp and mark the locations of the amino acid spots
• calculate the Rf value for each spot (distance spot travelled / distance solvent travelled)
• compare Rf values to data book to identify the amino acids present in the mixture
