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Infrared Spectroscopy Questions

Spectroscopy is the broad field of study that measures and interprets the electromagnetic spectra produced by the interaction of electromagnetic radiation with the matter as a function of wavelength or frequency.

Spectroscopy, particularly in the electromagnetic spectrum, is a fundamental exploratory tool in astronomy, chemistry, materials science and physics, allowing the composition, physical structure, and electronic structure of matter to be studied at the atomic, molecular, and macro scales, as well as over astronomical distances. Biomedical spectroscopy has important applications in tissue analysis and medical imaging.

Definition: An Infrared (IR) spectrum is a graph that plots the absorbed infrared light on the Y-axis against the frequency or wavelength on the X-axis.

IR Spectroscopy measures the frequencies of infrared light absorbed by a molecule. Molecules absorb these specific light frequencies because they correspond to the frequency of the vibration of the molecule’s bonds.

The energy required to excite a molecule’s bonds and cause them to vibrate with greater amplitude occurs in the infrared region. If a bond is polar, it will only interact with electromagnetic or infrared radiation.

The presence of distinct regions of partial positive and negative charge in a molecule allows the electric field component of the electromagnetic wave to excite the vibrational energy of the molecule.

Infrared Spectroscopy in Chemistry Questions with Solutions

Q1. Which of the following statements about infrared spectroscopy is correct?

a.) When the frequency of infrared light matches the frequency of bond vibration in a molecule, a peak appears on the spectrum.

b.) Infrared spectroscopy can be used to determine the size and shape of a compound’s carbon skeleton.

c.) An IR spectrometer illuminates a compound with infrared light and records the positions where the light is blocked by the compound. This results in the peaks of the spectrum.

d.) The fingerprint region of the spectrum can be used to identify functional groups.

Correct Answer – a.) When the frequency of infrared light matches the frequency of bond vibration in a molecule, a peak appears on the spectrum.

Q2. In an infrared (IR) spectrum, which of the following functional groups has the highest frequency?

a.) Ketone

b.) Aldehyde

c.) Ester

d.) Alcohol

Correct Answer – d.) Alcohol.

Q3. A carbonyl group will cause a sharp dip at about ____ cm–1.


a.) 1700

b.) 2800

c.) 3400

d.) 1200

Correct Answer – (a.) 1700

Q4. Which of the following causes the vibration of atoms?

a.) The number of protons contained in a nucleus.

b.) Electron movement to higher energy levels.

c.) The molecule’s total molecular weight.

d.) Dipole moments between atoms.

Correct Answer – d.) Dipole moments between atoms.

Q5.What is the range of the infrared spectrum?

Answer – The infrared spectrum extends from 700 to 1000 nm (wavelength), or 14,286 to 12,800 cm–1 (wavenumber).

Q6. What is the fundamental measurement in IR spectroscopy?

Answer – An infrared spectrum is a plot between measured infrared intensity versus wavelength (or frequency) of light, which is the fundamental measurement obtained in infrared spectroscopy.

Q7. Which solvent is ideal for infrared spectroscopy?

Answer – Carbon Tetrachloride (CCl4) and Carbon Disulfide (CS2) are the most commonly used solvents. Polar materials can be dissolved in chloroform, methylene chloride, acetonitrile, and acetone.

Q8. What are the different types of infrared spectroscopy?

Answer – The two types of infrared spectrometers are:

  • The dispersive infrared spectrometer (DS)
  • The Fourier transform infrared spectrometer (FTIS).

Q9. What is the region of the infrared spectrum?

Answer – The majority of the bands that indicate which functional group is present are found between 4000 cm–1 and 1300 cm–1. Their bands can be identified and used to determine the functional group of an unknown compound functional group.

The fingerprint region, from 1300 cm–1 to 400 cm–1, contains bands that are unique to each molecule, similar to a fingerprint. These bands are only used to compare the spectra of different compounds.

Q10. How does infrared spectroscopy work?

Answer – Infrared (IR) spectroscopy excites the molecules of a compound and produces an infrared spectrum of the energy absorbed by a molecule as a function of the frequency or wavelength of light.

Different types of bonds react differently to IR radiation. Triple and double bonds, for example, are shorter and stiffer than single bonds and thus vibrate at higher frequencies. The types of atoms in the bonds are also important. Because O-H bonds are stronger than C-H bonds, they vibrate at higher frequencies. As a result, IR spectroscopy allows us to identify the various functional groups present in a compound.

Q11. What is the necessary condition for IR spectroscopy?

Answer – According to the selection rule for IR transitions, the change in the electric dipole moment of the functional group present in a molecule or sample during the vibration is required for a molecule or sample to show an infrared spectrum.

Q12. How does the IR spectroscopy instrument work?

Answer – The infrared spectroscopy instrumentation is shown below.

First, an IR light beam from the source is split into two and passed through the reference and sample.

Both of these beams are now reflected in order to pass through a splitter and then a detector. After the processor deciphers the data passed through the detector, the required reading is printed out.

Q13. What are some of the applications of infrared spectroscopy?

Answer – Infrared spectroscopy is widely used in both industry and research. It is a simple and dependable measurement, quality control, and dynamic measurement technique. It is also used in forensic analysis, both civil and criminal.

The following are some of the most important applications of IR spectroscopy:

  • Identification of functional group and structure elucidation
  • Identification of substances
  • Studying the progress of the reaction
  • Detection of impurities
  • Quantitative analysis

Q14. What is the principle on which infrared spectroscopy works?

Answer – IR spectroscopy is based on the idea that molecules absorb specific frequencies that are unique to their structure. All atoms in molecules are in continuous vibration with respect to each other at temperatures above absolute zero. A sample’s IR spectrum is recorded by passing an IR radiation beam through it.

When the frequency of a specific vibration equals the frequency of the IR radiation directed at the molecule, the radiation is absorbed. The amount of energy absorbed at each frequency is revealed by examining the transmitted light (or wavelength). IR spectrometers can accept a wide variety of sample types, including gases, liquids, and solids, by using various sampling accessories.

Q15. What are the disadvantages of IR spectroscopy?

Answer – The disadvantages of IR spectroscopy are:

  • Sample Constraint: Infrared spectroscopy is not applicable to water-containing samples because this solvent absorbs IR light strongly.
  • Spectrum Complication: The IR spectrum is extremely complicated, and interpretation requires a great deal of experience. On the basis of a single IR spectrum, we cannot always determine the structure of a compound. Other spectroscopy techniques, such as Mass Spectrometry (MS) and Nuclear Magnetic Resonance (NMR), are still required to fully understand the specific structure.
  • Quantification: While infrared spectroscopy is effective for qualitative analysis of a wide range of samples, quantitative analysis may be limited under certain conditions, such as very high and low concentrations.

Practice Questions on Infrared Spectroscopy

Q1. Which of the following statements is correct?

a.) The fingerprint region is most easily used to determine the molecule’s functional groups.

b.) Molecular mass can be easily determined using IR spectroscopy.

c.) The frequency of vibrations between atoms is determined using infrared spectroscopy.

d.) The shape of the carbon backbone is determined using IR spectroscopy.

Q2. An alcohol group will cause a sharp dip at about ____ cm–1.

a.) 1700

b.) 2800

c.) 3400

d.) 1200

Q3. Why is the IR absorption spectra specific for a particular functional group?

Q4. Classify the IR spectrum region.

Q5. What are the advantages of IR spectroscopy?

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