Chemistry · Unit 4 · Paper 1

Chemical Reactions unit test

A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.

Paper 1 Paper 2 Paper 3

Each paper is built from this unit’s 17 terms and is the same for everyone, so a teacher can assign “Unit 4, Paper 1” and every student sits the identical test. Multiple choice is marked objectively; the written sections you mark yourself against the model answer and rubric.

Suggested time 37 min 34 points0/17 attempted

Solubility rules

Back titration

Acid-base neutralization

Equivalence vs endpoint

Oxidation number rules

Identifying reaction type

Percent yield

Balancing redox in acid

Limiting reactant

Gravimetric analysis

Net ionic equation

Titration calculation

Short answer 1. Define or explain: Titration

3 pts

Short answer 2. Define or explain: Spectator ions

3 pts

Short answer 3. Define or explain: Stoichiometry road map

3 pts

Short answer 4. Define or explain: Precipitation reaction

3 pts

Free response

10 pts

A reaction between maleic acid, H2C4H2O4, and sodium bicarbonate, NaHCO3, occurs in the presence of water: H2C4H2O4(aq) + 2 NaHCO3(aq) → 2 CO2(g) + 2 H2O(l) + Na2C4H2O4(aq) A student combines equal masses of H2C4H2O4(s) chunks and NaHCO3(s) chunks with sufficient water at 20.0°C, and determines that 0.0114 mol of CO2(g) is produced after the reaction goes to completion. The CO2(g) produced at 20.0°C was collected and found to have a pressure of 1.25 atm. The student then performs a second experiment identical to the first except that the chunks are ground into powder before being combined with water. The rate-determining step for the overall reaction is the dissolving of the solids. Additional trials: Trial Mass of H2C4H2O4 (g) Mass of NaHCO3 (g) Moles of CO2 produced 3 1.543 1.251 0.01489 4 1.543 1.686 0.02007 Molar masses: H2C4H2O4 = 116.07 g/mol, NaHCO3 = 84.01 g/mol, CO2 = 44.01 g/mol. The reaction has ΔS° greater than zero, and the temperature of the reaction mixture decreases as the reaction takes place, so the reaction is endothermic. Maleic acid is diprotic: H2C4H2O4 + H2O ⇌ HC4H2O4− + H3O+ Ka1 = 1.5 × 10−2 HC4H2O4− + H2O ⇌ C4H2O4²− + H3O+ Ka2 = 8.5 × 10−7

(a)(i) Calculate the number of grams of CO2(g) produced in the first experiment.

(a)(ii) Calculate the volume of the CO2(g), in liters, at 20.0°C and 1.25 atm.

(b)(i) What happens to the surface area of the reactants when the student grinds the chunks into powder?

(b)(ii) Would the time required for the dissolving of the solids in the second experiment be longer than, shorter than, or the same as the time required in the first experiment? Justify your answer based on the collisions between particles.

(b)(iii) When the reaction is complete, will the volume of CO2(g) at the end of the second experiment be greater than, less than, or equal to the volume at the end of the first experiment? Justify your answer.

(d) The reaction has a value of ΔS° greater than zero. Using particle-level reasoning, explain why the entropy increases as the reaction progresses.

(e) The student claims that the reaction is thermodynamically favorable at all temperatures because ΔS°rxn > 0 and the reaction is endothermic. Do you agree or disagree with the student’s claim? Justify your answer.

(f) Calculate the pKa2 value for the HC4H2O4− ion.

(g) A buffer solution with a pH of 7.00 is prepared using C4H2O4²− and HC4H2O4−. Calculate the ratio [C4H2O4²−] / [HC4H2O4−] in this solution.