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., $CaCO<em>3$, $CO</em>2$, $H_2O$).

  • 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):

  • $CaCO<em>3(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + H</em>2O(l) + CO_2(g)$

  • Or with hydronium ions:

    • $CaCO<em>3(s) + 2H</em>3O^+(aq) \rightarrow Ca^{2+}(aq) + 3H<em>2O(l) + CO</em>2(g)$

• 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.