module eight - revision deck Flashcards
environmental monitoring
the process of observing environments, often with analytical and scientific techniques that track changes to an environment
monitoring of ozone layer
ozone converts UV rays into heat, which is important as UV is hazardous to our skin and can cause cancer. ozone molecules contain three oxygen atoms and are an allotrope of carbon
damaged by the use of chlorofluorocarbons (production of deodorants and refrigerants). damage was realised by chemically analysing the structure of CFCs, particularly the carbon chlorine bonds which can break to release carbons atoms which then react with ozone
Cl + O3 –> ClO + O2
monitoring heavy metals in water and soil
poisonous if consumed
scientists regularly test water sources and soil samples to check for dangerous levels of chemicals or heavy metals
govt. can restrict access to certain areas or water to prevent heavy metal poisoning
particularly important since many children play in the soil in playgrounds and lead poisoning can impact the development of a child’s brain and potentially lead to ongoing intellectual difficulties
why flame tests work
the electrons in the sample absorb heat energy which excites them, so they jump to a higher energy level. When they fall back to their ground state, they emit energy in the form of light, and the colour of the light depends on the metal ion present
barium flame test colour
green
calcium flame test colour
red
magnesium flame test colour
bright white
copper 2 flame test colour
blue/green
iron 2 flame test colour
gold
iron 3 flame test colour
gold
potassium flame test colour
lilac
sodium flame test colour
yellow
lithium flame test colour
red
strontium flame test colour
red
limitations of flame tests
cannot detect all metals as some don’t produce colours and others are unsafe to test
can be difficult to distinguish between similar coloured flames
precipitation test for cations
- add HCl
If a precipitate forms:
add NaOH - brown precipitate (Ag), white precipitate (Pb)
If no precipitate forms:
Add NaOH - white precipitate (Al or Mg), pale green precipitate (Fe2+), brown precipitate (Fe3+), blue precipitate (Cu)
- no precipitate - add Na2SO4. forms precipiate. add NaF - white precipitate (Ba), no precipitate (Ca)
gravimetric analysis process
- find a precipitating agent, that is, a chemical that will react with the dissolved substance of interest to form a precipitate
- determine the initial mass of the sample
- add the precipitating agent in excess to the sample solution - add gradually using a burette to allow the precipitate to form and coagulate
- filter the solution and collect the precipitate
- thoroughly dry the precipitate until all of the water is evaporated
- weigh the dried filter paper and precipitate
- calculate the precipitate’s mass by subtracting the mass of the filter paper from the total mass of the dried filter paper and precipitate
to interpret gravimetric analysis data
- write a balanced chemical equation
- use the mass to moles formula to calculate the number of moles of precipitate
- use the molar stoichiometric ratio to calculate the number of moles of the ion
- use the moles to mass formula to calculate the mass of the ion
- find concentration or percentage
limitations of gravimetric analysis
filter paper and precipitate may not dry fully leading to an increase in mass and overestimation of amount of substance present
precipitate may be too fine and go through the filter paper leading to an underestimation of the amount of substance present
deciding when the precipitation reaction has reached completion is a qualitative observation
not suitable to analyse all salts
precipitates may form from other ions present in solution
precipitation titrations
colour change titrations are used where a colour changes occurs at the equivalence point due to a new precipitate being formed. the indicator forms a coloured precipitae with the solution being added from the burette once the equivalence point is reached
Mohr’s Method
used to calculate [Cl-] by using a solution of NaCl of unknown concentration and a solution of AgNO3 of known concentration. The equivalence point is where all chloride has precipitated out as silver chloride. A potassium chromate indicator is used to determine the end point
conductometric titrations
monitor the electrical conductivity of a solution. Electrical conductivity is at its lowest when the concentration of ions is at its lowest, which is the equivalence point. The change in conductivity before and after the equivalence point depends on the solutions used because different ions have different conductivities
Volhard’s Method
can determine the concentration of a range of ions, including chloride, bromide, iodide, phosphate, chromate and cyanide ions through a back titration
Volhard’s method steps
- a known sample of silver nitrate is added to the sample, which precipitates the anions
- the solution is titrated with potassium thiocyanate. fe3+ ions are used as the indicator, usually through ammonium iron (III) sulfate
once all the silver ions are gone, the next drop of potassium thiocyanate will react with the iron ions to form a deep red complex.
