Representations of Reactions - AP Chemistry

A student observes that mixing aqueous Na $_2$ S and aqueous Cd(NO $_3$ ) $_2$ produces a yellow solid CdS. Which balanced molecular equation correctly represents the reaction?
Answer: Na $_2$ S(aq) + Cd(NO $_3$ ) $_2$ (aq) CdS(s) + 2NaNO $_3$ (aq) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 2 Na, 1 S, 1 Cd, 2 N, 6 O on both sides. Verify neutral charges. Ensure states: aqueous to solid CdS, aqueous NaNO3. A tempting distractor is choice B, with incorrect coefficients and formula CdS2. Always check atom count and charge before worrying about reaction type.
A complete ionic equation is shown: 2Na $^+$ (aq) + CO $_3^{2-}$ (aq) + Ca $^{2+}$ (aq) + 2Cl $^-$ (aq) CaCO $_3$ (s) + 2Na $^+$ (aq) + 2Cl $^-$ (aq) Which statement correctly identifies the spectator ions in this reaction representation?
Answer: Na $^+$ (aq) and Cl $^-$ (aq) This question assesses the skill of representations of reactions. To check consistency across representations, count all atoms in the complete ionic equation to ensure balance. Verify charges: total +6 left and right. Identify spectators as ions unchanged on both sides. A tempting distractor is choice B, which are the precipitating ions, not spectators. Always check atom count and charge before worrying about reaction type.
A particulate diagram shows that aqueous ions remain in solution while a new aqueous molecular species forms: H $^+$ (aq) combines with OH $^-$ (aq) to form H $_2$ O(l). Which net ionic equation matches the diagram?
Answer: H $^+$ (aq) + OH $^-$ (aq) H $_2$ O(l) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 1 H, 1 O, 1 H on left to 2 H, 1 O on right. Verify charge: +1 (H+) -1 (OH-) to neutral H2O. Confirm formation of molecular water from ions. A tempting distractor is choice B, which reverses the reaction direction. Always check atom count and charge before worrying about reaction type.
A precipitation reaction occurs when aqueous iron(III) nitrate is mixed with aqueous sodium hydroxide, forming solid iron(III) hydroxide. Which net ionic equation correctly represents the reaction?
Answer: $\mathrm{Fe^{3+}(aq) + 3\,OH^-(aq) \rightarrow Fe(OH)_3(s)}$ This problem tests representations of reactions by asking for the net ionic equation of a precipitation reaction. The net ionic equation shows only the ions that form the precipitate: $\mathrm{Fe}^{3+}(aq) + 3 \mathrm{OH}^{-}(aq) \rightarrow \mathrm{Fe(OH)_3(s)}$ . Checking conservation: Fe ( $1 \to 1$ ), O ( $3 \to 3$ ), H ( $3 \to 3$ ), and charge ( $+3 + 3(-1) = 0$ on both sides). Choice C shows the complete molecular equation, not the net ionic equation—it includes spectator ions $\mathrm{Na}^{+}$ and $\mathrm{NO_3}^{-}$ . For net ionic equations, remove all spectator ions and show only the species that undergo chemical change.
A student claims that the net ionic equation for mixing aqueous ammonium chloride and aqueous sodium hydroxide is $\mathrm{NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(aq) + H_2O(l)}$ . Which molecular equation is consistent with that net ionic equation (including physical states)?
Answer: $\mathrm{NH_4Cl(aq) + NaOH(aq) \rightarrow NH_3(aq) + H_2O(l) + NaCl(aq)}$ This question tests the skill of representations of reactions by asking you to match a net ionic equation with its molecular form. To verify consistency, the molecular equation must produce the same net ionic equation when spectator ions are removed: NH₄Cl provides NH₄⁺ and Cl⁻, NaOH provides Na⁺ and OH⁻, and after removing spectators (Na⁺ and Cl⁻), we get NH₄⁺ + OH⁻ → NH₃ + H₂O. Choice C incorrectly shows NH₄OH as a product, but this compound doesn't exist as a stable molecule (ammonia exists as NH₃ in water). The correct answer A shows NH₄Cl(aq) + NaOH(aq) → NH₃(aq) + H₂O(l) + NaCl(aq), which yields the given net ionic equation when spectator ions are removed. To verify molecular equations match net ionic equations, write the complete ionic form and confirm that removing spectators gives the target net equation.
