AQA A Level Chemistry Year One - Required Practical’s Pt 1 Questions and Answers

AQA A Level Chemistry Year One - Required Practical’s Pt 1 Questions and Answers 1a) Describe how you should weigh out a required mass given a sample bottle 1a) Weigh the sample bottle to 2dp Transfer mass to beaker Reweigh sample bottle Record difference in mass 1a) Describe how you should make up a volumetric solution from a weighed out mass 1a) Add 100cm3 of distilled water to the beaker Stir with a glass rod to dissolve the solid Pour solution into a 250cm3 gradated flask via funnel Rinse funnel, beaker and glass rod Add washings to volumetric flask Using a dropping pipette of distilled water, make solution up to the mark Invert flask several times 1a) Why must the volumetric flask be inverted several times? 1a) To ensure a uniform concentration of solution 1a) Why is the beaker sometimes heated when making up a volumetric solution? 1a) Substance may not dissolve well in COLD water 1a) To which point should a volumetric flask be filled? 1a) The bottom of the meniscus sits on the line 1a) What is the problem with making up a volumetric solution of e.g. KMnO4 ? 1a) Dark coloured Difficult to see meniscus 1a) Why must hot solutions not be put in a graduated flask? 1a) The flask would expand The volume would be incorrect 1b) Describe how you would set up the equipment for an acid-base titration 1b) Rinse all equipment with distilled water and solutions that are to be added to them Pipette 25cm3 of alkali into conical flask Touch surface of alkali with pipette Fill burette with acid Add a few drops of indicator 1b) Describe how you would carry out an acid-base titration (equipment already set up) 1b) Note the burette reading Add acid to alkali and swirl conical flask Near the end point add the acid dropwise Note the burette reading Repeat titration until 2 concordant results 1b) What are concordant results? 1b) 2 readings that are within 0.1 of each other 1b) Why must you touch the surface of the pipette with the solution you are filling it with? 1b) To ensure the correct amount is added The pipette calibration assumed all the pipette is filled (no air gaps) 1b) Why must you only add a few drops of indicator? 1b) Usually weak acids Too many drops affect titre 1b) Why must you take the funnel out of the burette before a titration? 1b) Small drops of the liquid might fall from the funnel into the burette solution Lead to a smaller titre (inaccurate) 1b) Why might distilled water be added to the conical flask during a titration Does this affect the titre? 1b) To wash any acid/alkali on side of flask into solution so that they react No as water does not react or change number of moles 1b) If 2 or 3 titre values are concordant, what can be said about the titration experiment? 1b) It is repeatable and accurate Technique is good and constant 1b) To how many decimal places should burette readings to be taken to? 1b) 2dp Burette has intervals of 0.1 so readings either 0.00 or 0.05 1b) Why must you make sure the jet space in the burette is filled with acid? 1b) Leads to errors if it fills during titration Larger titre needed (inaccurate) 1b) Why must the burette be rinsed with the acid? 1b) So that it is not diluted by residual water So that it does not react with left over substances Concentration of substance would be lower Larger titre needed (inaccurate) 1b) Give the indicators with the pH range of the equivalence points and their colour change from acid to alkali 1b) Thymol blue 1.2 - 2.8 Red - Yellow Methyl orange 3.1 - 4.4 Red- Yellow Methyl red 4.4 - 6.2 Red- Yellow Bromothymol blue 6.0 - 7.6 Yellow - Blue Phenolphthalein 8.3 - 10 Colourless to Pink 1b) Why are conical flasks used for titrations? 1b) They are easier to swirl without spilling the contents 1b) Why is a white tile used underneath the conical flask in titrations? 1b) To help observe colour changes 1b) What are titrations used for? 1b) To work out the concentration of a substance e.g in quality control of batches of chemicals 1b) True or False If a solution contains more than one substance with acid/base properties, the concentration of each substance cannot be measured with certainity 1b) True You wouldn't be able to tell which substance (or if both) were reacting 2) Describe how to prepare the equipment for measuring the enthalpy change of a reaction 2) Rinse all equipment with solution to be used and dry Put polystyrene cup into beaker Measure out volume of solution with pipette and add to cup Clamp thermometer in place 2) Describe how you would calculate the enthalpy change of a reaction (equipment already prepared) 2) Measure (initial) temperature for 2 -3 minutes At 3 minutes Add 2nd reagent and stir Record temperature from 4 minutes onwards Plot a graph of results Draw lines of best fit for 0-3 and 4+ minutes 2) Why is there a problem if the reaction is too slow? 2) Cooling occurs simultaneously to reaction Temperature rise inaccurate 2) What are the possible errors in this experiment? 2) Energy transfer with surroundings (heat loss) Approximation of specific heat capacity (all solutions may not have same SHC as water) Neglecting SHC of calorimeter Incomplete or slow reaction If density of solution is NOT the same as water 2) Why is the initial temperature of the solution taken for 3 minutes? 2) Can take an average temperature More reliable 2) How do you counteract the problem of a slow reaction? 2) Take temperature at regular intervals Extrapolate back to find temperature rise on addition 2) Why must the solution be stirred? 2) Ensure all solution is the same temperature 2) Why must the polystyrene cup be placed in a beaker? 2) For insulation and support