gases + analytical techniques Flashcards
ideal gas
- the law (PV)/(nT) = R, is approximately true for real gases at low pressure and density. P = pressure (Pa - pascals), V = volume (L), n = number of moles, R = universal gas constant 8.314 J K^-1 mol^-1, T = temperature (K)
- an ideal gas is a gas that obeys this law
kinetic theory of gas - postulates
- gases are composed of particles that are in rapid continuous random straight-line motion
- attraction and repulsion between particles in gases is negligible
- the particles of a gas are wildly spaced such that the total volume of all the particles is negligible compared to the volume the gas occupies
- the particles of a gas have kinetic energy (Ek) given by, Ek = 1/2 mv^2, where the particles have a speed of motion - v and mass - m
- the average kinetic energy of the particles of a gas is proportional to its temperature and is the same for all gases at the same temperature
- particle collisions are elastic, that is over time as particles collide they do not lose speed or slow down thus particles do not lose Ek due to their collisions
how do gases have the ability to exert pressure?
the ability of gases to exert pressure is the result of multiple random collisions of its particles with their container walls, the pressure exerted by a gas increases with the frequency and force of these collisions
how does gas pressure increase?
- pressure increases with increasing temperature
- Pressure increases with increasing amount of gas
- Pressure increase with decreasing gas volume/volume of container
how does pressure increase with increasing temperature?
as gas temperature increases so does the average kinetic energy of its particles. this means greater average particle speed which results in a greater rate and force of particle collisions with the container walls. They increased force and rate of collisions with the container walls results in an increased pressure.
how does pressure increases with increasing amount of gas?
More gas means more gas particles in the container, hence there will be a greater rate of collisions between gas particles and their container walls. This causes the increased pressure.
how does pressure decrease with increasing gas volume/volume of container?
Increasing the container, volume means particles on average have a greater distance to travel before colliding with a container walls. This leads to a decreased rate of collisions and hence a decreased pressure. The force of particle collisions with the container walls remain unchanged.
boyles law
pressure is inversely proportional to volume
P1V1 = P2V2
pressure and volume of first situation, pressure and volume of second situation
charles law
volume (L) is directly proportional to temperature (K)
V1/T1 = V2/T2
combined gas law
(P1V1)/T1 = (P2V2)/T2
p= kPa, t= K, v= L
analytical techniques
- flame tests
- atomic absorption spectroscopy
- mass spectrometry
- chromatography
- thin layer chromatography
- gas chromatography
- high performance liquid chromatography
atomic absorption spectroscopy: explain
Atomic absorption spectroscopy is an analytical technique that relies on the unique nature of an element absorption spectrum to identify its presence and concentration in a mixture. AAS relies on the electron transfer between atomic energy levels and can be used to identify elements. It is related to emission spectroscopy, and can be used to perform Quantitative analysis to the find the amount of an element that is present. it uses absorption of light by electrons in atom, to measure how much of the element is present in the sample of a substance. absorption of light occurs when electrons jump from ground state to the excited state
atomic absorption spectroscopy: process
the Sample solution is sprayed onto the flame of the burner and a light from the cathode lamp that is made out of the element that is being tested for (might be in the sample) goes through the vaporised Sample, only the element being tested for will absorb this light, other elements will not absorb the light due to differences in energy levels and the electrons being unable to absorb the energy of the light. the light is focussed through a slit, then enters a monochromator which selects one wavelength of light for analysis, the detector analyses This wavelength, and displays as a number which is a measure of the amount of light that passed through the sample without being absorbed called the absorbance value
mass spectrometry: explain
mass spectrometry is an analytical Technique that uses the different masses of particles to measure their relative abundance in a Sample. This method is similar to atomic absorption spectroscopy but can determine isotopic composition of an element to tell which isotopes are present and the percentage of the isotope in the element and is not based on light or promotion of electrons to a higher energy level.
mass spectrometry: process
a vaporised Sample enters the mass spectrometer, and the Sample becomes ionised and is accelerated, the ions become fragments and an electromagnet separates and causes deflection of resulting ions according to their mass to charge ratio, lighter ions are deflected more by the magnetic field than heavier ions
mass spectrometry involves the ionisation of substances, and the separation and detection of the resulting ions. The spectra, which are generated can be analysed to determine isotopic composition of elements interpreted to determine relative atomic mass.
