practicals

• what is a standard solution?

  a standard solution is a solution of known concentration.
• how do you make a standard solution?

  • measure, using a balance, the mass of solid required. transfer this to a volumetric flask and rinse the remaining weighing bottle content (with distilled water) into the flask so no solid is lost.
  • add a volume of distilled water to dissolve the solid. swirl to mix.
  • then add more distilled water up to the line on the volumetric flask.
  • invert multiple times to mix.
• what is the weighing by difference technique?

  it is a method to weigh materials accurately

  • mass of substance = mass of weighing dish and substance - mass of dish after substance has been transferred
• what equipment is used to carry out a titration?

  • a pipette and pipette filler are used to accurately measure out the volume of a reactant before transferring it to a conical flask.
  • a burette is used to add small volumes of one reactant to the other reactant until the reaction has reached completion (end point).
• how do you carry out a titration?

  • once the pipette has been used to place one reactant into the conical flask, fill the burette with the other reactant.
  • record the initial volume then add a few drops of indicator to the conical flask.
  • open the burette tap and allow the reactant to flow into the conical flask, swirling to mix the contents.
  • close the burette tap once the expected colour change occurs (use a white tile so the colour change is easy to identify).
  • record final burette volume. repeat until you get concordant results, then calculate a mean titre.
• why are acid-base indicators used?

  to detect when a reaction reaches completion/becomes neutral, usually by the presence of a colour change.
• what are concordant results?

  titres that are within 0.1cm3 of each other.
• what are some common hazards and risks in the laboratory?

  hazard: bunsen burner

  • risk: burns
  • control: keep away from flammable chemicals and away from the edge of the desk

  hazard: chemicals

  • risk: irritant, corrosive , toxic, flammable
  • control: wear gloves and goggles, don't ingest

  hazard: glassware, i.e. beakers, test tubes

  • risk: may break and cause cuts
  • control: handle with care, keep away from the edge of the desk
• what is Hess's Law?

  the enthalpy change for a chemical reaction is always the same, regardless of the route from reactants to products.
• how can you reduce the uncertainty in the mass measurement?

  use a balance with a greater resolution, or use a larger mass
• what is percentage (%) uncertainty and how do you measure it?

  % uncertainty = uncertainty/calculated value x 100
• how do you calculate enthalpy change of reaction experimentally?

  • q=mcΔT
  • q is in joules, convert to kJ (divide by 1000)
  • ΔH=q/mol
• why may an experimental value for enthalpy change be different to the theoretical value?

  • heat loss to apparatus/surroundings
  • incomplete combustion
  • non-standard conditions
  • evaporation of alcohol/water
• how do you prevent heat loss to surroundings/apparatus?

  • insulate the beaker by placing it in a polystyrene cup with a lid
  • avoid large temperature differences between surroundings and calorimeter
  • use a bomb calorimeter
• other than preventing heat loss, how can the accuracy of this experiment be improved?

  • read the thermometer at eye level to avoid parallax errors
  • stir the solution so the temperature is evenly distributed
  • use a digital thermometer for more accurate and faster readings
  • use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty
• what is accuracy?

  the more accurate the data, the closer it is to the actual value.
• how can rate of reaction be measured?

  • initial rates method (i.e. iodine clock reaction)
  • a continuous monitoring method (i.e. measuring the volume of gas released in a reaction over time)
• why does the rate of reaction increase with increasing temperature?

  • as temperature increases, the kinetic energy of the molecules increases. this means more molecules have E≥Ea, and can therefore react.
  • also as kinetic energy of molecules increases, their velocities increase resulting in more frequent and successful collisions
• how would you investigate how rate of reaction varies with temperature?

  time how long it takes for a cross underneath a breaker to disappear, using a stopwatch. repeat this at 5+ different temperatures and compare times taken.
• what are some issues with the disappearing cross method?

  • the disappeared cross is judged by eye and can therefore be unreliable
  • there may be a delayed reaction time with the stopwatch
  • at low temperatures the reaction may be too slow
  • there may not be enough precipitate for the cross to disappear
• what is a control variable?

  a variable that is kept constant throughout the experiment.
• in an experiment investigating the effect of temperatures on rate of reaction, what variables will you control?

  • concentrations of reactants and products
  • volumes of reactants and products
  • use the same beaker and cross underneath
  • use the same person to decide when the cross has disappeared
• how can we keep a substance at a constant temperature?

  using a water bath.
• how do you test for group 2 metal cations?

  • place 10 drops of a group 2 compound in a test tube (0.1mol dm-3)
  • add 10 drops of 0.6mol dm-3 NaOH to test tube, and record any observations
  • continue to add NaOH so it is in excess, and record any observations
  • repeat with other group 2 compounds
• what are the results from these reactions?

  results:

  
• what are the results of group 2 compounds with sulfuric acid?


  results:

  
• what are the overall reactions for the group 2 reactions?

  results:

  
• how do you test for ammonium ions?

  • place 10 drops of NH4Cl into a test tube
  • add 10 drops of NaOH solution, and shake
  • warm the solution in the test tube
  • test the gas released with damp red litmus paper
  • if it goes blue, ammonium ions are present
• how do you test for group 7/halide ions?

  results:

  A) add nitric acid first

  
• how do you test for hydroxide (OH-) ions?

  • test a 1cm depth of solution in a tea tube with red litmus paper or universal indicator paper
  • record observations
  • sodium hydroxide will turn red litmus paper blue
• how do you test for carbonate (CO3 2-) ions?

  • put 2cm3 of Ca(OH)2 into a test tube
  • add 3cm3 Na2CO3 (0.5mol dm-3) in another test tube then add an equal volume of dilute HCl (1.0mol dm-3)
  • immediately put in a delivery tube with open end into the Ca(OH)2 test tube
  • if Ca(OH)2 goes cloudy, carbonate ions were present in the other test tube
• how do you test for sulfate (SO4 2-) ions?

  • add HCl and BaCl2 to the suspected sulfate solution
  • if sulfate ions are presents, a white ppt of BaSO4 will form
• what is the order of testing ions?

  carbonate -> sulfate -> halide

  • this prevents false positive results occurring, i.e. unexpected insoluble ppt's such as Ag2SO4, Ag2CO3, and BaCO3 could form
• what is distillation?

  distillation is a technique where a liquid is heated to create a vapour which is cooled by a condenser, causing the gas to condense into a liquid and drip into a separate flask. the species separate due to relative volatilities and boiling points.
• what equipment is used for distillation?

  distillation is either done with:

  • semi-micro distillation apparatus
  • quick fit apparatus (pear-shaped/round-bottomed flask with a Liebig condenser, still head, stopper, receiver adaptor fitted with a thermometer and collection vessel)
• what does a diagram of quick fit apparatus set up for distillation look like?


  diagram:

  
• oxidising agents are often used in distillation, what is an oxidising agent?

  • oxidising agents oxidise other chemicals by receiving electrons from them, in turn they are reduced
  • AKA: electron acceptor
• why are anti-bumping granules used in distillation?

  to allow smooth boiling by preventing the formation of large bubbles caused by vapour.
• how do you isolate a liquid product?

  you can use a separating funnel to isolate the organic layer from the aqueous layer. shake the flask. allow the layers to separate. then open the tap and run off one layer into a separate container.
• how do you test for an alcohol?

  • add 1cm3 of alcohol to a test tube, add a small piece of sodium
  • presence of an alcohol is indicated by a white ppt and effervescence