chem unit5 ans
TRANSCRIPT
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UNIT-V
PART-A
1. What is atomic spectroscopy?
Ground state of an atom means it is with normal electronic configuration. In
this state the atom remains in its lowest energy state and this is the most stable state
of the atom. Excited state of an atom refers to the electronic configuration availed
by an atom after absorbing certain definite amount of energy. The valence electrons are
promoted to some higher permitted energy level by absorption of energy. In the excited
state, the atom is unstable and the excited electron tends to come back to the original
position i.e., ground state. After about 104
sec. the electron returns to the ground state
by emitting the amount of energy absorbed during excitation. The energy is emitted orabsorbed in the form of electromagnetic waves of definite frequency i.e., of definite
wavelengths.
2. What is atomic absorption spectroscopy?
This is the analytical technique based on the phenomenon of light absorption
(UV or visible). It is applicable both to qualitative and quantitative analyses.
3. What are the parameters for expressing the absorption?
Transmittance is the ratio of the intensity of light transmitted to the intensity of
incident light.
Where
A = Absorbance,
Io = Intensity of incident light,
I = Intensity of transmitted light
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5. Mention the applications of UV.
(a) Qualitative:
Detection of conjugation
Detection of functional groupsDetection of geometrical isomers.
(b)Quantitative: Analysis of various samples (drugs, dyes etc.)
6. What are the different electronic transitions that take place on absorption of UV
light?
When a molecule absorbs UV radiations the electrons are excited to higher energy
levels. In the diagram below the electrons are represented
The following electron transitions take place:
* , n*, n* and *.
These electronic transitions are responsible for UV absorption of a molecule.
7. How does molecular spectrum arise?
It arises due to the interaction of electromagnetic radiation with molecules. This
results in transition between rotational, vibrational and electronic energy levels.
8. State Beer-Lamberts law?
According to this law, when a beam of monochromatic radiation is passed
through a solution of an absorbing substance, the rate of decrease of intensity of radiation
dI with thickness of the absorbing solution dx is proportional to the intensity incident
radiation (I) as well as the concentration of the solution (C). It is mathematically
4. Calculate the concentration of a substance A in an ethanolic solution of which the
absorbance in a 1 cm cell at its A, max 241 nm was found to be 0.890. The is 540 at
241 nm.
A= C l
0.890 = 540 x l x C
C = 0.00165 g/100 ml.
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Diagram
9. Name the important components of colorimeter?
1. Radiation source
2. Filter
3. Slits
4. Cell
5. Detector
6. Meter
10. Write the complexes formed in the colorimetric determination of Fe?
Fe3+
+ NH4CNS blood red colour complex
11. What are the important processes that occur in the flame photometry?
1. It should evaporate the solvent from the sample solution
2. It should decompose the solid into atoms
3. It should excite the atoms and cause them to emit radiant energy
12. What is finger print region? Mention its important uses?
The vibrational spectral region at 1400-1700 cm-1 gives very rice and intense
absorption bands. This region is called as finger print region. It can be used to detect the
presence of functional group and also to identify and characterize the molecule just as a
finger print can be used to identify a person.
represented as
- dI/ dx = k I C
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14. How is an I.R. spectrum recorded?
IR source: A Nernst glower, a rod of an allow of Zirconium, Yttrium
and Erbium oxides. The rod is electrically heated to 1750 K
Rock salt disc or KBr disc is used as glass and quartz absorb I.R.
Sample preparation: Either the sample with KBr is made to pellet or
Nujol mull is used. Nujol is hydrocarbon in nature.
Recording of spectra: The sample is placed in Rock salt cell in the path of
I.R. The change of intensity of light transmitted draws a graph which is
IR spectrum.
15. How will you distinguish CH3COOH from CH3COCH3 with the helpof I.R. spectra?
C = O (str.) peaks will be observed in both the spectra in the region of 1700 cm-1
.
But an absorption bond at 25003000 cm-1
(broad) will be observed in spectrum of
CH3COOH due to dimeric association of CH3COOH molecules through hydrogen
bonding.
16. What happens to a molecule when it is irradiated with (a) UV-Vis., and (b) IRlight.
UV-Vis., light causes electronic transition
IR light causes vibrational and rotational transitions
13. What are the principles of IR?
The atoms in a molecule bond are in a state of constant vibration and
rotation. They may be compared with two balls (atoms) joined by spring (bond). On
absorption of IR the bond may stretch, bend etc., as shown below. So stretching and
bonding of bonds are responsible for IR absorption.
