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AP Chemistry Practice Quiz: Representations of Reactions

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 7 questions to check your progress.

Question 1 of 7

The image provided shows a particulate representation of a chemical reaction. Which of the following balanced chemical equations best represents this process?

All Questions (7)

The image provided shows a particulate representation of a chemical reaction. Which of the following balanced chemical equations best represents this process?

A) 2X₂ + Y₂ → 2X₂Y

B) X₂ + 2Y₂ → 2XY₂

C) 4X + 2Y → 2X₂Y

D) 2X₂ + Y₂ → X₄Y₂

Correct Answer: A

The 'Before' container shows two molecules of X₂ (diatomic molecules of the larger spheres) and two molecules of Y₂ (diatomic molecules of the smaller spheres). The 'After' container shows two molecules of X₂Y and one unreacted molecule of Y₂. This means two X₂ molecules reacted with one Y₂ molecule to produce two X₂Y molecules, making Y₂ the limiting reactant. The balanced equation for the reaction that occurred is 2X₂ + Y₂ → 2X₂Y.

Which of the following particulate diagrams correctly represents the balanced chemical reaction for the synthesis of ammonia from nitrogen and hydrogen gas: N₂(g) + 3H₂(g) → 2NH₃(g)?

A) One N₂ molecule reacts with one H₂ molecule to form one NH₃ molecule and one H atom.

B) One N₂ molecule reacts with three H₂ molecules to form two NH₃ molecules.

C) Two N atoms react with six H atoms to form two NH₃ molecules.

D) One N₂ molecule reacts with one and a half H₂ molecules to form one NH₃ molecule.

Correct Answer: B

The balanced equation N₂(g) + 3H₂(g) → 2NH₃(g) indicates that one diatomic nitrogen molecule (N₂) reacts with three diatomic hydrogen molecules (H₂) to produce two ammonia molecules (NH₃). The particulate model must reflect this 1:3:2 stoichiometric ratio of molecules.

A student observes a particulate diagram representing a sealed container where ice, H₂O(s), is heated and transforms into water vapor, H₂O(g). Which statement best describes the change shown in a correct particulate model of this process?

A) The H₂O molecules break apart into H₂ and O₂ molecules.

B) The number of H₂O molecules decreases as they evaporate.

C) The arrangement of H₂O molecules becomes more disordered, and the distance between them increases.

D) The size and shape of the individual H₂O molecules change significantly.

Correct Answer: C

The process described is a physical change (sublimation or melting then boiling), not a chemical reaction. In a physical change, the chemical identity of the substance does not change. Therefore, the H₂O molecules themselves remain intact. The change is in their arrangement and kinetic energy. In the gas phase, molecules are far apart and move randomly, which is a more disordered state than the ordered lattice of a solid.

Consider the combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). A particulate diagram shows a sealed container with 1 molecule of CH₄ and 3 molecules of O₂ before the reaction. Which option correctly describes the contents of the container after the reaction goes to completion?

A) 1 molecule of CO₂, 2 molecules of H₂O, and 1 molecule of CH₄.

B) 1 molecule of CO₂ and 2 molecules of H₂O only.

C) 2 molecules of CO₂, 4 molecules of H₂O, and 1 molecule of O₂.

D) 1 molecule of CO₂, 2 molecules of H₂O, and 1 molecule of O₂.

Correct Answer: D

The balanced equation shows that 1 molecule of methane (CH₄) reacts with 2 molecules of oxygen (O₂). The initial container has 1 molecule of CH₄ and 3 molecules of O₂. Therefore, methane is the limiting reactant. The single CH₄ molecule will react completely with 2 of the O₂ molecules. This will produce 1 molecule of carbon dioxide (CO₂) and 2 molecules of water (H₂O). One molecule of O₂ will remain unreacted as the excess reactant. The final mixture is 1 CO₂, 2 H₂O, and 1 O₂.

A student proposes a particulate model for the decomposition of hydrogen peroxide: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). The student's 'After' diagram shows the formation of two H₂O molecules and two O atoms. What is the primary flaw in this representation?

A) The model violates the law of conservation of mass because atoms are lost.

B) The model incorrectly represents oxygen, which should be a diatomic molecule (O₂) in its elemental form.

C) The model shows the formation of too many water molecules according to the stoichiometry.

D) The model incorrectly depicts a physical change instead of a chemical reaction.

Correct Answer: B

The balanced equation indicates that one of the products is O₂, which is molecular oxygen. This consists of two oxygen atoms bonded together (a diatomic molecule). The student's model showing two separate, unbonded oxygen atoms (O) is an incorrect representation of the product. While the number of each type of atom is conserved, the chemical form of the oxygen product is wrong.

The particulate model shown represents a substance in a sealed container. If the container is cooled, causing the substance to deposit as a solid, which of the following statements would be true for the new particulate model?

A) The molecules would break down into their constituent atoms.

B) The molecules would be arranged in an ordered, crystalline lattice.

C) The number of molecules in the container would decrease.

D) The chemical composition of the molecules would change.

Correct Answer: B

Deposition is a physical process where a gas turns directly into a solid. This is a phase change, not a chemical reaction. The molecules themselves remain intact. In the solid state, particles are typically arranged in a fixed, ordered, and tightly packed structure known as a crystalline lattice. The distance between molecules decreases significantly, and their motion is restricted to vibrations.

A particulate model is created to represent the reaction between hydrogen gas and chlorine gas to form hydrogen chloride: H₂(g) + Cl₂(g) → 2HCl(g). The key for the model is: White sphere = H atom, Green sphere = Cl atom. Which description matches the correct particulate representation of the reactants and products?

A) Reactants: One molecule with two white spheres, One molecule with two green spheres. Products: One molecule with one white and one green sphere.

B) Reactants: Two white spheres, Two green spheres. Products: Two molecules, each with one white and one green sphere.

C) Reactants: One molecule with two white spheres, One molecule with two green spheres. Products: Two molecules, each with one white and one green sphere.

D) Reactants: One molecule with one white sphere, One molecule with one green sphere. Products: Two molecules, each with one white and one green sphere.

Correct Answer: C

The balanced equation is H₂ + Cl₂ → 2HCl. H₂ is a diatomic molecule, represented by two white spheres bonded together. Cl₂ is also a diatomic molecule, represented by two green spheres bonded together. These are the reactants. The product is 2HCl, which means two separate molecules are formed. Each HCl molecule consists of one white sphere (H) bonded to one green sphere (Cl). Therefore, the products are two molecules of HCl.