HESI A2 Chemistry Practice Questions

Correct Answer: B

Rationale: To balance the equation Fe + Cl2 → FeCl3, we need to ensure the same number of each type of atom on both sides. Starting with Fe, we have 1 Fe on the left and 1 Fe on the right, so Fe is balanced. Next, we have 2 Cl on the left from Cl2 and 3 Cl on the right from FeCl3. To balance Cl, we need to multiply Cl2 by 3 to get 6 Cl atoms on each side. Therefore, the correct balanced equation is: 2Fe + 3Cl2 → 2FeCl3. Choice A (2Fe + 2Cl2 → 2FeCl3) is incorrect because it doesn't balance the Cl atoms properly. Choice C (3Fe + 2Cl2 → 3FeCl3) is incorrect because it doesn't balance the Fe atoms. Choice D (3Fe + 3Cl2 → 6FeCl3)

Correct Answer: A

Rationale: Answer A (NH₃) does not name a polar molecule because it has a trigonal pyramidal shape with a lone pair on nitrogen, causing an uneven distribution of electron density. This results in a net dipole moment, making NH₃ a polar molecule. In contrast, choices B (H₂S), C (SO₂), and D (CO₂) all have symmetrical shapes or polar bonds that cancel out the dipole moments, making them nonpolar molecules.

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: C

Rationale: Failed to generate a rationale after 5 retries.

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.