AQA A-Level Chemistry - Practical’s Questions and Answers

AQA A-Level Chemistry - Practical’s Questions and Answers (1) Why do we use a white tile? To see indicator colour change better. (1) Why is a conical flask used? Can be swirled without losing any solution. (1) Why is swirling necessary? To ensure that solutions mix and so react completely. (1) Why does rinsing the flask with water not affect the end point? Water does not react --- doesn't affect number of moles present. (Other 1) Outline how to carry out a mass change experiment. 1. Crucible weighed on a balance and its mass is recorded (mass 1). 2. Some sample placed in the crucible. 3. Crucible and contents reweighed on a balance and its mass recorded (mass 2). 4. Sample heated strongly in Bunsen. 5. Crucible and contents reweighed on a balance and its mass recorded. 6. The sample heated and reweighed until mass no longer changed --- ensures reaction completed. (Other 1) Outline apparatus - diagram. Include: - Bunsen. - Tripod. - Pipe-clay triangle. - Crucible. - Lid. - HEAT. (Other 1) What things would we use mass change experiments for? 1. Formulae determination. 2. Water of crystallisation. 3. Decomposition. (Other 1) How can we ensure that the (mass change) experiment has gone to completion? Heat until constant mass. (2) Outline how to carry out a practical to measure Enthalpies of Combustion. 1. Fuel is burnt and the energy released is used to heat a set volume of water. 2. Fuel continues to burn until the temperature of water rises by a set change (20C etc.). 3. Mass of fuel used then measured using a balance. 4. Calculation. (2) What are the main errors in the experimental procedure when measuring Enthalpies of Combustion? How can we reduce heat loss? 1. Heat loss to surroundings (air/flask) 2. Incomplete combustion. 3. High temperature changes lead to evaporation of water. = Heat loss can be reduced by adding lagging to the flask. (2) Outline how to carry out a practical to measure Enthalpies of Neutralisation/Displacement. 1. Solution is measured out and placed in a polystyrene cup. 2. Initial temperature of solution recorded for a few minutes before second reagent is added (allows temperature to equilibrate). 3. Set, known quantity (mass/volume) of second reagent is added into the solution at fourth time. 4. Temperature recorded every minute until some cooling observed. NB= Polystyrene cup used to reduce heat loss. (2) How can we use a graph to calculate the temperature change (for Neutralisation/Displacement experiments)? 1. Plot a graph of temperature (y) against time (x). 2. Before the addition of reagent to solution (0-4 mins): - Join points for first 3 minutes with a straight line. - Extrapolate (extend) line to 4th minute. 3. After addition of reagent to solution (4-12 mins): - Draw a line of best fit through the points between the 5th and 12th minutes and extrapolate (extend) this line to 4th minute. = Temperature change can be measured - distance between 2 lines. Show it clearly on graph. (2) How can we limit heat loss for practicals measuring Enthalpies of Neutralisation/Displacement? What is the main source of apparatus error - how can this be reduced? 1. Lagging in polystyrene cup. 2. Add a lid to the polystyrene cup. Main source of apparatus error = thermometer --- can reduce this by increasing the temperature change. --- can increase temperature change by using a lower volume but higher concentration of solution. (3) Outline how to investigate how the Rate of Reaction changes with temperature for reactions with gas products. 1. Rate can be determined by measuring amount (volume/mass) of gas produced at regular time intervals (every 10/20s etc.). 2. Plot graph of volume/mass against time. 3. Rate calculated by determining gradient of a tangent to graph. = rate would have units based on the units of the axes on the graph. (3) Outline how to investigate how the Rate of Reaction changes with temperature for reactions with NO gas products. 1. Rate determined by measuring time taken for a set amount of product to be formed. (in this method Rate = 1/T --- units s-1.) 