A student writes the following equation to represent the formation of solid aluminum oxide from its elements: Al(s) + O $_2$ (g) Al $_2$ O $_3$ (s) Which balanced equation correctly represents the same reaction while conserving atoms?
Answer: 4Al(s) + 3O $_2$ (g) 2Al $_2$ O $_3$ (s) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 4 Al, 6 O on left to 4 Al, 6 O on right. Verify neutral charges. Ensure coefficients balance oxygen: 4 Al + 3 O2 to 2 Al2O3. A tempting distractor is choice A, with unbalanced coefficients for Al and O2. Always check atom count and charge before worrying about reaction type.
A particulate-level description shows that in aqueous solution, one Fe $^{3+}$ (aq) ion combines with three OH $^-$ (aq) ions to form an insoluble solid. Which equation best represents the process?
Answer: Fe $^{3+}$ (aq) + 3OH $^-$ (aq) Fe(OH) $_3$ (s) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 1 Fe, 3 O, 3 H on left to 1 Fe, 3 O, 3 H on right. Verify charge: +3 (Fe) -3 (3OH) to neutral Fe(OH)3. Confirm solid state for insoluble compound. A tempting distractor is choice B, with incorrect coefficient unbalancing O and H. Always check atom count and charge before worrying about reaction type.
A student writes the following balanced molecular equation for a precipitation reaction: Pb(NO $_3$ ) $_2$ (aq) + 2KI(aq) PbI $_2$ (s) + 2KNO $_3$ (aq) Which net ionic equation corresponds to the reaction?
Answer: Pb $^{2+}$ (aq) + 2I $^-$ (aq) PbI $_2$ (s) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms from the molecular equation: 1 Pb, 2 N, 6 O, 2 K, 2 I on both sides. Verify charge in net ionic: +2 (Pb) and -2 (2I) to neutral PbI2. Remove spectators like K+ and NO3- for the net form. A tempting distractor is choice D, which is the molecular equation, not net ionic. Always check atom count and charge before worrying about reaction type.
A particulate diagram shows diatomic hydrogen molecules reacting with diatomic chlorine molecules to form hydrogen chloride molecules. Which balanced molecular equation matches this particle-level description?
Answer: H $_2$ (g) + Cl $_2$ (g) 2HCl(g) This question assesses the skill of representations of reactions. To check consistency across representations, count the atoms in the particulate diagram and ensure the equation matches, with equal H and Cl on both sides. Verify that charges are neutral in this molecular equation, as all are gases. Confirm the coefficients reflect the diatomic nature, like 1 H2, 1 Cl2 to 2 HCl. A tempting distractor is choice B, which doubles H2 but not Cl2, violating atom conservation. Always check atom count and charge before worrying about reaction type.
A particulate description shows that one Al $^{3+}$ (aq) ion combines with three F $^-$ (aq) ions to form a solid ionic compound. Which net ionic equation matches the description?
Answer: Al $^{3+}$ (aq) + 3F $^-$ (aq) AlF $_3$ (s) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 1 Al, 3 F on both sides. Verify charge: +3 (Al) and -3 (3F) to neutral AlF3. Confirm solid state for the compound. A tempting distractor is choice A, with incorrect coefficient and charge for AlF2+. Always check atom count and charge before worrying about reaction type.
A student mixes aqueous solutions of calcium chloride and sodium carbonate. A white precipitate forms. Which balanced molecular equation best represents the reaction (including states)?
