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: 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: C

Rationale: The correct formula for iron III oxide is Fe₂O₃. In this compound, iron has a +3 oxidation state and oxygen has a -2 oxidation state. By applying the criss-cross method to balance the charges, we get Fe₂O₃. Choice A (IO) is incorrect as it represents a combination of iodine and oxygen. Choice B (FeS) is incorrect as it represents iron and sulfur. Choice D (OFe₂₃) is incorrect as it has the elements in the wrong order and incorrect subscripts. Fe₂O₃ is the correct formula based on the charges and combining the elements in the correct ratios.

Correct Answer: B

Rationale: The correct answer is B (H₂SO₄) because it is a strong acid that completely dissociates into ions in water, leading to a high concentration of H⁺ ions. This is indicated by its ability to donate multiple protons. Choice A (CH₃COOH) is a weak acid that only partially dissociates. Choice C (NH₃) is a weak base that accepts protons rather than donating them. Choice D (KOH) is a strong base that dissociates completely in water but is not an acid.

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)