2. Equal volumes of reactants used in each experiment to ensure time recorded is for same amount of product each time. 3. If ppt formed: - Reaction can be timed until enough ppt formed to obscure a cross on paper. If coloured product is formed: - Another reagent often added to reaction mixture which reacts with product as it is formed to produce a colourless substance. - When this additional reagent runs out, a colour change is observed = reaction then timed until the colour change is observed. = When investigating effect of temperature, the reaction mixture is heated to required temperature - could be using a water/sand bath. (3) Which other variables should be controlled? 1. Total volume 2. Concentration. 3. Surface area of any solid reactants. (3) What are some limitations? 1. Enthalpy change of reaction will affect temperature at which reaction is completed. 2. At high temperatures, difficult to measure rate as water will begin to evaporate. 3. At high temperatures, time for reaction is very short --- increases degree of uncertainty in the timer. 4. Human reaction times will affect the number of decimal places that can be recorded accurately if the times are measured manually (4) In ion tests, why do we add nitric acid before the reagent? - Reacts with other anions (and so removes them from the solution) --- may also produce a ppt such as CO3 2- /O 2- / OH-. - Many carbonates, hydroxides and oxides are insoluble so would also form a ppt (except those of group 1 metal ions). (5) Under what circumstances can we distil a product from a reaction? - Can distil a product from a reaction if it has a lower boiling point than the reactants. (5) Why does a reaction mixture containing an aldehyde product need to be frequently cooled in an ice bath? - To prevent vaporisation of aldehyde product from the test-tube/boiling tube. (5) Why are some organic reactions completed under reflux? = Can apply constant heat to a mixture without losing any of the liquid to evaporation - way of getting reaction to go to completion. = Many organic reactions need to be heated as this supplies the activation energy to break bonds and allow the reaction to occur. ^ Using a condenser allows this to happen whilst preventing the reagents from leaving the reaction vessel (they would otherwise evaporate). (5) How does Reflux work? 1. Reactants (and products) can be heated together constantly, over a period of time. 2. Components of reaction mixture don't escape from the reaction vessel (as heating causes them to evaporate/boil). 3. Vapour formed is cooled by the condenser --- condenses then falls back into the flask. (5) Draw reflux apparatus. Include: - Liebig condenser. - COLD water in (bottom right). - COLD water out (top left) - Pear-shaped flask. - Anti-bumping granules. - Open top. - HEAT. (5) Why do we use anti-bumping granules? - Prevent bumping (sudden release of a large bubble of vapour --- makes reaction mixture jump up the vessel). - Anti-bumping granules produce many small bubbles, rather than a large one --- prevent uneven boiling. (5) Why does reflux top need to be kept open? = Pressure (due to gas) can build up and could burst the glassware --- keeping the top open allows pressure release. (5) How can we heat reaction mixtures. 1. Bunsen Burner - although should be avoided if flammable reactants/products present. 2. Water bath - good for gentle heating but water will boil at 100C. 3. Sand bath - can achieve higher temperatures than a water bath. Often used in conjunction with a heating mantle. - Sand used to surround reaction vessel and conducts heat into vessel from sides as well = reduces need for stirring. (6) How can we test for a haloalkane? 1. Add aqueous silver nitrate. 2. Ppt forms (slowly - slow step is dependant on C-Hal bond being broken). 3. - Cl = white ppt. - Br = cream ppt. - I = yellow ppt. (7) Outline how to measure the Rate of Reaction by the initial rate method. 1. Rate determined by measuring time taken for a set amount of product to be formed. (in this method Rate = 1/T --- units s-1.) 2. Equal volumes of reactants used in each experiment to ensure time recorded is for same amount of product each time. 3. If ppt formed: - Reaction can be timed until enough ppt formed to obscure a cross on paper. If coloured product is formed: - Another reagent often added to reaction mixture which reacts with product as it is formed to produce a colourless substance. - When this additional reagent runs out, a colour change is observed = reaction then timed until the colour change is observed. or: 1. Can measure rate by monitoring the change in concentration of the substance over time. 2. Graph plotted and initial rate determined by tangent to curve at t=0. (7) Which variables should be kept constant? When would this method be often used? 1. Temperature. 