4 Researching Chemistry Flashcards
what are precise results?
if experiments can be repeated several times and their values are numerically similar
what are repetable results?
if the same experimenter obtains these numerically similar values
what are reproducible results?
if a number of different experimenters obtain these numerically similar values
what is a systematic error?
an error repeated in each measurement made by the experimenter
what is a random error?
they are made by the experimenter
how can you reduce random errors in experiments?
repeated measurements and averaging
what are uncertainties?
uncertainty of measurements results from the scale or digital reading not being an exact or precise measurement
what is the equation for percentage error?
uncertainty/actual measurement made X 100
what are types of spectral data which can be organised?
absorption spectroscopy
mass spectroscopy
IR spectroscopy
portion NMR spectroscopy
why is a control experiment carried out?
to validate a technique or procedure is accurate
what does parts per million refer to?
1 mg per kg
or
1 mg per L
how many mg are in a kg?
1,000,000 mg = 1 kg
what is percentage by mass?
the mass of solute made up to 100cm3 of solution
what is percentage by volume?
the number of cm3 of solute made up to 100cm3 of solution
how is percentage by yield reduced?
mass transfer or mechanical losses
purification of product
side reactions taking place
equilibrium position
what can gravimetric analysis determine?
the mass of an element or compound in a substance
what are weighing boats?
used to measure substance on a balance without contamination
what is the function ‘tare’ on a balance used for?
allows the balance to be set to zero when an object is on top of the balance pan
what is weighing by difference?
the mass of an empty weighing boat is measured and the chemical is added to the boat and reweighed. the difference is the mass of the substance in the bottle.
what is weighing accurately approximately?
weigh within the region of the calculated mass, but record exactly what you used
heating to a constant mass steps
the substance is initially weighed and recorded
the substance is heated to remove moisture
the substance is allowed to cool in a desiccator
the substance has its mass measured on a balance
repeating the steps of weighing, heating, cooling and reweighing until constant mass is obtained
what is the purpose of a desiccator?
prevents reabsorption of water
what is a constant mass?
the mass is no longer changing
what are two types of gravimetric analysis?
precipitation
volatilisation
what qualities must a product have in order for precipitation and filtration to occur?
a low solubility so that all the product is precipitated
a particle size large enough for filtration
be stable at temperatures of 100-105 so it can dry in oven
how is precipitation and filtration carried out?
two solutions added together which form a solid
the filtrate is tested to ensure reaction has completed
filter the solid by heating to a constant mass
cool in a desiccator and weigh
how do you test to make sure a precipitation reaction has gone to completion?
add excess of one solution
how can you ensure a precipitation and filtration reaction is completed to a high standard through human action?
carry out with extreme care to ensure all material is transferred and all apparatus dried and weighed with extreme care
(any excess mass will change the calculation results)
what happens during a volatillisation reaction?
the substance is heated and any volatile products (often water) are evaporated causing a loss in weight.
the substance is heated to a constant mass and the final mass is recorded.
what is a standard solution?
a solution of accurately known concentration
how can standard solutions be made?
weighing a primary standard accurately
dissolving in a small volume of solvent (usually deionised water)
solution is transferred with rinsings to a volumetric flask
make up to the graduation mark with water, stopper and invert.
what is an alternative way to make a standard solution?
by accurate dilution
how is accurate dilution carried out
pipette an appropriate volume of a standard solution into a volumetric flask
make up to the graduation mark with solvent (deionised water), stopper and invert
primary standard properties
high state of purity
stable when solid and in solution
soluble
reasonably high GFM
examples of primary standards
sodium carbonate
hydrated oxalic acid
potassium hydrogen phthalate
silver nitrate
potassium iodate
potassium dichromate
why can sodium hydroxide not be used as a primary standard?
relatively low GFM
unstable when solid as it absorbs moisture
unstable as a solution
what must you do to sodium hydroxide before using it in volumetric analysis?
standardise it
what does standardising a solution do?
determines the exact concentration (molarity) of a solution
types of titrations
acid-base
redox
complexometric
redox titration
occurs between oxidising and reducing agents
complexometric titrations
based on reactions using complexometric reagents like EDTA to form complexes with metal ions to determine the concentration of metal ions
what are back titrations?
they are used to find the number of moles of a substance by reacting it with an excess volume of a reactant of known concentration.
when are back titrations carried out?
substance is a solid
substance has low solubility
dilution factor equations
DF = final volume / initial volume
final concentration = initial concentration / DF
colorimetry relationship
colour intensity of a solution and the concentration of the coloured species present
how do you know if a solution is coloured?
it absorbs some but not all parts of white light passing through it
what happens to the parts of white light which isn’t absorbed in colourimetry?
they are transmitted through the solution and combine to give the colour we see
how do you know what colour will be absorbed in colorimetry?
the complementary colour of the colour we can see
what does the intensity of the colour in colorimetry depend on?
the concentration of the solution
the more concentrated - the darker the colour
what is used to measure the absorbance of light of a series of standard solutions?
colorimeter
spectrophotometer
describe a cuvette
it is where the coloured samples are placed in a colorimeter
one side is clear and faces the light source
other side is ribbed and is used to pick up/down
what is the first step of analysing a compounds absorbance?
a reference or blank sample (normally distilled water) is analysed
what value should a blank give in colorimetry?
