AP Chemistry Notes

Free Response Question #3 - 2023

Part A: Balanced Net Ionic Equation

  • Reaction involves calcium carbonate (CaCO_3) solid and hydrochloric acid (HCl) solution.

  • Net ionic equation rules:

    • Solids, liquids, and gases remain as they are (e.g., CaCO3, CO2, H_2O).

    • Aqueous substances are broken down into ions (including charges).

  • Hydrochloric acid (HCl) dissociates into hydrogen ions (H^+) and chloride ions (Cl^-).

    • 2HCl(aq) \rightarrow 2H^+(aq) + 2Cl^-(aq)

  • Calcium carbonate remains as (CaCO_3(s)).

  • Spectator ions: Ions that appear on both the reactant and product sides and can be canceled out.

    • In this case, chloride ions (Cl^-) are spectator ions.

  • Net ionic equation (two possible versions):

    • CaCO3(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + H2O(l) + CO_2(g)

    • Or with hydronium ions:

      • CaCO3(s) + 2H3O^+(aq) \rightarrow Ca^{2+}(aq) + 3H2O(l) + CO2(g)

  • Balancing:

    • Ensure the equation is balanced for both mass and charge.

    • Mass balance: One calcium, one carbon, three oxygens, and two hydrogens on each side.

    • Charge balance: Reactant side (+2), Product side (+2).

Part B: Identifying Inconsistent Data

  • Experiment: Studying factors affecting the rate of reaction between hydrochloric acid and calcium carbonate.

  • Variables:

    • Volume of hydrochloric acid: 50 mL

    • Temperature: 21.2 degrees Celsius

    • Mass of calcium carbonate: 1 gram

    • Concentration of hydrochloric acid (1 M and 3 M)

    • Particle size of calcium carbonate

  • Task: Explain why trial 5 is inconsistent with other trials. Consider concentration of (HCl) and particle size.

  • Explanation Approaches:

    • Compare trials 5 and 6: '‘ Same [HCl], different particle sizes. Trial 5 (smaller chunks) should be faster, but it's slower than trial 6 (larger chunks).

    • Compare trials 5 and 2: Trial 5 has higher [HCl] but is slower even with smaller particle sizes, which contradicts expectations.

Part C: Reaction Conditions and Collisions

  • Task: Explain the difference in reaction times between trials 2 and 3, addressing reaction conditions and collisions.

  • Reaction Conditions (Surface Area):

    • Tr ial 2: Small chunks (larger surface area).

    • Trial 3: Large chunks (smaller surface area).

    • Larger surface area in trial 2 exposes more calcium carbonate to hydrogen ions in the acid, leading to a faster reaction.

  • Collisions:

    • Trial 2 (small chunks): Larger surface area leads to more frequent and effective collisions between reactants.

    • More collisions result in a faster reaction time for trial 2 compared to trial 3.

Part D: Order of Reaction

  • Claim: The reaction is zero order with respect to (HCl).

  • Answer: Disagree with the claim.

  • Justification:

    • Argument against zero order:

      • Zero order means changing the concentration of (HCl) would not affect the reaction rate.

      • Comparing trials 1 and 4 : When ([HCl]) is tripled, the reaction time decreases, indicating it's not zero order.

    • Argument for first order (or other order):

      • Comparing trials 1 and 4: Tripling the ([HCl]) roughly divides the reaction time by three, suggesting a first-order relationship (although not definitively proven with only this data).

Part E: Molarity after Reaction

  • Given: (HCl) is in excess in all trials, volume stays at 50 mL.

  • Task: Determine the molarity of (HCl) after the reaction is completed in trial 2.

  • To calculate this answer we are good.