Answer: CaCl $_2$ (aq) + Na $_2$ CO $_3$ (aq) CaCO $_3$ (s) + 2NaCl(aq) This question assesses the skill of representations of reactions. To check consistency across representations, first count the atoms of each element on both sides of the equation to ensure conservation, such as 1 Ca, 2 Cl, 2 Na, 1 C, and 3 O in the balanced form. Next, verify that charges are conserved if ionic species are present, though this molecular equation shows neutral compounds. Additionally, confirm that the states of matter align with the description, like the white precipitate being solid CaCO3. A tempting distractor is choice A, which incorrectly shows CaCO3 as aqueous and NaCl as solid, reversing the solubility rules for these compounds. Always check atom count and charge before worrying about reaction type.
A student writes the balanced molecular equation $\mathrm{2\,Al(s) + 3\,Cl_2(g) \rightarrow 2\,AlCl_3(s)}$ . Which statement best describes the particle-level change represented by the equation?
Answer: Two Al atoms each bond with three Cl atoms, forming two units of $\mathrm{AlCl_3}$ from a total of six Cl atoms originally in three $\mathrm{Cl_2}$ molecules. This question tests the skill of representations of reactions by asking you to interpret a balanced equation at the particle level. To verify the interpretation, check that it matches the coefficients and formulas: 2 Al atoms react with 3 Cl₂ molecules (containing 6 Cl atoms total) to form 2 AlCl₃ units. Each AlCl₃ contains 1 Al and 3 Cl atoms, so 2 units contain 2 Al and 6 Cl atoms total, conserving all atoms. Choice A incorrectly describes the reverse reaction (decomposition rather than synthesis). The correct answer B accurately states that two Al atoms each bond with three Cl atoms, using six Cl atoms from three Cl₂ molecules to form two AlCl₃ units. When interpreting equations at the particle level, carefully track how atoms rearrange from reactant molecules to product molecules.
A student is shown a representation where two NH $_3$ (g) molecules react with one CO $_2$ (g) molecule to form one molecule of urea, CO(NH $_2$ ) $_2$ (s), and one H $_2$ O(l) molecule. Which equation matches this representation and conserves atoms?
Answer: 2NH $_3$ (g) + CO $_2$ (g) CO(NH $_2$ ) $_2$ (s) + H $_2$ O(l) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 2 N, 6 H, 1 C, 2 O on left to 1 C, 2 O, 2 N, 6 H on right. Verify neutral charges. Confirm states: gases to solid urea and liquid water. A tempting distractor is choice A, lacking coefficient for NH3, unbalancing N and H. Always check atom count and charge before worrying about reaction type.
A reaction in aqueous solution is described as: "Aqueous aluminum ions combine with aqueous phosphate ions to form solid aluminum phosphate." Which balanced net ionic equation best represents this reaction?
Answer: $\mathrm{Al^{3+}(aq) + PO_4^{3-}(aq) \rightarrow AlPO_4(s)}$ This problem tests representations of reactions for a precipitation reaction at the ionic level. The equation must balance atoms and charge: Al (1→1), P (1→1), O (4→4), with charge +3 + (-3) = 0 on the left and 0 on the right. AlPO₄ forms as a solid precipitate when Al³⁺ and PO₄³⁻ ions combine in aqueous solution. Choice C incorrectly shows Al²⁺, but aluminum forms +3 ions, not +2. When writing ionic equations, always use the correct charges for ions based on their position in the periodic table or polyatomic ion charges.
A reaction is described as: "Solid magnesium oxide is added to hydrochloric acid, producing aqueous magnesium chloride and liquid water." Which balanced molecular equation best represents this reaction, including physical states?
Answer: $\mathrm{MgO(s) + 2\,HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)}$ This problem requires the skill of representations of reactions to write a balanced equation matching a verbal description. Checking conservation: Mg (1→1), O (1→1), H (2→2), Cl (2→2), all atoms balance correctly. The equation also shows correct physical states: MgO is a solid metal oxide, HCl is aqueous acid, MgCl₂ is soluble (aqueous), and water is liquid. Choice D shows a different reaction—metallic Mg reacting with HCl to produce H₂ gas, not the acid-base reaction described. Always match the chemical species and physical states exactly to the problem description.
A student mixes aqueous solutions of calcium chloride and sodium carbonate and observes a white precipitate form. Which balanced molecular equation best represents the reaction that occurs (including physical states)?
