Researching chemistry

• Taring


  When an empty vessel (e.g. beaker, weighing boat) is placed on a balance, the balanced can then be "tared" (set to zero) such that the reading is only the substance being measured
• Weighing by difference


  Weighing by difference is when the empty vessel (e.g. beaker, weighing boat) is weighed, substance added and then weighed again. The mass of the substance is the difference between the two
• Gravimetric analysis by precipitation/filtration

  • The product must have a low solubility so that all the product is precipitated
  • The product must have a particle size large enough for filtration
  • The product must be stable at temperatures of 100-105°C so that it can be dried in an oven
• Gravimetric analysis


  The mass of an element or compound present in a substance is determined by changing that substance into another of known chemical composition that can be readily isolated, purified and weighed
• Gravimetric analysis by precipitation/filtration

  1. Equation

  2. Reaction to proceed to completion
• The accuracy of gravimetric analysis depends upon the dexterity of the person carrying out the procedure
• Gravimetric analysis by heating


  1. Weigh empty crucible

  2. Add sample and weigh

  3. Heat gently for 2 minutes

  4. Heat strongly for 10-15 minutes

  5. Allow to cool in desiccator and reweigh

  6. Repeat heating, cooling and weighing until constant mass
• Standard solution


  A solution of accurately known concentration
• Primary standard


  • High purity (>99.9%)
  • Stable in air and in solution (allows storage)
  • Reasonably high formula mass (reduces error)
  • Readily soluble (usually in water)
• Sodium hydroxide is unsuitable as a primary standard as it is hygroscopic
• Control


  A check to make sure the technique you're using works
• Blank


  A common aspect of spectroscopic techniques such as IR Spectroscopy and colourimetry, used to measure the absorbance of the solvent alone
• Complexometric titration


  Titrations based on the formation of a coloured complex by a transition metal ion, using EDTA as the titrant
• Complexometric determination of nickel using EDTA

  1. Make up nickel ion solution

  2. Titrate nickel solution with EDTA using murexide indicator
• Back titration


  A technique used to find the number of moles of a substance by reacting it with an excess volume of reactant of known concentration, then titrating the excess reactant
• Back titration of aspirin

  1. Treat aspirin with sodium hydroxide

  2. Titrate excess sodium hydroxide with sulfuric acid
• Colorimetry


  Uses the relationship between colour intensity of a solution and the concentration of the coloured species present
• Using a colorimeter or visible spectrophotometer

  1. Prepare a calibration curve using standard solutions

  2. Determine concentration of unknown solution from its absorbance and calibration curve
• Distillation


  A technique used for the identification and purification of organic compounds, based on their well-defined boiling points
• Distillation is used to purify a compound by separating it from impurities based on differences in boiling points
• Hydrolysis of aspirin


  1. Hydrolysis to ethanoic and salicylic acids

  2. Neutralisation of acids

  3. Excess alkali used

  4. Amount of remaining alkali determined by titration against sulfuric acid
• Colorimetry


  Relationship between colour intensity of a solution and the concentration of the coloured species present
• Using a colorimeter or visible spectrophotometer

  1. Prepare a calibration graph

  2. Determine concentration of 'unknown' solution from its absorbance and calibration curve
• Distillation


  • Powerful tool for identification and purification of organic compounds
  • Boiling point is a physical property used for identification
• Distillation


  Purify a compound by separating it from non-volatile or less volatile material
• Refluxing


  • Technique to apply heat energy to a chemical reaction mixture over an extended period
  • Vapours given off are immediately returned to the reaction vessel as liquids when they reach the condenser
• Using vacuum filtration methods

  1. Buchner, Hirsch or sintered glass funnel

  2. Carried out under reduced pressure

  3. Faster means of separating precipitate from filtrate

  4. Choice of filtering medium depends on quantity and nature of precipitate
• Recrystallisation


  • Laboratory technique used to purify solids, based upon solubility
  • Solvent carefully selected so impure compound is insoluble at lower temperatures
  • Impurities left behind in solvent
• Theoretical yield


  Expected quantity of product from known mass of reactant, calculated from balanced equation
• Actual yield


  Quantity of product actually obtained, usually less than theoretical yield
• Reasons actual yield is less than theoretical yield

  • Reaction has not gone to completion
  • Other competing reactions may have occurred
  • Separation of desired product may be difficult
  • Product may be impure or some lost on purification
• Percentage yield


  Actual yield / Theoretical yield x 100%
• Industrial processes require high percentage yield and high purity of product, so unconverted reactants are often recycled for further reaction
• Thin Layer Chromatography (TLC)

  • Similar to paper chromatography but uses a fine film of silica or aluminium oxide as stationary phase
  • Solvent is the mobile phase
  • Rf values calculated same as paper chromatography
  • Colourless spots can be detected using fluorescent substance or spraying with chemical
• Thin Layer Chromatography


  Used to determine purity of substance - a pure product will only show one spot
• Chromatography


  Separation process that depends on partition of substances between stationary and mobile phases
• Paper Chromatography


  • Sample mixture spotted on paper, solvent drawn up paper as mobile phase, water in paper is stationary phase
  • Separation depends on solvent, solubility, and partition between mobile and stationary phases
• Rf value


  Distance travelled by component / distance travelled by solvent
• Gas-Liquid Chromatography


  • Stationary phase is high boiling point liquid held on inert solid support
  • Mobile phase is a gas, usually carrier gas
  • Separation occurs as sample passes through column, detected at other end
  • Retention time used to identify components
• Melting point


  • Physical property used to identify organic compounds
  • Presence of impurities lowers melting point and extends melting temperature range