C373 General Chemistry I with Lab
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Free C373 General Chemistry I with Lab Questions
The hybridization of carbon in C₂H₄ is:
- sp
- sp²
- sp³
- sp³d
Explanation
In ethene (C₂H₄), each carbon atom forms three sigma (σ) bonds—two with hydrogen atoms and one with the other carbon atom—and one pi (π) bond as part of the double bond. The formation of three sigma bonds requires three hybrid orbitals, meaning each carbon atom is sp² hybridized. The remaining unhybridized p orbital on each carbon overlaps sideways to form the π bond.
Correct Answer:
sp²
Which molecule is an exception to the octet rule with only 6 electrons?
- BF₃
- PF₅
- SF₆
- XeF₄
Explanation
Boron trifluoride (BF₃) is a well-known exception to the octet rule. In this molecule, boron forms three single bonds with fluorine atoms, resulting in only six electrons around the boron atom. Because boron lacks sufficient valence electrons to complete an octet and does not have available d orbitals for expansion, it remains electron-deficient. In contrast, PF₅, SF₆, and XeF₄ all involve central atoms that have expanded octets due to available d orbitals in period 3 or higher.
Correct Answer:
BF₃
Which gas effuses fastest at the same T and P?
- He
- Ne
- Ar
- Kr
Explanation
According to Graham’s law of effusion, the rate of effusion of a gas is inversely proportional to the square root of its molar mass. He (helium) has the smallest molar mass (4 g/mol) compared with Ne (20 g/mol), Ar (40 g/mol), and Kr (84 g/mol). Therefore, helium effuses the fastest under the same conditions of temperature and pressure.
Correct Answer:
He
The Lewis structure of BeCl₂ shows beryllium with how many electrons around it?
- 4
- 6
- 8
- 10
Explanation
Beryllium is an exception to the octet rule. In BeCl₂, beryllium forms two single bonds with chlorine atoms, each bond containing two shared electrons. Therefore, beryllium is surrounded by a total of four electrons (two pairs). Because it lacks the necessary orbitals to expand its octet, BeCl₂ remains electron-deficient, making it an example of a compound where the central atom has less than an octet.
Correct Answer:
4
Which element can form more than an octet (expanded octet)?
- C
- N
- P
- Be
Explanation
Elements in period 3 or beyond of the periodic table can form expanded octets because they have available d orbitals that can accommodate additional electrons beyond the usual eight. Phosphorus (P), being in period 3, can form compounds such as PCl₅ and PF₅ where it has ten electrons in its valence shell. In contrast, carbon, nitrogen, and beryllium are limited to an octet or less since they do not have available d orbitals.
Correct Answer:
P
In the best Lewis structure of O₃, one oxygen has a formal charge of:
- 0
- +1
- –1
- +2
Explanation
Ozone (O₃) has resonance structures where the central oxygen atom forms one double bond and one single bond with the other two oxygens. To satisfy the octet rule, formal charges are distributed as follows: one terminal oxygen atom has a formal charge of –1, the central oxygen has a charge of +1, and the other terminal oxygen has a charge of 0. These resonance forms alternate between the two terminal oxygen atoms, giving the molecule an overall neutral charge.
Correct Answer:
–1
Which compound contains a coordinate covalent bond in its Lewis structure?
- H₂O
- NH₃
- NH₄⁺
- CH₄
Explanation
A coordinate covalent bond (also known as a dative bond) occurs when one atom donates both electrons in a shared pair to form a bond. In the ammonium ion (NH₄⁺), this occurs when an NH₃ molecule donates a lone pair from the nitrogen atom to bond with a hydrogen ion (H⁺), which lacks electrons. This forms a coordinate covalent bond between nitrogen and the added hydrogen, resulting in NH₄⁺. The other compounds—H₂O, NH₃, and CH₄—have only normal covalent bonds.
Correct Answer:
NH₄⁺
What type of reaction is used in a breathalyzer (ethanol → acetaldehyde)?
- Dehydration
- Oxidation
- Reduction
- Esterification
Explanation
In a breathalyzer, ethanol is converted to acetaldehyde. This involves the loss of hydrogen atoms from ethanol, which is an oxidation process. Oxidation in organic chemistry often refers to increasing the number of bonds from carbon to oxygen or decreasing the number of bonds to hydrogen. Therefore, the reaction of ethanol to acetaldehyde is an oxidation reaction.
Correct Answer:
Oxidation
As you move left to right across period 3, what happens to electronegativity?
- Decreases
- Remains constant
- Increases
- First increases, then decreases
Explanation
Electronegativity refers to an atom’s ability to attract electrons in a chemical bond. Across a period from left to right, the number of protons in the nucleus increases while the atomic radius decreases due to stronger attraction between the nucleus and electrons. This stronger pull causes atoms to attract bonding electrons more effectively, so electronegativity increases from left to right across period 3.
Correct Answer:
Increases
Which molecule requires resonance structures?
- CH₄
- SO₂
- BF₃
- SiH₄
Explanation
Resonance structures are needed when more than one valid Lewis structure can be drawn for a molecule due to the delocalization of electrons. In SO₂, sulfur can form double bonds with either of the two oxygen atoms, creating two equivalent structures. The actual structure is a resonance hybrid, with the double-bond character delocalized between both sulfur–oxygen bonds. Compounds like CH₄, BF₃, and SiH₄ do not require resonance structures because their bonding arrangements are fixed and cannot be represented by multiple equivalent Lewis structures.
Correct Answer:
SO₂
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