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18. Name two fuel gases used in flame photometry?
1. Acetylene
2. Propane
19. What is the main application of flame photometry?
Analysis of alkali metals, particularly in biological fluids and tissues.
20. How can you identify an unknown element from emission spectrum?
By comparing the unknown spectrum with the spectrum of a known
element.
21. Find out the absorbance of a solution if the transmittance of a solution is
18.5%.
Solution:
% Transmittance = 18.5 %
T = 0.185
Absorbance, A = - log T
A = - log 0.185
= 0.733
17. How to calculate the number of vibrational modes for a different types of
compounds?
i) For a non-linear molecule containing n atoms, the number of vibrational
modes (3n-6)
ii) For a linear molecule containing n atoms, the number of vibrational modes
(3n-5)
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1. Explain flame photometry with a neat diagram?
Components
The various components of the flame photometer are described as follows:
Burner
The flame must possess the following functions:
(i) it should evaporate the solvent from the sample solution
(ii) it should decompose the solid into atoms
(iii) it should excite the atoms and cause them to emit radiant energy.
Mirror
The radiation from the flame is emitted in all directions in space. In order
to increase the amount of radiation reaching the detector, a concave mirror is used
which is set behind the burner.
Slits
Entrance slits: It is kept between the flame and monochromator. It permits only
the radiation coming from the flame & mirror.
Exist slits: It is kept between the monochromator and detector. It prevents the
entry of interfering lines.
Monochromator
It allows the light of the required wavelength to pass through but absorbs
the light of other wavelength.
PART B
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Amplifier & Recorder
The current coming out from the detector is weak, so it is amplified and
recorded.
2. Expalin the estimation of sodium by flame photometry.
The instrument is switched on. Air supply and gas supply are regulated.
First distilled water is sent and ignition is started. After the instrument is warmed
up for 10 min, the instrument is adjusted for zero reading in the disply. Since
sodium produces a characteristic yellow emission at 589 nm, the instrument is set
at max= 589 nm and the readings are noted.
A series of standard NaCl solution (1, 2, 3, 4, 5, ppm) is
prepared and is sent one by one and the readings are noted. The calibration graph
is drawn between the concentration vs intensity of the emitted light. A straight
line is obtained.
Now the unknown sodium solution is sent and the reading is noted. Then
the concentration of sodium in the water sample is determined from the
calibration curve.
Detector
The radiation coming from the filter is allowed to fall on the detector,
which measures the intensity of the radiation falling on it. Photo cell or photo
multiplier is used as detector, which converts the radiation into an electrical
current.
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Components
The various components of a visible UV spectrometer are as follows:
Radiation source
In visible UV spectrometers, the most commonly used radiation sources are
hydrogen or deuterium lamps
Monochromators
Monochromator is used to disperse the radiation according to the wavelength. The
essential elements of a monochromator are an entrance slit, a dispersing element and an
exit slit. The dispersing element may be a prism or filter.
Cell
The cells, containing samples or reference for analysis, should fillfil the following
conditions:
i) They must be uniform in construction
ii) the material of construction should be inert to solvents
iii) they must transmit the light of the wave length used.
Detectors
There are three common types of detectors used in UV-Vis., spectrometers. They
are barrier layer cell, photomultiflier tube, photo cell.
Recording system
The signal from the detector is finally received by the recording system. The
reading is done by recorder pen.
3. Draw a neat diagram of UV-Visible soectrophotometer and explain the each
component ? Explain the working of UV-Visible spectrophotometer.
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The radiation from the source is allowed to pass through the monochromator unit.
The monochromator unit allows a narrow range of wavelength to pass through an exit
slit. The beam of radiation coming out of the monochromator is split into two equalbeams. One half of the beam is directed to pass through cell containing a solution of the
compound to be analysed. The another half is directed to pass through an identical cell
that contains only the solvent. The instrument is designed in such a way that it can
compare the intensities of the two beams.
If the compound absorbs light at a particular wavelength, then intensity of the
sample beam (I) will be less than that of the reference beam (I 0). The instrument gives
output graph, which is a plot of wavelength vs absorbance of the light. This graph is
known as an absorption spectrum.
Working
2. State Beer Lamberts law and explain its applications.
According to this law, when a beam of monochromatic radiation is passed through a
solution of an absorbing substance, the rate of decrease of intensity of radiation dI
with thickness of the absorbing solution dx is proportional to the intensity incident
radiation (I) as well as the concentration of the solution (C). It is mathematically
represented as
- dI/ dx = k I C
- dI/ I = kC dx
Integrating the above equation between limits.
