Physical Chemistry : Physical Chemistry

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #5 : Quantum Numbers

Which of the following is true regarding ethene? (The electrons in answer choices refer to carbon electrons)

Possible Answers:

The electrons in  bond have the same set of quantum numbers

The electrons in the  bond have the same orbital angular momentum number

The electrons in the  bonds and the  bond have different principal quantum number

The electrons in the  bonds have the same set of quantum numbers

Correct answer:

The electrons in the  bond have the same orbital angular momentum number

Explanation:

Ethene, or , has a carbon-carbon single () and double () bond. Recall that a bond can be found in hybridized orbitals whereas a  bond cannot. This means that the carbon atoms in ethene hybridize the single ‘s’ orbital and two of the ‘p’ orbitals, forming a  hybridization. The  bonds () are found in these three hybridized orbitals. The remaining unhybridized ‘p’ orbital will house the two electrons in the  bond.

The orbital angular momentum number is the second quantum number and it signifies the type of orbital. An electron found in a ‘p’ orbital will always have an . Since both electrons in the  bond are found in the ‘p’ orbital, the ‘l’ value for both electrons is the same.

The principal quantum number is the first quantum number and it signifies the shell or energy level of an electron. All electrons involved in bonds are found in carbon’s outermost shell (2nd shell); therefore, they will all have an . Remember that no two electrons can have the same set of quantum numbers (regardless of whether the electrons are found as lone pairs, in  bonds, or in bonds).

 

 

Example Question #21 : Nuclear, Quantum, And Molecular Chemistry

Which of the following is/are true regarding the principal quantum number?

I. Principal quantum number signifies the energy level

II. Principal quantum number can never equal zero

III. Principal quantum number can be positive or negative

Possible Answers:

I and II

I, II, and III

I and III

II only

Correct answer:

I and II

Explanation:

Quantum numbers are fancy coordinate systems that describe the potential location of an electron within an atom. The first quantum number is called the principal quantum number and it signifies the shell the electron is located in. Recall that electron shells are discrete orbits in an atom that have discrete energy; therefore, the principal quantum number signifies the energy level of an electron.

The principal quantum number is always an integer and is always greater than zero. If  the electron is found within the first shell, if  then the electron is found within the second shell, and so and and so forth. Also, since it is always greater than zero, the principal quantum number can never be negative.

Example Question #21 : Quantum Chemistry

How many subshells are there with n = 4 in an atom? 

Possible Answers:

Correct answer:

Explanation:

The types of subshells, from smallest to largest, are as follows: s, p, d, and f. These four subshells correspond respectively to the following quantum numbers: 0, 1, 2, and 3. The total number of sublevels with n = 4 is n or 4: 4s, 4p, 4d and 4f.

Example Question #3 : Quantum Numbers

Which of the following set of quantum numbers is not valid?

Possible Answers:

n=4

l=2

m=-3

n=3

l=1

m=0

n=2

l=1

m=1

n=3

l=2

m=-1

n=2

l=1

m=0

Correct answer:

n=4

l=2

m=-3

Explanation:

The principle quantum number (n) and the angular quantum number (l) are acceptable. However, the magnetic quantum number (m) is restricted to lie between -l and l. Therefore, for l=2, the only possible numbers for m are -2, -1, 0, 1, 2. 

Example Question #22 : Quantum Chemistry

Alkynes exhibit a distinctive peak in the  region of an IR spectrum, caused by the stretching of the triple bond. After performing a synthesis of ethyne in the laboratory, she takes an infrared spectrum of the synthesized compound and no such peak appears. Which of the following statements best explains the reason for this phenomenon? 

Possible Answers:

The compound synthesized was not ethyne, and therefore the IR spectrum is incorrect

Ethyne does not contain triple bonds

Ethyne is has a center of symmetry about its triple bond, making it IR inactive

There was not enough sample for the detector to read

The instrument is malfunctioning; ethyne should definitely show a peak at 

Correct answer:

Ethyne is has a center of symmetry about its triple bond, making it IR inactive

Explanation:

Infrared spectroscopes uses dipole changes as a means of recording data. Because ethyne has a center of symmetry about its triple bond, there is no net dipole change when the bond stretches, and therefore no signal is measured by the detector. Such molecules are called IR inactive and a Raman spectrum must be obtained in order to observe the desired peaks. 

Example Question #1 : Covalent Bonding

Which statement best characterizes a covalent bond? 

Possible Answers:

Electrical conduction

Crystal structure

Sharing electrons between atoms 

Cations and anions

Electron sea

Correct answer:

Sharing electrons between atoms 

Explanation:

To achieve an octet of valence electrons, atoms can share electrons so that all atoms participating in the bond will have full valence shells. Covalent bonds, by definition, result from the sharing of one or more pairs of valence electrons.

Example Question #2 : Covalent Bonding

How many single covalent bonds would the element sulfur be expected to form in order to obey the octet rule?

Possible Answers:

Correct answer:

Explanation:

The key to this problem is that electrons in covalent bonds are shared and therefore "belong" to both of the bonded atoms. Sulfur is a nonmetal in group 6A , and therefore has 6 valence electrons. In order to obey the octet rule, it needs to gain 2 electrons . It can do this by forming 2 single covalent bonds.

Example Question #3 : Covalent Bonding

How many single covalent bonds would the element selenium be expected to form in order to obey the octet rule? 

Possible Answers:

0

Correct answer:

Explanation:

The key to this problem is that electrons in covalent bonds are shared and therefore "belong" to both of the bonded atoms. Selenium is a nonmetal in group 6A , and therefore has 6 valence electrons. In order to obey the octet rule, it needs to gain 2 electrons. It can do this by forming 2 single covalent bonds.

Example Question #4 : Covalent Bonding

Which of the following constitutes a covalent bond?

Possible Answers:

All of these

Constructive interference between atomic orbitals

Electron pair shared between two neighboring atoms

Spin pairing of electrons

Atoms of similar electronegativities

Correct answer:

All of these

Explanation:

A chemical bond is considered covalent if there is sharing of one or more pairs of electrons between atoms. As opposed to a covalent bond, an ionic bond can involve the transfer of electrons from one atom to another resulting in a high charge differential between two atoms in order for them to acquire a full octet.

As described by the Pauli Exclusion Principle, every pair of electrons must consist of spin-up paired with spin-down. It states that no more than two electrons may occupy an orbital, and in full electron orbitals, the spin of one must cancel the spin of the other so their spins will have a zero net spin/angular momentum.

Chemical bonds are made up of orbitals, which are simply waves that have wave functions. Wave functions tell us the likelihood that an electron can be found within an orbital. Constructive interference of two wave functions/orbitals occur when the two waves are in phase and result in a new wave function, or in other words a chemical bond.

Example Question #1 : Molecules

Which of the following molecules does not contain polar bonds? 

Possible Answers:

Correct answer:

Explanation:

Polar bonds form between all atoms of different electronegativity. Bromine is a diatomic molecule and thus both atoms of bromine have the same electronegativity. This bond between bromine is perfectly nonpolar, meaning that both atoms share the electron density equally.

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