vaporisation, ionisation, acceleration, deflection, detection
chromatography: explain
chromatography is very useful technique for finding the concentration of each substance in a mixture. Very small amounts of analyte are required down to 1 x 10^-6 L micro-litres are sufficient and they can also detect 1 x 10^-12 g picogram amounts. The separation of individual components from mixture relies on the differing solubility is due to intermolecular forces in the moving liquid, mobile phase compared to the tendency to adsorb to the stationary phase. Substances that tend to stay dissolved in the moving liquid phase, become separated from those that adhere (tend to adsorb) onto the stationary solid phase.
chemical basis of chromatography
The chemical basis for techniques of chromatography are different substances will adsorb onto a surface and desorb into a solvent at different rates.
factors that produce differences in the analyte
- the different types of polar groups
- the amount of charge and polar chemical groups present
- it’s molecular weight
- it’s geometry
- the positions and numbers of carbon-carbon double bonds.
analyte
sample being analysed
STP
standard temperature and pressure
real vs ideal gases
- particles of an ideal gas have a negligible volume whereas particles of a real gas occupies space and the volume is sometimes significant when compared to the volume occupied by the gas as a whole
- particles of an ideal gas have negligible attraction for one another. Whereas the particles of real gases do have forces of attraction for one another and these forces of attraction can become significant
thin layer chromatography: explain
thin layer chromatography is used to separate, and identify a wide range of organic (molecular) mixtures. thin layer chromatography is similar to paper chromatography, but uses a thin glass plate coated with finely powdered stationary phase for example, aluminium oxide or silicon oxide which are both polar. The mobile phase is a solvent chosen for its ability to dissolve and separate the components of the mixture. as most of the components analysed by TLC are molecular, their polarity will affect the solubility in the chosen solvent.
thin layer chromatography: process
in TLC, a plastic, glass or aluminium sheet is coated with a thin layer of silica gel. A very small amount of solution of the substance to be analysed is applied in the small spot with a capillary tube, around 1 cm from the bottom of the TLC plate. The mobile phase moves up the stationary phase, carrying the various components of the mixture at different rates depending on the polarity. This results in the component spread out across the coated plate. The separations produced with TLC are sharper and better defined than those from paper chromatography. This makes TLC a more reliable separation technique, then paper chromatography.
retardation factor (Rf)
Rf equals to the distance moved by substance (solute), divided by the distance moved by the solvent.
gas chromatography: explain
gas chromatography is used to separate and identify, and find the concentration of a variety of complex mixtures
gas chromatography is a separation technique for small organic molecules that can with stand relatively high temperatures (can be vaporised).
gas chromatography: process
The samples injected into a chromatograph through the oven where the inert carrier gas (usually N2, non polar) pushes the Sample into the long, thin column (polar e.g SiO2). In the column, smaller particles and those that are adsorb onto the stationary phase the least leave the column first. Large particles and those that adsorb more readily take longer to leave the column (elute). A detector is used to produce a chromatogram which is produced from the chromatograph. The chromatogram shows peak height on the y-axis and retention time on the X axis. Retention time (Rt) is the time it takes for the Sample to leave the column higher peaks on the gram mean higher concentration. Area under the peak indicates amount of substance present.
retention time
the time that a solute spends in a column before eluting
which analytes have a longer retention time?
Low boiling point analytes that is those with weaker intermolecular forces, have high volatility and tend to remain in the carrier gas. These exit the column sooner and have a lower retention time. Higher boiling point analytes, that have stronger intermolecular forces and thus a higher retention time.
retention time values vs calibration curve
retention time values are used to identify compounds, standards are run to confirm the identity and a calibration curve is used to determine the amount of the substance present
analytical and forensic applications of chromatography
monitoring air and water pollutants, drug testing of urine and blood samples, and testing for food additives and quality.
high performance liquid chromatography: explain
high-performance liquid chromatography is used to separate and identify and find the concentration of a variety of larger organic complex mixtures. compounds that decompose when heated and do not easily vaporise are analysed using HPLC.
high performance liquid chromatography: process
HPLC uses a liquid solvent as a mobile phase e.g. water-methanol mixes, the stationary phase consists of a tightly packed column of fine particles such as SiO2. A high pressure pump is used to increase the flow rate by forcing the solvent mixture through the column, resulting in high performance. Polar analytes have a greater retention time as they have a greater tendency to adsorb onto the polar stationary phase rather than dissolve in the non-polar mobile liquid phase, thus taking longer to get out the column (elute). Retention time are used to identify compounds. Area under the peak indicates amount of substance present.