I
- dI/ I = kC dx
I
-ln I/ I = kCx
Where k = Molar absorption coefficient
ln I/I =kCx
2.303 log I/I = kCx
log I/I = k/2.303 Cx
Where E = k/2.303 (Molar extinction coefficient )
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Beer Lamberts law can be used to determine the concentration of unknown solution.For that first we measure the absorbance of standard solution. The absorbance will be
As = ECsx --------(1)
Now measure the absorbance of unknown solution. The absorbance will be
Au = ECux ----------(2)
From 1 and 2
Au / As = Cu/ Cs
Cu = Au / As Cs
From the values of Au , As and Cs we can calculate the concentration of unknown
slolution.
5. How is nickel estimated by AAS?
The instrument is switched on. Air supply and gas supply are regulated.Now prepare
stock solution of nickel by dissolving one g of Nickel nitrate in dilute nitric
acid.When a blank solution is aspirated into the flame, the meter of the AAS
assembly is adjusted to zero absorbance.From the ststandard solution of Nickel, a
series of standard solutions are prepared by appropriate dilutions.Now these standard
solutions are aspirated one by one into the flame and absorbance are measured at 232
nm.Now a calibrated graph is plotted between absorbance and concentration of
nickel in ppm. The nickel solution of uknown concentration is now aspirated into the
flame and absorbance is measured under the same conditions as above. From the
absorbance, the concentration of the unknown nickel ion sample is determined.
A = ECx
Where A = log I/I
Application:
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Block diagram:
Components
Radiation source
The main sources of IR radiation
(a) Nichrome wire
(b) Nernst glower
When they are heated electrically at 1200 to 2000C, they glow and produce IR
radiation.
Monochromator
It allows the light of the requires wave length to pass through, but absorbs thelight of other wavelength.
Sample cell
The cell, holding the test sample must be transparent to IR radiation.
Detector
IR detector convert thermal radiant energy into electrical energy. There are photo
PART C
1. Explain the principle, working of IR spectroscopy with a block diagram?
Principle
The atoms in a molecule bond are in a state of constant vibration and
rotation. They may be compared with two balls (atoms) joined by spring (bond). On
absorption of IR the bond may stretch, bend etc., as shown below. So stretching and
bonding of bonds are responsible for IR absorption.
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Working of IR Spectorphotometer
The radiation emitted by the source the source is split into two identical beams
having equal intensity. One of the beams passes through the sample and the other
through the reference sample.
When the sample cell contains the sample, the half-beam travelling through it
becomes less intense. When the two half beams (one coming from the reference and the
other from the sample) recombine, they produce and oscillating signal, which is measured
by the detector. The signal from the detector is passed to the recording unit and
recorded.
conductivity cell, thermocouple, Pyroelectric detector.
Recorder
The recorder records the signal coming out form the detector.
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The excited atoms which are unstable, quickly emit photons of different wave
lengths and return to the lower energy state. Then the emitted radiation is passed through
the filter, which permits the characteristic wave length of the metal under examination. It
is then passed into the detector, and finally into the recorder.
Block diagram
Components
The various components of the flame photometer are described as follows:
Burner
The flame must possess the following functions:
i) it should evaporate the solvent from the sample solution
(ii) it should decompose the solid into atoms
(iii) it should excite the atoms and cause them to emit radiant energy.
Mirror
The radiation from the flame is emitted in all directions in space. In order to
increase the amount of radiation reaching the detector, a concave mirror is used which is
set behind the burner.
2. a) Draw a neat diagram and explain the principle, working of flame photometry?
Principle
When a metallic salt solution is introduced into a flame, the following processes
will occur.
1. It should evaporate the solvent from the sample solution2. It should decompose the solid into atoms
3. It should excite the atoms and cause them to emit radiant energy
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Slits
Entrance slits: It is kept between the flame and monochromator. It permits only the
radiation coming from the flame & mirror.
Exist slits: It is kept between the monochromator and detector. It prevents the entry of
interfering lines.Monochromator
It allows the light of the required wavelength to pass through but absorbs the light
of other wavelength.
Detector
The radiation coming from the filter is allowed to fall on the detector, which
measures the intensity of the radiation falling on it. Photo cell or photo multiplier is used
as detector, which converts the radiation into an electrical current.