Answer: $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2\,NaCl(aq)}$ This question tests the skill of representations of reactions by asking you to identify the correct molecular equation with proper physical states. To check consistency across representations, verify that atoms are conserved: reactants have 1 Ca, 2 Cl, 2 Na, 1 C, and 3 O atoms, which must equal the products. Also verify charge is balanced: all species are neutral compounds, so charge is automatically balanced. Choice A incorrectly shows NaCl as a solid when it should be aqueous (NaCl is highly soluble), and CaCO₃ as aqueous when it should be solid (CaCO₃ is insoluble). The correct answer B properly shows CaCO₃(s) as the precipitate and NaCl(aq) remaining dissolved, with a coefficient of 2 for NaCl to balance the equation. When analyzing reaction representations, always verify both atom conservation and correct physical states based on solubility rules.
In a beaker, aqueous potassium sulfate is mixed with aqueous barium nitrate. A solid forms. Which balanced molecular equation best matches this observation?
Answer: $\mathrm{K_2SO_4(aq) + Ba(NO_3)_2(aq) \rightarrow BaSO_4(s) + 2\,KNO_3(aq)}$ This question requires the skill of representations of reactions to match observations with a balanced molecular equation. Checking atom conservation: K (2→2), S (1→1), O (4→4), Ba (1→1), N (2→2), all atoms balance correctly. Charge is conserved as all species are neutral compounds. Choice C incorrectly shows Ba(SO₄)₂ which doesn't exist—barium has a +2 charge and sulfate has a -2 charge, so they combine in a 1:1 ratio as BaSO₄. Remember to check both atom balance and correct chemical formulas based on ion charges.
A student writes the following net ionic equation for a precipitation reaction: $\mathrm{Pb^{2+}(aq) + 2\,I^-(aq) \rightarrow PbI_2(s)}$ . Which balanced molecular equation corresponds to this net ionic equation?
Answer: $\mathrm{Pb(NO_3)_2(aq) + 2\,KI(aq) \rightarrow PbI_2(s) + 2\,KNO_3(aq)}$ This question tests representations of reactions by asking you to match a net ionic equation with its corresponding molecular equation. The net ionic equation shows Pb²⁺ + 2I⁻ → PbI₂(s), so the molecular equation must provide these ions from soluble compounds. Checking atom balance: Pb (1→1), N (2→2), O (6→6), K (2→2), I (2→2), all balanced correctly. Choice C fails to balance iodine atoms (only 1 I on the left but 2 I in PbI₂). Always verify that coefficients in the molecular equation produce the correct number of ions shown in the net ionic equation.
A word description states: "Solid iron(III) oxide reacts with carbon monoxide gas to form solid iron and carbon dioxide gas." Which balanced molecular equation matches the description?
Answer: Fe $_2$ O $_3$ (s) + 3CO(g) 2Fe(s) + 3CO $_2$ (g) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 2 Fe, 3 O, 3 C, 3 O on left to 2 Fe, 3 C, 6 O on right. Verify neutral charges. Ensure states: solid Fe2O3, gas CO to solid Fe, gas CO2. A tempting distractor is choice A, lacking coefficients, unbalancing C and O. Always check atom count and charge before worrying about reaction type.
A student describes a reaction: "Solid zinc metal is placed into a solution containing copper(II) ions. Copper metal forms, and zinc ions appear in solution." Which balanced net ionic equation best represents this reaction?
Answer: Zn(s) + Cu $^{2+}$ (aq) → Zn $^{2+}$ (aq) + Cu(s) This question assesses the skill of representations of reactions. To check consistency across representations, count atoms like 1 Zn and 1 Cu on both sides. Verify charge conservation, with +2 on left (Cu2+) matching +2 on right (Zn2+). Ensure states match the description: solid Zn to aqueous Zn2+, aqueous Cu2+ to solid Cu. A tempting distractor is choice A, which omits charges and shows both as solids without ions. Always check atom count and charge before worrying about reaction type.

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