MCAT Exam  >  MCAT Notes  >  Practice Passages for MCAT  >  Practice Passage Test - 2

Practice Passage Test - 2 | Practice Passages for MCAT PDF Download

Passage

Molecules are not rigid, unchanging structures. Their atoms are in constant motion even relative to each other, ceaselessly oscillating around their average bond lengths and bond angles. For instance, in non-linear triatomic molecules there are three possible modes of vibration. There is the symmetric stretch in which both bonds in the molecule lengthen and contract in unison. In the asymmetric stretch, one bond lengthens while the other contracts. Finally, there is the bend in which the bond angle alternately widens and narrows.

Figure 1 Vibrations of a Triatomic Molecule Figure 1 Vibrations of a Triatomic Molecule 

More generally, each atom in a molecule is capable of moving in three distinct directions, often represented by x, y and z. In a molecule with N atoms, there will be 3N possible atomic movements. However, if all the atoms in a molecule move in the same direction, translational movement and not vibration will result. Likewise, there are some combinations of atomic motions that result in rotation of the molecule and not vibration. Taking this into account, in a molecule containing N atoms there will be 3N - 6 normal modes of vibration in non-linear molecules and 3N - 5 normal modes of vibration in linear molecules.

If we make the rough approximation that atoms in a molecule are harmonic oscillators, then the energy of their vibration is given by:

Practice Passage Test - 2 | Practice Passages for MCAT

where v is the quantum vibrational number, h is Planck's constant, k is the force constant of the bond which increases with bond strength, and u is the reduced mass of the molecule. Changes in the vibrational quantum state are associated with energies similar to infrared photons. Thus, IR spectroscopy is the study of the energetics of a molecule's vibrational quantum states. However, only those normal modes of vibration that induce a change in the dipole moment of a molecule can be excited with IR light.

Table 1 Bond Energies of Select Diatomic Elements

Practice Passage Test - 2 | Practice Passages for MCAT

Question for Practice Passage Test - 2
Try yourself:Which of the following molecules has nine normal modes of vibration?
View Solution

Question for Practice Passage Test - 2
Try yourself:A change in which of the following combinations of molecular movement can never produce a peak in an IR spectrum?
View Solution

Question for Practice Passage Test - 2
Try yourself:Assuming their reduced masses are the same, which molecule will have the highest energy of vibration in the v = 0 state?
View Solution

Question for Practice Passage Test - 2
Try yourself:All of the following molecules will display absorption peaks in an IR spectrum EXCEPT
View Solution

Question for Practice Passage Test - 2
Try yourself:Nitrate is best described by a resonance average of three structures:

Practice Passage Test - 2 | Practice Passages for MCAT

View Solution

The document Practice Passage Test - 2 | Practice Passages for MCAT is a part of the MCAT Course Practice Passages for MCAT.
All you need of MCAT at this link: MCAT
55 docs

Top Courses for MCAT

55 docs
Download as PDF
Explore Courses for MCAT exam

Top Courses for MCAT

Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

Extra Questions

,

Objective type Questions

,

Free

,

Previous Year Questions with Solutions

,

ppt

,

shortcuts and tricks

,

Practice Passage Test - 2 | Practice Passages for MCAT

,

Important questions

,

video lectures

,

Exam

,

Semester Notes

,

mock tests for examination

,

Summary

,

practice quizzes

,

Sample Paper

,

Practice Passage Test - 2 | Practice Passages for MCAT

,

Viva Questions

,

Practice Passage Test - 2 | Practice Passages for MCAT

,

MCQs

,

past year papers

,

pdf

,

study material

;