2. Total volume. 3. Concentration. 4. Surface area of any solid reactant. = Often used to help determine the order of a reaction with respect to a reactant. (7) Outline how to measure the Rate of Reaction using a continuous monitoring method - Gas product. 1. Rate can be determined by measuring amount (volume/mass) of gas produced at regular time intervals (every 10/20s etc.). 2. Plot graph of volume/mass against time. 3. Rate calculated by determining gradient of a tangent to graph. = rate would have units based on the units of the axes on the graph. (7) Outline how to measure the Rate of Reaction using a continuous monitoring method - Quenching method. 1. Sample removed at regular time intervals from mixture and then quenched with another reagent. 2. Other reagent may be something that reacts with the catalyst, or dilutes the solution so the rate is effectively reduced to 0. 3. The mixture is then titrated to determine the number of moles present in the sample which was removed. (8) Under what circumstances would the cell EMF drop towards zero? = When the reaction reaches 0. (9) Outline how to investigate how the pH changes when an acid reacts with a base. pH meter method. 1. pH meters first calibrated as after being stored for a while, the meter does not give accurate readings. 2. Using a series of buffer solutions with known pH, we record pH meter reading. 3. Simple calibration cuve plotted - pH meter reading vs pH of buffer solution. 4. Once pH measured - the calibration curve is then used to calculate the true pH. 5. pH meters can also be calibrated by using a small dial on the meter (although this can be a little fiddly). (9) Outline how to investigate how the pH changes when an acid reacts with a base. Determination of Ka method. 1. Set volume of acid pipetted into a conical flask. 2. Alkali added 1cm3 at a time. 3. Mixture stirred between each addition. 4. pH measured and recorded after each addition. 5. As equivalence point approaches, the alkali should be added dropwise to allow for a more accurate determination of the equivalence point. 6. Once equivalence point passed, alkali is then added 1cm3 at a time until it is in excess. 7. Curve of pH (y) plotted against volume of alkali added (x) 8. Find volume added at equivalence point. 9. Halve this value to calculate the volume at half-equivalence. 10. Use the curve to read off the pH at this volume = value of pKa. --- at this point, calibration curve used to adjust meter pH to actual pH. (10) Outline practical procedure for the reaction to prepare a pure organic solid/liquid. 1. Reactants mixed in a suitable apparatus --- could be a beaker, conical flask, Quick-Fit apparatus. 2. If reactants are to be cooled, ice bath used. 3. If reactants need to be heated, reflux usually used (with anti-bumping granules) --- constant heating with no evaporation. 4. Reactions can be heated using water/sand baths or Bunsens - but they have limitations etc as previously mentioned. (10) Outline how to collect a SOLID product. 1. Solid collected by filtration from the reaction mixture. 2. Solid then separated by filtration under reduced pressure using Buchner apparatus. 3. Once solid is isolated, washed using a suitable solvent. 4. Solvent needs to be able to wash away or dissolve any impurities on the isolated product but the residue must not dissolve in the solvent. (10) Outline how to purify a SOLID product. = Purified by recrystallisation. 1. Crude product dissolved in minimum amount of hot solvent = creates a saturated solution at a high temperature - enables a high yield of product to recrystallise when solution cooled. 2. Solution filtered whilst still hot to remove any insoluble impurities (apparatus must be heated as crystallisation will occur if solution cools). 3. Saturated solution cools slowly to room temperature and then in ice --- product then crystallises out of solution while other impurities remain dissolved. = Cooling in ice allows an increased amount of crystals to be formed. However, if saturated solution cooled too quickly, impurities crystallise out as well. 4. Solid then isolated using Buchner apparatus and then washed with some cold solvent to remove any soluble impurities which may have crystallised out of solution.