Amplifier & Recorder
The current coming out from the detector is weak, so it is amplified and
recorded.
Working of the Flame photometer:
Air, at a given pressure, is passed into an atomiser. The suction so-produced
draws some solution of the sample into the atomiser. Air+sample solution is then mixed
with fuel gas in the mixing chamber. The Air+sample solution +fuel gas mixture is then
burnt in the burner. The radiation, emitted by the burner flame, is passed successively
through the lens, filter, detector, amplifier and finally into a recorder.
The above experiment is first carried out using a series of standard solutions and
the reading for each solution is noted. The graph is drawn between the concentration
against intensity of emitted light. The test solution is taken and similar experiment is
carried out. From the graph the concentration of the unknown sample can be determined.
b) The percentage transmittance of 0.0005 M solution of disodium fumarate in a
1 cm cell is 19.2 %. Caluculate (i) the absorbance (ii) the molar extinction coefficient
().
Solution:
Given
% T = 19.2 %
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4. Explain the principle, working of colorimeter with a block diagram. How
will you estimate iron using colorimetry
Principle
This colorimteric method is convenient for the coloured substances. The intensity
of colour can be easily measured by using a photoelectric colorimeter, from which the
concentration of coloured solution can be obtained using Beer-Lamberts law.
If the substance is colourless, then a suitable complexing agent is added to the solution so
that a coloured complex is obtained which can absorb the light.
Working
In a colorimeter, a narrow beam of light is passed from radiation source through
the test solution towards a sensitive detector. Usually, colorimeter is provided with the
arrangement of filter and slits, which select the light of required wavelength. The detector
generates the current, which is proportional to the amount of light transmitted by the
solution.
The amount of light transmitted depends on the depth of colour of the testsolution. Thus the current from the photo cell will be more when the light transmitted is
more. This will be possible only if the coloured solution is most diluted.
Current light transmitted 1 / Concentration
T = 0.192
C = 0.0005 M
l = 1 cm
(i) Absorbance
A = - log T
= - log 0.192
= 0.717
(ii) Molar extinction coefficient
= A / C . l
= 0.717 / (0.0005 1)
= 1.434 103mol dm
-3
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Components:
Radiationsources :
The wavelength range of visible light les between 400 750 nm. In this region, atungsten-filament lamp is most widely used.
Filter or monochromator :
It is a instrument, which allows the light of the required wavelength to pass
through,but absorbs the light of the other wavelengths.
Slits:
(a) Entrance slit: It provide a narrow source of the light.
(b) Exit slit: It select a narrow band of dispersed spectrum for observation by the
detector.
Cell:
The cell, holding the test sample (usually a solution), should be transparent. For -
visible region the cell is made of colour-corrected fused glass.
Detector :
It is used for measuring the radiant energy transmitted through the sample.
Photosensitive devices are used to detect radiations. These detectors produce current,
which is directly proportional to the intensity of the incident radiation.
Meter:
It is used to measure directly the fraction of light absorbed.
The transmitted light is now a days are allowed to send through a meter, which is
calibrated to show not the fraction of light transmitted but the fraction of light absorbed.
The light absorbed is proportional to the concentration of the test solution.
Block diagram
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Reagents required
Standard iron solution
0.865 gms of FAS is dissolved in distilled water 5-10 ml of conc. HCl is added
and the solution is diluted to 1 litre. 1 ml of this solution contains 0.1 mg of Fe.
KCNS solution
20 gms of KCNS is dissolved in 100 ml of water
1:1 HCl
50 ml of conc.HCl is added to 50 ml of distilled water
Procedure
A series of standard solution of Fe3+ are prepared by adding KCNS with small
amount of 1:1 con.HCl. then the colorimeter is set at zero absorbance using a blank
solution , with a proper filter. Now absorbance of each standard solution is then measured
using the same filter. A graph is plotted between absorbance vs concentration. This plot is
called as calibration curve and will be the straight line passing through origin. This is
according to Beer-Lamberts law:
A = C l
WhereA is absorbance
C is concentration
l is path length
is molar extinction coefficient
Estimation of iron
Principle
Fe3+
is a colourless but Fe3+
forms blood red colour complex with KCNS or
NH4CNS.
Fe3+
+ KCNS [Fe(CNS)6]3-
+ 6K+
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Similarly, the absorbance of test solution is measured using the same colorimeter. From
the calibration curve, the concentration of the unknown ferric iron solution can be
evaluated.
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