HESI A2 Chemistry Practice Questions

Correct Answer: B

Rationale: The correct answer is B: London dispersion force. This is the weakest intermolecular force because it occurs between temporary dipoles induced by momentary fluctuations in electron distribution. It is present in all molecules, regardless of polarity. Dipole-dipole interactions (A) are stronger as they involve permanent dipoles, hydrogen bonding (C) is stronger and specific to molecules with hydrogen directly bonded to highly electronegative atoms, and ionic bonding (D) is the strongest intermolecular force due to the electrostatic attraction between oppositely charged ions.

Correct Answer: C

Rationale: To calculate the molar mass of hydrogen iodide (HI), we add the atomic masses of hydrogen (1 g/mol) and iodine (127 g/mol). Therefore, the molar mass of HI is 1 + 127 = 128 g/mol. Rounding to the nearest whole number, the mass of one mole of hydrogen iodide is 128 g/mol, making choice C (87 g/mol) incorrect. Choices A (2 g/mol) and B (58 g/mol) are also incorrect as they do not reflect the correct molar mass of hydrogen iodide.

Correct Answer: B

Rationale: In a single displacement reaction, an active element displaces a less active element in a compound, resulting in the formation of a new compound. This is because more reactive elements have a higher tendency to replace less reactive elements in a compound. The other choices are incorrect because: A: A compound decomposing into two substances describes a decomposition reaction, not a single displacement reaction. C: Formation of a precipitate from the reaction of two solutions describes a double displacement reaction, not a single displacement reaction. D: The change in oxidation states of atoms in the reactants is not a defining characteristic of a single displacement reaction.

Correct Answer: A

Rationale: The correct answer is A: Arrangement of atoms. Allotropes are different forms of the same element with different structures of atoms. This distinction is based on the specific arrangement of atoms in each allotrope, leading to varying physical and chemical properties. Gram atomic mass (B) is related to the weight of an atom, not the arrangement. Physical state (C) refers to the form of matter (solid, liquid, gas), not the atomic structure. Stability (D) may be a factor influenced by the arrangement of atoms, but it is not the primary distinguishing feature among allotropes.

Correct Answer: B

Rationale: The correct answer is B: Pyridine (C5H5N) because it is not a hydrocarbon. Hydrocarbons are compounds composed of only carbon and hydrogen atoms. Pyridine contains nitrogen in addition to carbon and hydrogen, making it a heterocyclic compound. Methane, ethane, and propane are all hydrocarbons as they only contain carbon and hydrogen atoms. Pyridine is not a hydrocarbon as it contains nitrogen in its chemical structure, making it the only non-hydrocarbon option among the choices.