0
how do you decide the colour of the filter in colorimetry?
it should be complementary to the colour of the solution being analysed
how do you find the value of the absorbance for a sample in colorimetry after the experiment has occurred?
the difference between the absorbance of the coloured solution and the blank
what is needed in order to create a calibration graph?
a series of standard solutions prepared at different concentrations
measure their absorbance and plot it on the graph
what is essential when carrying out a calibration graph?
the concentration of the coloured species being tested must lie within the straight line section
what is distillation?
identifies and purifies organic compoundsh
how does distillation identify the substance being tested?
it can determine its melting point
how can distillation purify the substance being tested?
it can separate the substance form less volatile substances in the mixture
what is reflux?
a technique used to apply heat energy to a chemical reaction mixture over an extended period of time without volatile substances escaping
how is reflux set up?
draw diagram
liquid reaction mixture placed in a round bottom flask along with anti-bumping granules with a condenser at the top
check diagram
how is distillation set up?
draw diagram
check diagram
how does reflux work?
the flask is heated vigorously over the course of the reaction. any vapours which are given off are immediately returned to the reaction as liquids when they reach the condenser
what type of flasks does vacuum filtration use?
buchner
hirsch
sintered glass funnel
what does vacuum filtration do?
they are carried out under reduced pressure and provides a faster means of separating the precipitate from the filtrate
how do you choose an appropriate filter medium for vacuum filtration?
depends on the quantity and nature of the precipitate
what is recrystallisation?
used to purify solids
recrystallisation steps
the impure compound is dissolved gently in the minimum volume of hot solvent
hot filtration occurs to remove any insoluble impurities
the filtrate is allowed to cool slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
the crystals are isolated by filtration, washing with cold water and drying the pure crystals
conditions of the impure compound when choosing a solvent for recrystallisation
soluble at high temperatures
insoluble at lower temperatures
solvent qualities when choosing a solvent for recrystallisation
immiscible with the liquid or solution (usually water)
one in which the solute is more soluble in than the liquid mixture or solution
volatile to allow the solute to be obtained by evaporation of the solvent
unreactive with the solute
what does immiscible mean?
incapable of mixing
when do you use solvent extraction?
if the desired product is present in an aqueous mixture (water is in the solvent)
properties of the 2nd solvent in solvent extraction
must be immiscible with water (non-polar)
the solute is more soluble than water
volatile to allow the solute to be obtained by evaporation of the solvent
unreactive with the solvent
how does solvent extraction work?
ageous mixture and non-polar solvent are all placed in the funnel and vigorously shaken for several minutes
the desired product moves to the non-polar solvent
it is then ‘ran off’ from the funnel and collected
solvent evaporated leaving a pure sample
equipment used in solvent extraction
separating funnel
why is solvent extraction repeated?
to maximise the quantity of solute extracted
what is good practice in a solvent extraction?
using smaller volumes of the chosen solvent
if 50cm3 available - do two using 25cm3
how does equilibrium work in solvent extraction?
the desired product distributes itself between two layers which sets up an equilibrium
the concentration of the solute can be determined in both layers and the equilibrium constant K calculated
what is a the melting point of a substance?
the temperature range over which the solid first starts to melt, to when all of the solid has melted
how do you know if your compound is pure when finding its melting point?
it can be confirmed by comparing the experimentally determined melting point with a literature or known mp value
how does impurities effect a mp?
broades and lowers
caused by disruption in intermolecular bonding in the crystal lattice
what does mixing a pure product and some of your own product together do in mp determination?
mp value and the range of the melting temperature can be used to determine if the product and pure compound are the same substance.
what is chromatography and what does it use to do its purpose?
separates non-volatile components present within a mixture
it does this by using the differences in their polarity or molecular size
how is TLC set up?
it is performed on a sheet of glass, plastic or aluminium foil which is coated in a thin layer of absorbent material usually silica gel or aluminium oxide
stationary phase of TLC
the solid plate
mobile phase TLC
liquid (solvent)
how does the substance travel up the plate in TLC?
the more strongly a component of a mixture is absorbed onto the stationary phase, the less time it will spend in the mobile phase and the more slowly it will migrate up the plate
what does the distance the compounds carry depend on in TLC?
how soluble the compounds are in the chosen solvent
how well they adhere to the plate
what do you do if your spots don’t show up on your TLC plate?
a developing agent or ultraviolet light in required
how to calculate Rf value?
distance travelled by the sample / distance travelled by the solvent
will a compounds Rf value ever change?
not if the conditions are kept the same and within experimental error
(temp, solvent, saturation levels)
how can you identify a compound using TLC?
compare the experimentally determined Rf value with literature values determined under the same conditions
make a direct comparison on a TLC plate between the compound being tested and the pure substance where a co-spot has been used
how does TLC identify the purity a the substance?
a pure substance should appear as a single spot when developed
what does the presence of multiple spots on a TLC plate show?
impurities are present
