HESI A2 Chemistry Practice Questions

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: +1. In KCl, potassium is a group 1 element, which always has an oxidation state of +1 in its compounds. Chlorine, a group 17 element, has an oxidation state of -1 in most compounds. Therefore, the oxidation state of potassium in KCl must be +1 to balance the -1 oxidation state of chlorine. Choices B, C, and D are incorrect because they do not align with the known oxidation states of potassium and chlorine in this compound.

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: The correct formula for calcium carbonate is CaCO₃. Calcium carbonate consists of one calcium ion (Ca²⁺) and one carbonate ion (CO₃²⁻). The calcium ion has a 2+ charge while the carbonate ion has a 2- charge, so they combine in a 1:1 ratio to balance the charges. Therefore, the correct formula is CaCO₃. Choice A (CaSO₃) is incorrect because it represents calcium sulfite, not calcium carbonate. Choice C (Ca(OH)₂) is incorrect as it represents calcium hydroxide, not calcium carbonate. Choice D (CH₃OH) is incorrect as it represents methanol, not calcium carbonate.

Correct Answer: A

Rationale: The correct answer is A: H⁻. A hydride ion is formed when a hydrogen atom gains an extra electron, resulting in a negatively charged ion. The notation for a hydride ion is H⁻, indicating the presence of a single hydrogen atom with a negative charge. This is the only choice that accurately represents a hydride ion. Explanation of incorrect choices: B: H⁺ - This represents a hydrogen ion with a positive charge, not a hydride ion. C: OH⁻ - This represents a hydroxide ion, not a hydride ion. D: HCl - This is a compound consisting of a hydrogen atom and a chlorine atom, not a hydride ion.