Researching Chemistry Flashcards
weighing by difference
mass of the crucible is measured before adding the substance and the final mass of the substance is determined by subtracting the mass of the crucible from the original mass
heating to constant mass
process of heating the substance, allowing it to cool in a desiccator (dry atmosphere) and then reweighing it all in a crucible
gravimetric analysis
- mass of the analyte present in a substance is determined by changing that chemical substance into solid by precipitation of known chemical composition and formula
- precipitate needs to be readily isolated, purified and
weighed, easier to remove the analyte by evaporation - final product has to be dried completely which is done by “heating to constant mass”- crucible lid should be left partially off to allow the water to escape, blue flame should be used to avoid build up of soot on the outside of the crucible which could affect the mass, heating should be started off gently and then strongly
- sample is weighed by difference
- process of heating, drying and weighing is repeated until a constant mass is obtained- shows that all the water has been driven off
characteristics of primary standard solution
- high state of purity
- stability in air and water
- solubility
- high GFM
Preparation of standard solution
- calculate the mass of the primary standard required to make the concentration of solution required in the appropriate volume of solution
- make sure glassware is clean
- place beaker on 2 d.p. balance and tare it, accurately weigh out and record approximately the mass of solid requred
- add distilled water to beaker and stir until all of the solid dissolves and transfer solution to a volumetric flask using a funnel
- take 2/3 rinsings of the beaker and funnel to ensure all of the chemical is transferred into the volumetric flask
- add distilled water up to 2cm3 short of graduation mark on flask and top up until bottom of meniscus is touching mark using wash bottle
- stopper and invert flask 2/3 times to completely mix
controls
validates a technique and may consist of carrying out a determination on a solution of known concentration
Complexometric titration
form of volumetric analysis in which the formation of a coloured complex is used to indicate the end point of a titration, particularly useful for the determination of a mixture of different metal ions in solution
back titrations are used if
- analyte is insoluble
- reaction between analyte and standard solution is slow
- end point of direct titration is unclear or there is no suitable indicator
back titration process
known excess amount of standard solution is added to the analyte until it all reacts, excess amount can then be titrated with another standard solution to work out how much hadn’t reacted, knowing how much was added and how much is left unreacted we can work out how much reacted with the analyte
colorimetry
used to determine conc of coloured substances, the more concentrated the solution, the greater the absorbance of light
construction of calibration curve
preparing solutions of various concentrations (standard solutions) and recording their absorbance values, straight line section of calibration curve should cover the dilution range likely to be used in determination
distillation
process of heating a liquid or mixture until boiling and then collecting and cooling the resultant vapours
uses of distillation
to purify a compound by separating it from a non-volatile or less volatile material
to identify compounds because the boiling point is a well defined physical property
steam distillation
distillation for temperature sensitive materials like
natural aromatic compounds
fractional distillation
separation of a mixture into its component parts (fractions)
refluxing
technique used to apply heat energy to a chemical reaction mixture over an extended period of time without volatile reactants/ products escaping into the atmosphere
process of refluxing
The liquid reaction mixture is placed in a round-bottomed flask along with anti-bumping granules with a condenser connected at the top. The flask is heated vigorously over the course of the chemical reaction; any vapours given off are immediately returned to the reaction vessel as liquids when they reach the condenser.
vacuum filtration
can be carried out using a Buchner, Hirsch or sintered glass funnel, carried out under reduced pressure and provide a faster means of separating the precipitate from the filtrate
recrystallisation
laboratory technique used to purify solids based on solubility
process of recrystallisation
- impure solid is dissolved in minimum volume of an appropriate hot solvent and the mixture is heated, solvent is added until solid fully dissolves
- any insoluble impurities are removed by hot filtration (filter funnel and conical flask are heated before use)- prevents solvent mixture from cooling prematurely and forming crystals during filtration
- as the solution cools down, the product crystallises out and impurities remain dissolved in solvent
- once mixture is crystallised, it is filtered
solvent for recrystallisation must be
insoluble at lower temperatures but completely soluble at higher temperatures
solvent extraction
method of extracting a crude product from a reaction mixture based on relative solubility of a compound
solvent for solvent extraction requirements
- solute must be more soluble in it than the reaction mixture
- must not react with solute
process of solvent extraction
The two solvents form two separate layers in the separating funnel and the lower layer is run off into one container and the upper layer is poured out into another container. The quantity of solute extracted depends on the partition coefficient and on the number of times that the process is repeated.
risk of solvent extraction
pressure build up but can be handled by opening the stopped at the top of the funnel routinely while mixing
thin layer chromatography
can be used to assess product purity- pure substances will show up as one spot, more than one spot indicates impurities
to determine identity of a substance
to separate mixtures of substances (faster than paper chromatography)
stationary phase in TLC
a fine film of silica or aluminium oxide spread over glass or plastic
process of TLC
When setting up a TLC plate, a pencil line is drawn, usually 1 cm up from the bottom of the plate. The sample solution is then spotted on several times to get a concentrated spot on the plate. The plate is then placed in a suitable solvent (which acts as the mobile phase) making sure that the solvent is below the level of the spot. The solvent then travels up the plate and the components of the sample separate out according to their relative attractions to the stationary phase on the plate and the mobile phase
rf value equation
distance travelled by spot/ distance travelled by solvent
why can determination of melting point give an indication of the purity of a substance?
the presence of impurities lowers the melting point and extends its melting temperature range
crystalline substance melting point
sharp falling between narrow temperature range
mixed melting point determination
used as a means of identifying the product of a reaction
mixed melting point process
the product can be mixed with a pure sample of the substance and a melting point taken of the mixture
If the melting point range is lowered and widened, it means that the two are different compounds. If the melting point stays the same it means that
the two compounds are likely identical.