volhard’s method limitations
- only works at low pHs
- some silver salts are more soluble than silver thiocyanate
- to combat solubility issues, we can remove the first silver precipitate from the analyte before adding potassium thiocyanate
Fajan’s Method
can be used to determine the concentration of chloride, bromide, iodide and thiocyanate ions through a direct titration. an anionic adsorption indicator is used, which is an indicator that adsorbs, or sticks, to the surface of a precipitate
colourimetry
a method for determining the concentration of a chemical in solution. it enables the concentration of a particular species to be identified using the fact that the colour intensity is proportional to concentration
process to conduct a colourimetry experiment
- light from a bulb is passed through a filter to produce the required wavelength that is absorbed by the species being analysed
- the solution is placed in a cuvet. the filtered light passes through the solution and some is absorbed
- the amount of light is measured by a detector
- a cuvet of distilled water is used to calibrate the instrument by setting it read to zero
- the filter used depends on the colours that are absorbed by the species being investigated. the complementary colour of the solution is used for the filter as it is the most strongly absorbed
uv vis spec
uses the UV visible part of the spectrum (400-800nm). relies on the electrons in the molecules being excited to a higher energy state
when sample molecules are exposed to the wavelength of light energy that corresponds to an electron transition between energy levels, the light energy will be absorbed.
chromophores
a group of covalently bonded atoms that are responsible for the colour of a particular molecule. they absorb a unique wavelength of electromagnetic radiation, typically in the UV vis region. the particular wavelength absorbed by a chromophore depends on the unique bonds of the chromophore
process for UV vis spec
the light source provides wavelengths between 200-800nm
the light is focused on a diffraction grating monochromator that splits into different wavelengths
the different wavelengths are shone through two cells - one containing the sample to be analysed and the other containing only the pure solvent used in preparing the sample
process for UV vis spec
the light source provides wavelengths between 200-800nm
the light is focused on a diffraction grating monochromator that splits into different wavelengths
the different wavelengths are shone through two cells - one containing the sample to be analysed and the other containing only the pure solvent used in preparing the sample
process for UV vis spec
the light source provides wavelengths between 200-800nm
the light is focused on a diffraction grating monochromator that splits into different wavelengths
the different wavelengths are shone through two cells - one containing the sample to be analysed and the other containing only the pure solvent used in preparing the sample
process for UV vis spec
the light source provides wavelengths between 200-800nm
the light is focused on a diffraction grating monochromator that splits into different wavelengths
the different wavelengths are shone through two cells - one containing the sample to be analysed and the other containing only the pure solvent used in preparing the sample
uv vis spec can be used to:
- confirm the identity of a substance by comparing the spectrum of a sample to the spectrum of a pure sample or standard of the compound
- calculate concentrations of a known substance using a calibration curve of known concentrations of analyte
AAS
quantitative analysis technique that uses the absorption of light by electrons in atoms to measure how much of an element is present in a sample of a substance
process for AAS
the lamp for this process is made out of the same element that is being tested
an electric current is passed through a gaseous sample of the element so it will emit light
the sample being tested is vaporised, changing the substances it contains into atoms. when the light passes through the vaporised sample, only the element being tested for will absorb the light from the lamp. other elements won’t because the energy levels of all other atoms are different and their electrons cannot absorb the energies of the light present
the light passes through a sample and is focused through a slit before entering a monochromater
this selects just one wavelength of light for analysis by the detector.
the detector measures the light, which is then displayed as a number
limitations of AAS
- only works for metals as the cathode only works if the element can conduct electricity
- must choose metal before starting
- can only detect one metal at a time
